I'm going to open up this can of worms again, but with the wing at zero (relative to the datum line) then would not the horizontal stabilizer be at 3 degrees positive with the leading edge of the H-stab higher?

Bill, AMA 4720
WACO Brotherhood #1

Shimming the stab leading edge up will give the wing neg. incidence ifthe wing is level.

And once again............
The stab is set according to how far the wing is ahead of it, and above or below it. And what airfoil the wing is made with. It also matters how fast you expect to fly the airplane most of the time. And how heavy the airplane is expected to be.

This is not sound byte simple.

Take a look at every model you have in your hangar. There is a good chance that no two of them will have the same decalage.

But you will see some general "agreement" with similar type planes.

Airplane designers usually figure out what AOA their chosen wing is probably going to fly at most of the time. They then plan on attaching the wing to the fuselage so the fuse flies at it's least drag angle when the wing is at the estimated AOA.

When the wing is doing that chosen job, it creates downwash behind it. Depending on how far back the stab is, and how it's oriented up or down to the wing, it sees the downwash however that is.

Depending on the wing, the stab has more or less work holding that wing from pitching. A "lifting" wing with non-symmetrical airfoil will need more "stability" from the stab than a symmetrical wing. And the symmetrical wing will wind up being attached to it's fuselage at a different angle than a lifter would be. So to try and describe what angular difference to expect from all airplanes is a losing task unless you consider the wing's airfoil.

So the stab needs to be angled based on the downwash it sees, and the necessary force needed based on the wing type. And BTW, stabs differ also.

Okay, let's say that you have a biplane with both top and bottom wings (Clark-Y) set at zero (to the datum line). If built per instructions the tail will have 3 degrees positive incidence. When seen flying at cruise throttle, it appears to be dragging it's behind, while the full scale flys much as does the cub or the DC-3 (tail high). Would you say that it presents too much wing to the airstream?

Bill, AMA 4720
WACO Brotherhood #1

I'd say it drags it's behind at cruise.

As for why, there could be a bunch of reasons.

Going back to the paragraphs that describe how the wing's AOI is figured from it's AOA.............. and expanding that in light of your observed behavior from that biplane (let me guess, it's a WACO, right)...................

You've observed the final product of wing and stab rigging in cruise.
You've seen the fuselage is not level.
The first and most powerful thing flying in that assemblage of parts is the wing(s). It's going to find it's required AOA and DO IT. And all else is going along for the ride. Those two wings have found where they wish to be and they're happy. They will do that no matter what you do with all the rest. They will be influenced by other things, like the extra induced drag from a fuselage that's cocked down in back, but take that away, and those two wings won't shift their pitch enough that you'd be able to see the difference in AOA from the ground.

So the first thing I'd figure to fix was the AOI of the two wings. And I'd start by getting an accurate measurement of the wings relative to each other. And then see if I'm reading each of their AOIs correctly.

It can't. The wings decide how much they want to "present" and do it. They assume whatever AOA is needed to carry whatever load that airplane is at whatever speed you've got the engine pulling with whatever load is coming from a CG that might be off or a tail that's being forced to "present too much wing" and is generating a bunch of excess induced drag.

I would WAG that the two wings are screwed up in decalage (to each other, not to the stab) and causing this problem. WACOs don't normally fly around looking like a waterskier.

If that airplane flies around with it's tail say 5degrees down, then moving both wings' AOI up 5 degrees would be one of the very first things to consider.

That tail is flying behind two wings. One high above it, one somewhat below it. Both of which are creating downwash AND airflows that affect each other's airflows. Lord knows what that tail sees for airflows. But that tail is having to deal with the pitching moments of TWO cambered wings. And it's doing it in a very different environment than a monoplane's tail sees.

BTW, the lift needed for our models is really very easily provided by most cambered wings so they seldom assume much AOA to provide it. Increase the load by screwing up the AOI of the tail, and the induced load and drag increase still probably won't be enough that the wing needs to compensate with lots more AOA. So the tail down cruise is probably more the result of AOI being off than anything else.

But heck, this stuff really isn't sound byte easy.

No mention has been made of the engine's angles.

Rock and Stick

Keep it up,i'm learning alot.


My Cubs motor angle is 4 degrees Down thrust
and 2 degrees Right thrust.

Thanks,

Bob

Here's a good one for you, The one that is on the building table will have a variable incidence horizontal stab. It uses a servo, and will utilize the flap switch to make incremental changes to try and eliminate the droopy tail. We'll see how it works. By the way, the wings are both at zero to the datum line just as the plan calls out. That is, if I am to trust my incidence guage. But all things being relative, if the datum line checks at zero, and both wings check at zero with zero decalage, then I suppose it is.

Bill, AMA 4720
WACO Brotherhood #1

When do you figure that trimmable tail will be flying. Am interested in hearing your results.

In the meantime think about what was said above.

If your trimmable tail does bring the fuselage up to fly level, what do you think is going to happen to the wings that are rigidly attached to that fuselage? Let's say those ClarkY wings were at 2degrees AOA to carry the load. The load isn't changing. But now an outside force is going to bring the fuselage level and that's going to crank the nose of those wings down however much the fuselage was dragging along at. And if say for example the fuselage was hanging 5 degrees down, those wings are now going to be pitched 5 degrees negative. That's around the zero lift angle for a ClarkY. What's going to carry the load now?

Dign',

These guys all have it right.. you shouldn't need an incidence meter with this airplane.. I built one a few years ago. It was actually my first build (probably a little over my head at times) but turned out pretty good.. I did use an incidence meter to check the wings after all was done to make sure I hadn't built in any warps.. Anyway, what motor setup are you going to use with this plane? I used the suggested geared t600 setup and was VERY displeased with the power output of this setup.. Mine wouldn't do a rolling takeoff from short grass to save it's life. Anyway, took all that out a few months ago and put in a great planes rimfire outrunner, lipo and associated ESC.. Cut 10 ounces of wt, and doubled the thrust.. Now it's a SuperCub!

John

John


I'm leaning toward the Rim fire set up, for $$ reasons.
An Axi motor and Jedi esc are in the wish list also, but
more $$. IMO the T600 motor recommendation is OLD
lol. Thanks for your help.

Bob

It shouldn't be too long. The fuse and tail group are completed, and I'm about to begin on the wing assemblies. This is not the first one of these that I have built (obviously), but it is the first one with the adjustable incidence. Look in Vintage and Antique forum WACO YMF thread, to see photos of the unit. The full scale airplane has adjustable incidence, so why would it not be a viable addition to the model? Also I don't use any jury rigging on the engine thrust any more. The full scale does not use them either. You have to watch application of power, and learn to better utilize the rudder.

Bill, AMA 4720
WACO Brotherhood #1

Who said it would not be?

Hundreds and hundreds of fullscale designs use tail trim. They don't happen to use it to lift up a hanging tail however. Nor to hold the wings at zero lift AOAs.

Right Rock, They use it to trim the airplane. I have noticed that a lot of biplane models must use a lot of trim in the elevator in order to fly level (kinda). I think that this will allow me to fly without the elevator deflected. I plan to start with the h-stab set at the plan setting, and make minor adjustments at cruise and at full throttle. I think that I'll probably go back to the stock setting again for landing. That's the plan for now, but the best thing about this set-up is that I can put it where it does best.

Bill, AMA 4720
WACO Brotherhood #1