Dudes!

Bob Thomasson sent me some excellent questions about proposed modifications to the Bearhawk a couple of weeks back. While I recommended NOT doing most of them, I think you may learn something useful reading this and realize what all becomes involved when you think about making a "small" modification. The big one to watch out for is what I call the "design propagation nightmare" which can happen with just about any modification. Please remember that unless you know at least as much as the designer about aircraft design (and some of us may), it's best not to mess with it.

I have included Bob's original questions with his permission. Hope you enjoy it. I enjoyed putting it together and it gave me something to do on a long transcontinental airline trip other than just "eating my way across the country."

Russ

-------------Forwarded Message-----------------

From: Russ Erb
To: Robert L. Thomasson,

Date: 8/24/97 4:17 PM

RE: Answers to your questions

Bob-

Ive been off to a Society of Flight Test Engineers Symposium for the last week. Here's your original questions along with my answers.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Russ,

I was browsing the Chapter 1000 web page and discovered that you're building a Bearhawk. I've bought the plans, but haven't started on it yet. I have a couple of aerodynamic type questions. The purpose of my plane will be mostly long trips with heavy loads to Alaska, the Northwest Territories, etc., with floats eventually. I don't see trying to land on very many 100' long sandbars unless I have to. Efficient cruise speed is probably a higher priority than extreme STOL capabilities.

What do you think of the Bearhawk wing airfoil? At Oshkosh I noted that there are endless variations in airfoils for similar type planes. Some have flat profiles on the wing bottom, some are concave and some convex. The Glastar wing is the most interesting to me, with the same airfoil as the Glasair with constant chord. It seems to work well and the Fowler flaps evidently help with STOL. How come nobody else has used this type wing with a "bush type" aircraft? I don't have any specific problem with the existing Bearhawk airfoil, I'm just hesitant to start bending metal when I don't have much of an understanding of the wing. I'm trying to avoid expending 9 million hours of labor only to decide I really wanted another wing airfoil. Did you modify yours at all, or consider any changes?

Same goes for those huge 50 degree flaps. I asked why the flaps don't start until about a foot away from the fuselage when all the other flaps I've looked at start very close to the wing root. The Bearhawk people told me that they flaps are built like that for ease of construction and had a surprise benefit. The propwash flows through the gap to the tail when the flaps are deployed and gives better low speed controllability. What do you think? It also seems to me that the flaps might be too wide and with the way they are hinged will sacrifice too much in wing area when deployed - only conjecture, but what do you think? I'm thinking of spending the extra time to build Fowler flaps. Did you modify the flaps in your aircraft?

Other changes I'm considering are some windows in the roof and a Wittman type gear, since I don't anticipate too much real rough field landings.

Thanks for your time, and I understand if you're too busy to reply. I really enjoyed the Chapter 1000 web site. Good luck with your Bearhawk. What are you going to power it with?

Regards, Bob Thomasson EAA 474976

Bob-
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

You've asked several excellent questions. That shows you're thinking, which is a good thing.

The short answer: don't change anything (with one exception).

The long answer:

Glasair/GlaStar Airfoil:

The airfoil you refer to in the Glasair and GlaStar wing is either the GAW-1 or GAW-2. A friend of mine refers to it as the "Gawd Awful Wing," and everyone I've talked to with opinions I respect feels that this airfoil is highly overrated. It has a very high pitching moment, which will factor into the stability and control of the aircraft. For instance, it would probably require a readjustment in the incidence angle of the horizontal tail or the wing incidence angle. Are you ready to deal with that?

Another problem with the GAW series of airfoils is they are very sensitive to proper shape. In other words, the wing skin must be rather stiff to maintain the shape of the airfoil in order to get the expected performance. On the Glasair, the composite skins are probably sufficiently stiff to accomplish this. You may be thinking "But the GlaStar uses an aluminum wing." But that doesn't mean it's right. I'm guessing that Stoddard-Hamilton used the GAW airfoil on the GlaStar simply because that's what they were used to on the Glasair. Another concern I have with the GlaStar wing is that it uses primarily hat section stringers to maintain the shape of the wing instead of ribs. A local GlaStar builder told me that he did not find a single such stringer in his kit that was the proper shape. He had to make a forming block to force each of them into the proper shape. Since the stringers and skins are pre-punched, there is a reasonable chance that the wing will eventually get to the proper shape, at least to begin with. I'm not yet convinced about how stiff the wing skins will be.

An increasingly more publicized "failure" of an implementation the GAW airfoils is the Piper Tomahawk. When Piper was designing the Tomahawk, they were looking for any advantage they could gain over the Cessna 150 while using the same engine. One of the things they did was to use the "new" GAW airfoil to try to get slightly better performance. This worked out reasonably well on the prototype, which was used to do the certification. Unfortunately, in a story repeated far too often in history, the production engineers thought they were smarter than the design engineers and built the production aircraft with about half as many ribs as the wing originally had. (During a sheet metal workshop, I had the opportunity to de-skin a Tomahawk wing, and was surprised at how few ribs it had. This was before I learned this story.) Sounds good: less weight, lower parts count, right? Wrong! Remember what I said about this airfoil being very sensitive to the proper shape? The end result was that the skins were not stiff enough and would "oil can" under air loads, disrupting the airfoil shape. Wing bending under aero loads would also distort the airfoil. The biggest problem was with the stall characteristics. In the prototype, they were acceptable. On the production birds, the stall was unpredictable and would change from time to time. I did my flight training in a Tomahawk, and have most of my logged time in one (again, this was before I learned about the wing problem). Stalls in a Tomahawk are not a nice, gentle g-break like a Cessna 150. Instead they were characterized by a fairly sharp (violent?) wing drop, which seemed like at least 45ø of bank and was unpredictable in direction. I'm not sure about FAR 23, but I'm pretty sure that the stall characteristics would fail the appropriate Mil Specs. My flight instructor tried to convince me that it was designed that way to "improve training," but I don't buy that anymore. I certainly wouldn't want that sort of "training" low to the ground during a botched turn to final, whereas a g-break would probably be recoverable. The clearest indicator to me was that I was sufficiently scared of the stall characteristics that I refused to practice any stalls in the airplane after I received my certificate. On the other hand, stalls in other aircraft, such as Cessnas or even the Piper Cherokee series are non-events. I recently read that the NTSB is calling for a re-certification of the Tomahawk stall and spin characteristics. The stories I have read correlate well with what I remember. I may have close to 100 hours in the Tomahawk, but based on what I know now, I really have no desire to ever fly in one again. Are you sure you want this airfoil?

One last thing on the GAW airfoils: the undercamber on the lower surface (the concave section) will increase the difficulty of construction and require redesign of the flaps and ailerons. The ailerons on the Bearhawk are of a very nice Frise aileron design. You will note that when the aileron is deflected trailing edge up, the nose of the aileron will poke out the bottom of the wing. The added drag of this nose offsets the additional drag of the opposite aileron (deflected trailing edge down, increasing lift and thus induced drag) and results in a reduction of adverse yaw. That means less rudder is required to coordinate turns. I wouldn't want to mess that up.

Airfoil as designed:

Bob Barrows told me that the Bearhawk airfoil is essentially an NACA 4412 airfoil, which was actually what I expected. This is a very well known, time-tested and proven airfoil with well documented characteristics. You can find it in the classic airfoil text "Theory of Wing Sections" by Abbott and Von Doenhoff (Dover Publications). It's a blue paperback book about 1-1/2" thick that just about any aeronautical engineer will own.

The NACA 4412 has a turbulent boundary layer, which is actually a good thing for an aluminum wing. You may have heard a lot of hype about laminar flow airfoils and their lower drag, especially with respect to composite aircraft. The whole laminar flow business got started back in the early days of WWII, and is best remembered with respect to the development of the P-51. The P-51 was the first aircraft to be designed with an early laminar flow airfoil, which was supposed to give large gains in performance. However, it didn't turn out quite like it was planned. Laminar flow airfoils require very smooth surfaces to work properly. The P-51 had an aluminum wing constructed traditional techniques, and all of those flush rivets and any waviness in the skins were enough to trip the boundary layer to turbulent flow. If not in the beginning, certainly after a little bit of hangar rash and dirt got on the surface. Traditionally, laminar flow airfoils have not worked well with aluminum structures. They have worked with composite structures because the skin could be made extremely smooth and stiff.

The turbulent boundary layer actually helps keep the flow attached to the surface and improves the stall characteristics. Laminar flow airfoils tend to have lower stall angles of attack because of the sharper leading edge and the fact that laminar boundary layers are not as likely to stay attached. The NACA 4412 has a nice rounded leading edge, and a two-dimensional unflapped stall angle of attack of 12 to 16 degrees. Why do I keep harping on stall characteristics? First, a high stall angle of attack indicates that the airfoil will be able to operate at a high angle of attack, which is necessary for STOL. Second, because a STOL aircraft will land and take off with a reduced margin above stall speed, it is imperitive that the aircraft have gentle stall characteristics so that you can get yourself out of trouble shouldst you get into it.

Another nice benefit is that the bottom surface is almost flat, which makes the construction easier.

The 4412 has a design lift coefficient (i.e. minimum drag lift coefficient) of 0.4, which for a 2300 lb Bearhawk, would occur at 97 KCAS. I'm expecting with a 220 HP engine a cruise speed of 140 KCAS. If the aircraft was to get heavier, the profile drag of the airfoil would actually decrease as the lift coefficient increased toward 0.4. Of course, the induced drag will increase, but the overall drag wouldn't change much. With this airfoil, there is still "growth" room.

As it turns out, airfoil selection has very small effect on aircraft performance compared to other factors. Generally you don't turn to tweaking airfoil performance until everything else has been optimized, because the improvements realized are typically no more than a percent or two. (As a side note, wing tips are another area that seems to be popular for changing, but again usually with small or no improvement, and sometimes a degradation in performance. After all, if it was so easy to improve, why wouldn't the designer do it that way in the first place?) Read the August 1997 Sport Aviation article on the Nemesis and Shadow on page 75. Note how small a gain is expected for using different airfoils. In air racing where fractions of a knot matter, the results are noticeable. For the type of flying I expect to do, you'll never notice the difference.

Changing the airfoil leads to a design propagation nightmare. When you change the airfoil, you'll probably change the thickness of the wing at the spar location. Thus, the spars won't be same height, which means you would have to redesign both spars. Assuming that wing got thinner, the spar would be heavier to have the same strength (weight is almost always bad). Well, when you changed the spars, you changed the whole structural analysis, and pretty soon youve designed an entirely new wing with unknown structural characteristics. In addition, you would have to redesign the flaps and ailerons to fit the airfoil, along with the flight control cables, pulleys, bellcranks, and pushrods. The designer wont recognize it as a "Bearhawk," and you'll be on your own as far as builder support. If you really want to do that, then design your own airplane. At least then you'll understand the entire system and the tradeoffs involved. If you "don't have much of an understanding of the wing" the last thing you want to do is to start redesigning major components!

You asked "How come nobody else has used this type wing with a "bush type" aircraft?" I think by now you should be convinced the reason is because it is not suitable for the "bush type" mission. Remember that the Glasair has very different mission in life. If one airfoil were truly the "best" for everything, then why do we have so many different airfoil designs?

Flaps:

Let's start this discussion by reviewing the purpose of flaps:

1. To increase lift (reduce stall speed)
2. To increase drag to allow steeper approaches without gaining airspeed
3. To lower the deck angle on approach so that you have a chance of seeing over the cowling

Starting the flaps close to the fuselage would only gain about 9 percent more flap area, which would probably result in less than 1 knot stall speed reduction. The current design allows a good integration of the wing root with the fuselage. While I cant verify the claim that it improves low speed controllability, it does not sound unreasonable based on my other experiences. Aeronca had problems in the past with flaps that came all the way to the wing root causing buffeting on the tail.

Deflecting the flaps does not result in a loss of wing area. While the planform area seems to be reduced (what you would see from above), the chord that the air "sees" doesnt change when flaps deflected. It just exists in more of a curved path. Overall lift and drag still go up when the flaps are deflected, otherwise the flaps wouldn't be deflected that far.

The flaps may seem large to you, but they are only about 23 percent chord, and the typical size of flaps for most aircraft is 20 to 30 percent.

According to Budd Davisson's article on the Bearhawk in the October 1995 Sport Aviation (that convinced me to order the plans), Bob Barrows originally considered fowler flaps for the Bearhawk, but decided to go with plain flaps for simplicity (read: reliability) and the resulting small change in longitudinal trim. Fowler flaps cause a much larger change in pitching moment, which results in the pilot having to make bigger trim changes when extending the flaps. Using fowler flaps would also require extending the upper surface of the wing back to where the flaps would roll back to. This extension would require designing additional structure (here we go again), which would take up room in the airfoil, forcing the flaps to be thinner, resulting in less strength. Then, how would you mechanize them? The two basic choices: 1) external hinges, which result in increased drag and weight, not to mention a hazard on the ground for you to bash your head into, or 2) Cessna style internal flap tracks, which are a design nightmare, and would probably lead to serious questions of strength and fatigue problems. I figured out how much reduction in stall speed you would expect to get after all of this work according to a mathematical model I have built of the Bearhawk: about 1 knot! That's way too much effort with no noticeable payoff for me, not to mention the maintenance headaches you've created for yourself. Remember the designer's credo: Keep it simple, keep it light.

Windows in roof:

Here's the one area that I would agree with you. The April 1996 issue of Beartracks has already approved installation of a skylight back to the rear spar. I still havent decided if I want to do this. Flying around in fighter-type aircraft with big bubble canopies I find that the sun still makes it awful hot. It's wonderful if you want a greenhouse, but bad for comfort. Living in the desert, this is a significant problem. I've noticed many Navions and other aircraft that allow you to see straight up have some sort of sun shade over the pilot and passengers.

Wittman gear:

I wouldnt recommend it. While it is true that it is very simple, it has virtually no inherent damping (shock absorption). As a result, when you try to plant the mains on the ground in any sort of landing, you'll probably bounce a few times. The landing gear as designed does very well for this type of aircraft. It is very clean and very appropriate. It is not just a bungee system like a Cub. The Bearhawk gear as designed has an integral spring and damper in the strut that keeps the gear from spreading. Thus you can plant it reasonably hard without bouncing. This system is very similar to what is used on the Maule, and has worked very well for that aircraft. If you are concerned about the additional drag of the additional support arm, then add a lightweight non-structural streamline fairing over it.

Another point to consider is that the Wittman gear will transmit the loads to the fuselage in different amounts at different points than the gear as designed. I'm not sure how this would affect it, but I'm pretty sure it wouldn't be good.

As for rough field landings, it doesn't matter if you don't expect too many. One rough field landing is all you need to bounce the landing and possibly damage something. The landing gear as originally designed is much more tolerant of this type of operation. I still recommend it.

In general, as an aeronautical engineer with a background in aircraft design, and having a father who has much more experience in aircraft design and is more concerned with analyzing the design than I have been, you would expect me to critically evaluate the design of any aircraft I considered building. I have been pleasantly surprised and impressed with the design of the Bearhawk, finding it to be very sound. In fact, so far the only modification I have made is to add a landing light in the wing leading edge outboard of rib 10. Prior to doing this, I checked with Bob Barrows for the best location to do that. I realize this will reduce the wing strength slightly, but we are working hard to minimize the impact and the propagating design changes. It has resulted in moving one of the nose ribs in each wing about 2 to 3 inches, which I wasn't real thrilled about. I also plan to add navigation lights to the wingtips and tail, but those should have minimal impact on the structure of the aircraft.

[Web "master's" note: due to the size of this document, I have chosen not to follow my convention of putting other people's writing in italics. Hopefully this will make it easier to read the whole thing (you really should...), which was written entirely by Russ Erb and originally appeared on the Bearhawk email list.]

Greetings, Bearhawkers!  Below is an article I wrote at Mike's request for
inclusion in Bear-Tracks.  For various reasons not worth repeating, it's
been more than a year from when he requested it.  As you'll see, the
article grew until it was way too big for Bear-Tracks, so I'm releasing it
in this forum.  I'm sure that it will soon appear in Del's FAQ file.

Russ Erb
#164, Edwards CA


So You Want To Be An Aircraft Designer?

That's the real question you are asking when you take up that time honored
homebuilder's hobby of aircraft design modifications. If you feel you are
qualified to do this and understand the risks involved, then do so and
enjoy.  If you have not had the appropriate engineering training, and your
engineering techniques are best described as "eyeball engineering" or
"TLAR" (that looks about right), realize that there are risks involved in
design changes, and you're probably better off just building your aircraft
to the plans.  Besides, if you have to make major design changes to make
the aircraft do what you want it to do, maybe you're building the wrong
aircraft.

Purpose

I am writing this article in response to a request from Mike Meador.  After
reading some of my previous writings, he felt that I would be able to
respond with the benefits and pitfalls for some of the many modifications
that builders have asked Bob Barrows to approve. 

It never ceases to amaze me how many unqualified people think they can
"improve" a design, but don't understand the design process.  Of course,
with sufficient study they could learn enough about aircraft design to be
successful, but typically they don't bother.  After all, college students
spend years learning about engineering and design.  

Think about how a designer might respond when, after years of effort making
everything work together, some yahoo comes along saying the design is okay
but here's how to make it better.  Usually said yahoo does not see the
disadvantages of his proposed modification, or its impact on other parts of
the design.  Thus, the purpose of this article is to give you some insight
into the design process and hopefully make you think twice about that
modification you might be proposing.

Not All Modifications Are Bad

However, before I get any farther into this, let me make the point up front
that I am not saying that all modifications are bad.  The Bearhawk plans
intentionally leave a lot of latitude for the builder to personalize his or
her airplane.  In fact, you can't get away from it.  You may have noticed
that drawings 29-32 detailing the firewall forward engine installation were
not included in your plans package.  There's a good reason for this--they
don't exist!  Bob realized that the biggest variation in Bearhawks would be
what engine was installed, so he didn't even bother drawing up his
installation because he knew yours would be different. While I don't share
Bob's allergic reaction to electricity, I do know that my arms are allergic
to an improperly hand-propped engine. Therefore, I will be installing an
electrical system, complete with starter.  However, we will see that there
is a difference between modifications which don't affect the primary
aircraft structure and those that do.

Defining Your Mission

No, this isn't trying to describe the Alamo or San Jose.  This is simply
deciding what you plan to do with your completed aircraft.  Knowing what
your planned mission is will help you answer many questions.  For instance,
if your desire is to cruise at 25,000 feet at 300 knots, you will know that
you don't want to build a VP-1 Volksplane.  The Glasair III is a fine
aircraft, but would not be a good choice if you want four seats to carry
yourself, your spouse, and your two kids.  When deciding what aircraft you
want to build, you should start with an aircraft design reasonably capable
of completing the mission without modification.

The "Design Propagation Nightmare"

Design is characterized by a series of compromises. A design is not
optimized to do any one thing, but to do everything adequately.  A design
optimized for strength would probably be too heavy. A design optimized for
minimum drag would probably be difficult to build and maintain.

A design is like a house of cards--if you move one card, it affects all of
the other cards.  It has been said that changing the number of screws in
the spinner would eventually require a new tailwheel. Changing one item
will change another, which will change another, which will change another,
until an incredible amount of seemingly unrelated things have been changed.
 This is what I call the "Design Propagation Nightmare."

A classic historical example was designing the Spirit of St. Louis. 
Charles Lindbergh went to San Diego to visit the Ryan Company and was
suitably impressed with the Ryan M-2  Mailplane.  It looked like it would
suit his mission of flying from New York to Paris. It was a three place
aircraft with low drag, an efficient design for it's day, and a good,
reliable engine.  Sounds perfect,  except it had one minor problem--it
didn't have sufficient range to fly from New York to Paris, which, of
course, was a major mission requirement.

Now at this time, a typical homebuilder might think "No problem!  We'll
just put in a larger fuel tank."  If only it were that simple. The Spirit
of St. Louis makes such a good example of the Design Propagation Nightmare
because Lindbergh was not interested in increasing the endurance of the
airplane by a couple of hours.  He needed to increase the fuel capacity by
750%! Adding this much fuel impacts the design in two major areas--volume
and weight.

Volume was a problem because there wasn't enough room in the wings or a
header tank for that much fuel.  The large amount of fuel compared to the
aircraft's empty weight (fuel fraction) meant that it was possible that the
aircraft's center of gravity could change drastically as the fuel was
consumed.  To minimize the change in cg, all of the fuel tanks were
concentrated around the cg.  Additionally, Lindbergh did not want any fuel
tanks behind the cockpit for fear of getting trapped between fuel tanks in
a crash, especially during takeoff.  The most obvious effect of this
decision was that the fuel tanks totally blocked his forward visibility. 
Lindbergh decided that this was an acceptable impact for several reasons.
Taildraggers are notorious for poor visibility on the ground, so what's a
little less visibility?  Most of the flight would be over water on
instruments with no landmarks to see anyway.  Lindbergh was also confident
that there would not be any other aircraft on his route that he would need
to see and avoid.

The increased weight dictated that more wing area was needed to keep the
wing loading to an acceptable value for the available power and to keep the
cruising speed close to the best range speed.  Donald Hall, the designer,
chose to keep the same chord, allowing him to use the existing wing ribs
and the same fuselage attachment.  Wing area was increased by increasing
the wing span.  With a larger wing span, the longitudinal and directional
stability were both decreased.  The solution to both of these problems was
to either enlarge the tail surfaces or to lengthen the fuselage.  In this
case, Donald Hall decided it was easier to lengthen the aft fuselage than
to redesign the tail surfaces.  Lengthening the aft fuselage threw the cg
out of whack, so the nose had to be extended to compensate.

The increased span also decreased roll performance, but since roll
performance was not important to this mission, the ailerons were not
changed.  

The increase in span did create an increase in wing bending moments, which
required beefing up the wing structure, which added more weight.

The increase in gross weight required a stronger landing gear, which again
adds more weight, and may cause more drag. More weight and drag requires
more fuel, which increases the weight, and the cycle repeats.

The end result was that the NYP (as Ryan Aircraft designated the design)
was a totally new airplane except for the tail feathers, all because of a
"simple little change." 

The vast majority of modifications, especially those typically added by
EAAers, all have one thing in common--they add WEIGHT!  In my extensive
studies, I have only found one instance where adding weight is a good
thing.  Racing sailplanes carry water ballast to increase their gross
weight, which increases the airspeed for best glide without changing the
glide ratio.  Even in this case, the water ballast is jettisoned prior to
landing to restore the landing speed and weight back to an acceptably low
value.  In every other case I have ever looked at, adding weight will be a
detriment to aircraft performance.

Now that we've discussed modifications in general terms, let's look at some
specific modifications that have been proposed by Bearhawk builders,
presented here in no particular order.

Cockpit Adjustable Rudder Trim Tab

Very few if any similar production aircraft have rudder trim tabs. For the
flight conditions that a Bearhawk is likely to see, there is very limited
usefulness for one.  During takeoff, you are actively controlling the
rudder in an effort to remain lined up with the runway, so rudder trim
would be of little use.  During the climb, a constant rudder deflection
will probably be required to keep the ball centered, but this is only for a
limited duration (If you can't hold the rudder for a few minutes, maybe you
should visit the gym more frequently).  For cruise flight, a fixed tab can
be ground-adjusted to trim the rudder.  You're probably going to cruise at
pretty close to the same conditions most of the time, and the propeller
effects on directional trim are small over the limited range of cruise
airspeeds.  For descent, you will probably be at similar airspeeds to the
cruise conditions, so rudder trim changes would be minimal.  In the landing
phase, the rudder will be actively controlled as in the takeoff.

A cockpit adjustable rudder trim tab could be installed without
significantly affecting the primary structure.  However, the benefit gained
would be very small compared to the time and effort required to install it
and the weight that would be added at the tail end of the airplane, which
is probably the worst location to add weight (because of its effect on the
cg).

Another possible way to get the same result much simpler would be to
install some sort of bungee trim system on rudder system.  This could be as
simple as a lever in the cockpit attached to a spring attached to the
rudder cable.  It doesn't affect primary structure, is much simpler than
implementing a tab on the rudder, and the added weight is closer to the cg.
 A similar system could be considered for aileron trim as well.

Folding Wings

Folding wings have been popularized in recent years by the Kitfox and Avid
lines of aircraft.  This popularity is primarily based on a marketing
concept of being able to land at the airport, fold the wings, tow the
airplane home, and store it in your garage.  To a lesser extent, folding
wings could allow the airplane to be stored in a smaller hangar.

There are several things that they don't tell you, however. The effort
required to fold the wings may quickly convince you not to bother.  It's
the same line of thinking that a car is much more likely to be put in the
garage if the garage has an automatic door opener.  Otherwise the benefit
of putting car away is quickly overpowered by effort to get out and open
door.  (Of course, some of you may be unfamiliar with the concept of
putting a car in a garage--after all, the purpose of a garage is to give
you a place to build airplanes, right?)

Though Skystar shows the airplane being towed on its landing gear, they
will also quickly tell you that you don't want to do that any farther than
a mile or two.  Remember that the landing gear bearings and suspension are
designed to taxi slowly to the runway, then roll at high speed for the
length of the takeoff or landing.  They aren't intended for high speed over
long distances.  Skystar will tell you that if you want to go any farther
than a couple of miles you'll need to put the airplane on a trailer.

I don't know how big garages are in Nampa, Idaho, but a Kitfox would not
fit in the garage of any of the last three houses I've lived in.  Well, it
would fit in the two car garage, but it would have gone in diagonally and
taken up the whole garage.  I seriously doubt many Kitfoxes are towed home
from the airport and stored in the garage, regardless of what Skystar's
marketing says.  What makes you think your Bearhawk, which is much larger
than a Kitfox, will follow you home and live in your garage? It sure won't
fit in mine!

There's also that minor detail that any vehicle on the road is limited to a
width of 8 feet.  Folding the Bearhawk wings directly rearwards, pivoting
at the rear spar (as the Kitfox does), would result in a vehicle 12 feet
wide!  You could design a pivot to fold the wings against the fuselage as
in the F6F Hellcat or TBF/TBM Avenger, but that pivot would still have to
carry all of the flight loads.

Still think you want to fold the wings?  Here are a few more items you'll
want to consider.  Will the trailing edges of the wings overlap in the
folded position?  You might be able to move the flaps and ailerons out of
the way, but what about the back rib area at the root and the wingtips?  Is
the fuselage in the way of the folding wing?  Getting fuselage clearance
may require redesign of primary structure.  An additional strut would be
needed to support the wing during the folding process, and the attach point
for the strut would have to be moved onto the folding axis.

I'm estimating that the wing weighs about 100 pounds.  This weight, plus
the weight of the fuel (up to 165 pounds per wing) adds up to a large,
heavy object that I would rather not try to muscle around. A vent system
for the fuel tanks would need to be designed that would properly vent with
the wings in the extended position and at least not let fuel drain out in
the folded position.  The aileron control system of the Bearhawk does not
lend itself to wing folding, so it would have to be redesigned. Most of
all, you would be messing with a critical structural area, and adding at
least one more critical preflight item.

Then there's the horizontal tail. It is 10 feet wide, which is too wide to
tow down the road unmodified.  You would either need to be able to remove
the horizontal tails, which would again require disconnecting flight
controls, or redesign the tail to also fold.  If you redesign the
horizontal tail to shorten its span, you would have to increase the chord
to maintain the same area.  Actually, you would have to increase the area
because the lower aspect ratio is aerodynamically less effective, and the
elevators would also be less effective.  The structural load paths would
also change, requiring more redesign.  Of course, the tail would look
different, and would probably not look as good.  Aesthetics are a big part
of how well your airplane is accepted.  After all, you don't want to fly
around with a bag over your head, do you? 

Spring Steel Landing Gear

Spring steel landing gear change the paths of the landing loads into the
fuselage from 3 points to 2 points for each side. A tapered rod landing
gear further reduces it to 1 point.  Because of these differences, changing
the landing gear style would require the redesign of major fuselage
structure. According to Bob Barrows, large steel plates would be required
to distribute the loads, which would add more weight.

Even if spring steel gear were installed, I'm not convinced it would be an
improvement.  As the name says, the landing gear is a big spring with very
little damping. As a result, you will probably bounce more landings than
with the as-designed damped landing gear, which allow heavier landings with
the dampers preventing the landing gear springing the airplane back into
the air.

The purported benefit of spring steel landing gear is reduced drag over
tubular landing gear, especially landing gear with exposed bungees, such as
on the Piper Cub.  There would only be a slight difference in drag between
the spring steel landing gear and the Bearhawk gear. 

Stinson 108 Landing Gear

The existing landing gear is plenty big enough. While I am not familiar
with the Stinson 108 landing gear, I am told that it is heavier.  Besides,
unless it would be a pure bolt-on replacement, using it would create more
work than it would save, because you would have to redesign the landing
gear attach points on the fuselage.

Fabric Covered Wings

This is not as easy as just leaving off the aluminum skin and covering the
wing in fabric.  Have you ever noticed that fabric covered wings always
have two struts per wing while aluminum wings only have one strut? The
difference arises from differences in how the torsion (twisting) loads are
handled. 

An aluminum skin will resist torsion.  To see this, try twisting a soda
can.  It will resist the twisting, even if you partially flatten it so that
it looks more like an airfoil.
Now try twisting a leg of panty hose.  The fabric offers virtually no
resistance.  To prevent twisting, a fabric covered wing requires two
struts, one to each spar.

Two Strut Wings

As mentioned above, two struts are only needed with fabric covered wings
for torsion resistance.  A second strut adds nothing but drag and weight to
an aluminum covered wing.   

36 Foot Wing Span

The current wing span is about 33 feet, so a 36 foot wingspan would be an
increase of 1.5 feet on each side.  I'm not sure of the reason for this
proposal.  Perhaps more wing area for shorter takeoffs and landings?  The
Bearhawk already has as good or better STOL performance than most any
aircraft out there, such as the Cessna 180. Perhaps looking for a higher
gross weight? On the contrary, if you extend the span without changing the
structure, you would actually DECREASE the maximum gross weight because the
wing bending moment would increase because of the longer moment arm.  The
highest bending moment for a strut braced wing is at the strut attachment
point.  If the wing span is increased without moving the strut attachment
point, the longer wing would increase the bending moment.  The bending
moment might be reduced somewhat by moving the strut attachment point
outboard, but this has the drawback of worsening the bracing angle of the
strut.  As the bracing angle changes, the tension loads in the strut and
the compression loads in the wing between the strut and the fuselage
increase. Additionally, since the portion of the wing between the strut and
the fuselage is longer, its resistance to buckling under compression is
reduced.  The bottom line is that if you are interested in increasing the
wingspan with the idea of increasing the maximum gross weight, you will
need to totally redesign (beef up) the wing structure. 

Steel Wing Struts

A steel strut with the same tensile strength as the specified aluminum
strut would be of a smaller cross section.  While this might seem
advantageous for reducing drag, it is weaker than the aluminum strut in
compression (i.e. more likely to buckle) because of the smaller cross
section.  Wing struts are occasionally under compression, such as when you
are sitting on the ground, the occasional hard landing (though you never do
that, of course), and during that nasty big down draft that bounced you off
of your seat belt last week.  Because the wing strut is very slender for
its length, it is much weaker in compression (buckling) than in tension. 
Reducing the cross section size (i.e. making it more slender) makes the
problem worse.

Switching to a steel strut may or may not reduce the weight when sized
strictly for tensile loads.  When a steel strut is sized to handle the
compression loads, it will very likely be heavier than the aluminum strut. 
Either that or you will have to add a jury strut as seen on Piper Cubs,
which adds back the drag you were trying to get rid of. 

Then there's the corrosion problem.  Seen a few Piper ADs lately?  The
Bearhawk runs the aileron control cable up the inside of the wing strut. 
If you set up a steel strut the same way, water would soon get inside the
strut and start rusting it away from the inside.  Tough to detect and even
tougher to repair.  Alternatively, you could seal the strut by welding the
ends closed, and then run the control cable externally through fairleads on
the back of the strut, as was done on early Piper Cubs.  However, this will
probably add as much drag as you saved.  Additionally, if you get into
icing conditions, you're going to get some serious flight control problems
really quick.  Incidentally, such a configuration is no longer certifiable
under current FARs for just that reason, which can be interpreted to mean
that it's not a very good idea.

Extruded Wing Spars

I don't see the benefit in this, unless you happen to have the equipment to
make the extrusion dies and do the extruding.  Even then, you would not
want the spar to have a constant cross section from root to tip.  The spar
design is beefiest where the greatest loads are (at the strut attach
point), and thins out where the loads are smaller.  Any constant cross
section extrusion strong enough to handle the loads at the strut attach
point will be heavier than the built-up spar.  Either that, or the extruded
spar will require extensive machining to remove the extra unneeded weight. 
The built-up spar is actually a very simple and effective design.  I think
it is even simpler than the RV spar which uses bigger rivets (3/16") and
has multiple webs.  Of course, the RV spar has to be bigger, since the wing
is cantilevered. 

Tricycle Landing Gear

You're on your own for this one.  Mike Meador tells me "We will never live
to see the day Bob caves in to this."  That tells me that your first
problem would be to find a new name for your aircraft, since a "Tri-Gear
Bearhawk" or a "Tri-Bearhawk" or even a "Bearhawk-A" are all oxymorons. 
The next (and biggest) problem would be redesigning the fuselage structure
because the landing gear loads are now in totally different locations. 
Also, the tail would probably be over 10 feet tall. Take a look at the
Piper Tri-Pacer to get the idea.  Might be tough to get in your T-hangar
door as well.

Besides, if you are building a Bearhawk because of its STOL capabilities or
its ability to operate from grass or unprepared strips, there are numerous
reasons why a conventional gear (the proper name for a "taildragger")
arrangement is better, which I won't go into here.

If you're concerned that you don't know how to fly a taildragger, find an
appropriate instructor and go take some lessons.  You can learn
how--consider that virtually every pilot up through World War II learned to
fly taildraggers.  If they could do it, then you can too.

Wooden Airframe

This question has actually been raised.  Simply replacing the steel tubes
with similar sized sticks of spruce won't cut it.  Find a toothpick and a
similarly sized nail.  Try to break each one with your hands.  Which one
broke?  Which didn't?  Point made?

While it's true that large aircraft have been constructed of wood (e.g. the
Hughes HK-1 Hercules, a.k.a. the "Spruce Goose"), typically they use a
totally different construction method.  The DeHavilland Mosquito used a
monocoque or semi-monocoque construction where wood formed the outer shell
of the aircraft and this shell carried the loads, much like the shell of an
egg. The Corby Starlet fuselage is built from sheets of plywood reinforced
by a wood truss.  The majority of the loads are carried through the skin
acting as shear panels.  The primary purpose of the truss is to keep the
unsupported panel areas small enough to prevent buckling.

Another type of wood construction uses a large number of stringers held in
place by wooden formers, covered by fabric. The Sopwith Camel was typical
of this type of construction (Many Fokker aircraft used welded steel tube
construction--Anthony Fokker was a pioneer of this method).  Most Guillow
rubber-powered airplane models use the stringer and former construction.

Both of these methods are significantly different from the method used in
the Bearhawk fuselage. Thus, choosing to change to a wooden fuselage would
necessitate designing an entirely new fuselage.  If you really want to do
that, you might as well design your own airplane.

Elimination of the Front Strut on the Tail

I'm not sure what would be gained by this. The left and right horizontal
tails attach to the fuselage by a tube slipping over a tube, held in place
with a bolt.  While this setup handles forces sufficiently well, it doesn't
handle moments very well.  It's actually rather similar to the wing root,
where the pin (bolt) carries the forces and wing strut counteracts the
moments.  The three struts on the horizontal tail keep it from flapping up
or down.

Because the horizontal tail is fabric covered and thus has no load bearing
skin, it is not exceptionally stiff in torsion. Therefore, two struts are
used on the bottom just as with a fabric covered wing.  Both the forward
and aft strut systems are necessary.  The forward strut is large enough to
take loads 
in tension and compression.  The aft struts are thinner, but only need to
take loads in tension because of the upper and lower struts.  Removing any
of these struts would compromise the torsional rigidity of the tail. 

All Aluminum Tail Feathers

You could do this, but you would be designing the whole thing yourself. The
structure would be much more like the wing than the current tail feathers. 
Aluminum tail feathers would definitely be more complex, take longer to
build, and might even be heavier.

All Aluminum Flaps and Ailerons

The only benefit that I could see for this would possibly be in a bush
flying scenario where the flaps and ailerons might frequently be hit by
debris.  Of course, by that same logic, you'd probably want to cover the
fuselage in aluminum too.  Even then, you're trading fabric tears
(temporarily repairs in an emergency with duct tape) for aluminum dents
(which may be more difficult to repair).  Aluminum covered flaps and
ailerons would be heavier, since the aluminum sheet is heavier than the
fabric.  The ailerons would also pick up additional weight because of the
increase in ballast required to balance them.

Wet Wings
 
While wet wings are doable, the increase in fuel volume would be minimal,
while the increase in complexity and building time and effort would be very
large.  You wouldn't be able to make the tank fill more rib bays without
significantly changing the load distribution and redesigning major portions
of the wing, such as the flap actuation mechanism.  

There are benefits of having non-integral tanks.  For instance, fabrication
and plumbing are easier.  If maintenance is required, such as finding a
fuel leak, the tank can be removed for inspection and repair.

Skylight

Here's a change of pace--this one has actually already been approved!  See
the April 1996 Bear-Tracks, where Bob said that the transparent portion
could be continued over the fuselage back to the rear wing spar.  The
covering in this area is non-structural.

Of course, there is a down side to this--you will increase the sun
radiation load in your cockpit significantly.  In other words, it can get
HOT!  Take a flight with a buddy on a sunny day in a Long EZ, RV, or any
other aircraft with a bubble canopy.  You'll see what I mean.  The point
here is that you'll want to think about providing a sun shade for those
days when the sun is oppressive.

Stretching the fuselage

This will launch you down the path of redesigning the fuselage.  If your
idea was to increase the payload, see the previous discussion on increasing
the wing span.

Electric Flaps

Electric flaps are usually mechanized by placing a jack screw in place of
the flap lever.  This adds one more gadget to fail on you at the worst
time.  One advantage of electric flaps are that you can set them up to be
able to stop at any deflection.  Even so, with four positions available on
the manual flap lever, this benefit is minimal at best.  My experience has
been that manual flaps can be set to a different position faster than
electric flaps if desired.  You might find the faster manual flaps a
benefit if you like to retract the flaps quickly after landing to "plant"
the airplane on the ground.

So What Should I Do?

As you can probably see by now, your best bet if you don't consider
yourself an aircraft designer is to build your Bearhawk according to the
plans for the parts where the plans exist.  There are plenty of
non-structural areas and areas not specified in the plans, such as the
engine installation and the instrument panel layout, where you can express
your individuality with your own design, while still feeling confident
about the primary structure around you.  If you still feel that the
Bearhawk is not right for you without some major design change, then maybe
you should be building some other design.

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