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#1 Cougar429 Nov 14, 2012 09:53 AM

Question on Wing Washout
I'm hoping someone can answer this seemingly simple question. I'm usually a builder, but I recently had to work through what was left of my T/T Rare Bear ARF, (yeah, right!). The wing structure was particularly weak and on the second flight the joiner tube in the LH wing let go. Interestingly enough the plane went into an inverted flat spin from 100' into the beans and was relatively unscathed.

During the rebuild I fabbed fixtures to ensure both wings came out to a -2.5 degree washout. I used this process and washout amount with my Harvard, (Canadian version of the AT-6 Texan) and with the exception of split flaps this plane has a similar tapered wing plan. The wing loading is also close to identical.

Due to weather and other factors I have yet to get the Bear airborne again and I wonder if it will suffer the wicked low-speed stall characteristics of the Harvard.

That 2-1/2 degrees has been the norm for all my standard straight wings, but I was wondering if the tapered planform requires a more drastic twist to allow the stall to start inboard. The strange thing about all of this is that even with the Harvard snapping either way to vertical I still had very good aileron control.

#2 Chad Veich Nov 14, 2012 10:32 AM

Two and a half degrees of wash out is a substantial amount and should be more than ample in my opinion. I don't think I've ever used more than two degrees in any of my designs and, in fact, I only used about a degree on my Yak-11 racer and it has a very highly tapered wing. Even so it is very docile around the stall and does not have a violent wing drop at slow speed nor is it prone to high speed snaps. I would say that it is imperative to make sure that you build accurately so that both wings have identical wash out. Beyond that the best thing you can do to prevent snapiness is build light. (Not really an option with an ARF, I understand that.) Anyway, I've successfully used somewhere between 1 and 2 degrees of wash out on two different Yak-11's, a Ki-61 Tony, and a Grumman Hellcat. Also, I have a Midwest Harvard which I think has around 2 or 2.5 degrees and it is not prone to snapping though it will drop a wing if you get it too slow. However, it can be picked right back up again with proper use of the rudder and is not what I would consider violent. (And I've had a few airplanes that were violent at the stall so I know what that looks like!) Anyway, I think you will be just fine on the big, fat Bearcat wing with 2.5 degrees of wash out. My .02 cents anyway.

#3 Cougar429 Nov 14, 2012 11:39 AM

The first flight of the Harvard was interesting, to say the least. Due to the terrible tailwheel design and my own error of no toe-in it was necessary to plant the tail during acceleration till at least some rudder control. Unfortunately the plane launched far earlier than expected and climbed to 10' where it dropped the right wing to vertical. I was able to immediately level the wings, then at about 4' the left wing dropped vertical. I strongly suspected I was bringing home the plane in a bag, but was able to level it again. By now there was absolutely no forgiveness height, so I chopped the throttle, dropped the nose and planted it back on the grass, bending the main gear aft a bit.

When straightening it I set some toe in and the next launch was much more benign. With the wing setup there was the need for absolutely no roll trim.

I do note the nose up and high power were a recipe for advanced stall, something Bonanza owners may know about, but with no flap for takeoff I was surprised with the behavior. Normally this would move the stall much quicker outboard towards the ailerons, but as I stated I had immediate and strong aileron control. I guess not having a stick in your hands to give some feedback and the fact things can happen so much faster here caught me by surprise.

I'm hoping the Bear is a bit less twitchy. My biggest concern is on landing as it lacks flaps as with the Harvard and therefore will need a bit more speed on approach. Low and slow with the need for a quick power up are definitely the math required for rekitting a plane.

#4 Zor Nov 14, 2012 02:18 PM


Did my pictures satisfy a response to your question ?


#5 epoxyearl Nov 14, 2012 07:24 PM

I built a Ziroli F8F-2 Bearcat..Folding wings,Scale retracts,,all the things to make it heavy...I was successful in that....something like 42 lbs.
The wallowing on landing,and at slow speeds had me not flying (or landing) slowly...
I had the recommended wash out,and the experts at Warbirds decided that I needed to raise the ailerons about 3/16" while landing.
One of the computer wizzes took my radio for a bit,and fixed me up.
_Their reasoning-when the flaps went down,that negated the washout already in the panels,and I needed more if I was gonna slow down to land...Popping the ailerons 'up' when the flaps went down solved the problem.

#6 Cougar429 Nov 14, 2012 10:04 PM

Zor, from the pics I still cannot figure out how the top wing attaches to the cabane.

epoxyearl, as I stated the Bear lacks flaps so will have to keep the speed up. My question leans more to what kind of stall behavior I can expect and if I have enough washout. With the Saito 125 there is more than enough power and the plane can launch at half throttle. During the first two flights I kept the plane on the wheels till some speed on, but air work may have some yankin' and bankin' and a high speed stall can be pretty nasty if not enough height to get it back.

#7 packardpursuit Nov 14, 2012 11:05 PM


Not going to say that your Bearcat fix was wrong (how could it be if it worked??) but my understanding is that symetrical flap deployment of a given wing system usually ENHANCES washout effect. Could it be that moving the ailerons " up' re-attached a separated airflow?

Speaking of turbulet airflow, highly tapered wings and washout, there was an interesting fix to a small rubber scale model over on Hip Pocket. Fellow added a 1/32 sq strip at at extreme top aft TE. Provided enhanced washout effect.

Good to see you still around. Any progress on the Mustang?

#8 Chad Veich Nov 14, 2012 11:34 PM


Originally Posted by packardpursuit (Post 23272412)
Good to see you still around. Any progress on the Mustang?

Hi Charles,

Lots of Mustangs being slowly pieced together actually. The gentleman in Italy who originally commissioned the design has the tail surfaces and fuselage well underway on his prototype. For the life of me I can't find the pictures he sent me though! Another builder here in the States has started construction of two at the original 1/4 scale and two more reduced down to 1/5 scale! I think the pic below is one of the reduced scale fuselages. (I believe that is a Platt B model in the background.) I'd love to build one myself but 1/4 scale is just a bit too big for my budget and storage space!


#9 epoxyearl Nov 15, 2012 05:30 AM

Re: report #7
packard pursuit-agreed...and remember the Bearcat has those oil cooler inlets at the base of the wing ,which should encourage stalling at the wing root first.

The solution defied logic,but it did work..

And I was probably at a critical wing loading with my "fat'cat"...

#10 Cougar429 Nov 17, 2012 08:20 PM

Well guys. I had the chance to take the plane up this afternoon and the wing worked out fine. It was an overly touchy elevator that made life interesting, to say the least. Finally had to do a gear up as I did not trust it to land without ripping the retracts out.

Anyway, the wing only required a notch or two of aileron trim to remain level and with the wonky pitch tendencies I was sure if there were any boogums lurking in the aerodynamics they would have come out to bite me big time.

Now if I can only figure out why the Harvard has such a wicked stall life would be good.

#11 epoxyearl Nov 18, 2012 02:26 AM

If you're experiencing swift pitch changes,I'd suggest adding some expo, actually reducing the throw (low rates),or adding some nose weight.

I have settled down some 'bears' to a gentler landing speed by adding nose weight....the nose heaviness requires a little up elevator at slower speeds,which increases the angle of attack and slows down the aircraft..

My favorite test is to "land" at 400' slow her down,adjust the throttle to just keep it from stalling,and apply up until it does. Any horrible tendencies will be revealed at a safe altitude.

#12 Cougar429 Nov 18, 2012 06:56 AM

That's what the reprogramming involved. Already had expo, just increased it a lot and cut the elevator ATV down on both low and high rate.

I bolted back on the small piece of lead I used to compensate for the lack of a cowl for the first few flights as the plane tended to end up on its nose. With some repairs to the nose I had thought this was a no-brainer as I had triple-checked the CofG before this flight and again after landing. Was bang on recommendation, but that doesn't mean a tad nose-heavy would not be welcome for the first few flights again.

From the long list of problems the tail on this plane has been reported to be the weak link, not the wing. It was a structural issue due to horrible assembly exacerbated by too long a set of C/F rods that would break the ribs away if you were too aggressive during installation. I guess they would fail long before other gremlins showed up. I had reinforced mine slightly earlier on even before I read the woes, but due to the fact the flying surfaces were prebuilt and only required assembly there were lots of places that needed to be checked and/or improved. Still, the elevators are huge for the size of the stabs and I had reset the throws to recommended, (on high rate).

I've done your test procedure frequently on new craft with the first few flights normally pretty benign till I get used to the plane. Between my friend I taught to fly and myself I put well over a dozen maidens away this season alone. Some, such as the Corsair, were much larger with little forgiveness if you pushed it too hard, but most tests are conducted with controls I can at least manage to be predictable. This one was so sensitive it reminded me of heli flying. At least I did not have to worry about ailerons as well. I did a first flight of his prebuilt Sig Fazer a few years ago and it was one hairy flight. The radio did not have dual-rates and spun like a drill bit till we cut the mechanical throw to less than 1/3 of what it came with.

#13 kerwin50 Nov 18, 2012 09:51 AM

It sounds like your Harvard is way over weight.
What is your wing loading.
Lead sleds do not fly well.

#14 epoxyearl Nov 18, 2012 10:17 AM

That's where I get my experience-Yesterday I couldn't SPELL test pilot and now I ARE one!
Hairy flights are nearly a thing of the past,since with all the testing,models sort of fall into categories.Light wing loadings seldom cause problems.Aerobatic and low wing civilians are exciting..Warbirds always turn on the adrenalin "armed" switch ! It is my habit to always balance Warbirds at the forward range of the C/G,or a little ahead of a singular mark,for first flights.
Continued good luck with Rare Bear....I'm a fan.

#15 Cougar429 Nov 18, 2012 01:37 PM

I don't normally push another forum, but if anyone wants to read through the build reports on the Harvard and/or Bear give me a PM.

As for weight, the Harvard works out to 11.5 lbs, approx 1 lb over the recommended range. All the mods I made are the reason, one in particular dealing with the abysmal number of retract failures. This plane will take off on half throttle with the OS 91 Surpass. It's just that rather than having a predictable stall behavior the wing can immediately snap either way.

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