Incidence setting ?
 

I'm building a GP electric Cub and the Plans
call for 3 degrees of + incidence with the Aft
deck at 0 degrees. This is on the Horizontal
Stab. I'm not sure how their measuring on
the plans? (L.E or T.E.) . I have ordered a
I. gauge, but it is not arrived yet.

This is my 3rd build after a 22 years of being
out of the Hobby, talk about changes. lol

Thanks in advance!

Bob


dignlivn

RE: Incidence setting ?
 

With the rear of the airplane jacked up to the point that the leading edge and the trailing edge of the wing are equal, the leading edge of the horizontal stabilizer should be 3 degrees higher than is the trailing edge. In other words, with the airplane flying level with no input required from the radio, the tail should sit higher than dead level. This is to prevent the airplane from climbing uncontrollably when under full power. You really do need your incidence meter to measure this. If you have a flat bottom airfoil on the wing, you can get it close by having the bottom of the wing level, and then check that the Horizontal stab is 3 degrees up in the front. Do this before you install the elevator, or let the elevator droop out of the way. This measurement applies only to the non moveable part of the stabilizer.

Bill, AMA 4720
WACO Brotherhood #1

RE: Incidence setting ?
 

Wouldn't it be the other way around? With the horizontal stabilizer set at 0 deg, the leading edge of the wing will be higher than the trailing edge, and the angle of the wing will be 3 degrees. I haven't heard of a model with -3 degrees of incidence unless it's a biplane with the top wing set at -3 degrees and the bottom wing set a 0 degrees, or a 3 degree difference between the two.

Hogflyer

RE: Incidence setting ?
 

Without knowing the airfoil, almost everything is wild speculation.

And cambered airfoils AOA and incidence is measured from the middle of the LE through the middle of the TE, not along the flat bottom.

So don't look for good answers until some more details are known. And don't bust a gut with that indicence meter until you've got an understanding of what you're measuring.

The incidence of the wing and the incidence of the horizontal stabilizer are set after taking a number of different things into consideration. You've got to know the AOA you want for that airfoil to present the proper lift for your model weight at your cruise speed. And then figure out the downwash from that wing and what of that downwash the stab is going to see. And then take that stab's performance numbers into consideration to set it's incidence.

Or you could look at the plans and match up what they show.

RE: Incidence setting ?
 

WOW!!! I am a dummy not sure what you said. What is AOA? How do I know about downwash? Thanks, I think

RE: Incidence setting ?
 

LOL ! I worked many years in aviation, and this stuff still gives me a headache. AOA= Angle of Attack

RE: Incidence setting ?
 

Thanks for the replys, here are the Specs of my Cub.


SPECS: Wingspan: 58.75" (1490mm)
Wing Area: 471.5 sq in (30.4 sq dm)
Wing Loading: 16-17oz/sq ft (49-52g/sq dm)
Fuselage Length: 37" (940mm)
Weight: 3.25-3.5lb (1470-1590g)
Airfoil: Flat-bottom, high-wing
Center of gravity: 2-7/8" back from the leading edge of wing at fuse.
(not from inside the windshield area)
Control Throws- High Rate Low Rate
Elevator: Up and Down 3/4" 5/8"
Ailerons: Up 1/2" Down 3/8" Up 3/8" Down 1/4"
Rudder: Left and Right 3/4" 1/2"
Note: If your radio does not have dual rates, we recommend setting
the throws between the low and high rate amount.
Incidence- Wing: +2°
Stabilizer: +3° with the aft deck base at 0°
Motor: 4° down thrust and 2° right thrust

Looking at the plans the LE of the H. Stab is higher than the TE.
I'm using a Protractor for the time being, but still don't see where
the +3 degrees is with the Aft deck at 0 degrees.
Still confused lol.

Bob


dignlivn

I'm new at getting old.

RE: Incidence setting ?
 

What I said was just an attempt to show that most of what we do ain't sound byte simple. And it isn't.

Lots of guys want every problem to have one simple answer. Most want the answer to be a 3 word sentence. Ain't gonna happen. (Now there's a 3 word sentence that works.) [sm=wink_smile.gif]

AOA is what was just answered. But it ain't that simple. We need to know the AOA for cruise, and then we use that to set the wing to the fuselage. And we call that angle the AOI, or angle of incidence. The angle a wing or stab is attached to the fuselage is called the AOI.

And the angle the stab is attached (it's AOI) is chosen with the understanding that although the wing might be hitting the air headon, the air the stab sees has been pushed off at an angle by the wing. And the air between the wing and the stab is now "downwashing". So the stab's AOI is set considering that the stab sees a differently angled airflow than the wing. whew...........

And since the stabs need to be aligned to the airflow they see, in order to hold the wing steady in pitch, and usually need to be creating a downward balancing force, they'd usually be angled down a bit in front. But since the airflow is already being downwashed they might not need so much angle, but might.................

So basically, the original question isn't going to get a sound byte answer that would have a hope in hell of actually being true.

Everybody always wants a simple answer. No problem. Do whatever the plans say. OK 3 words. Follow the plans.

RE: Incidence setting ?
 

Bob,
The beauty of this hobby is that lots of layouts fly no matter what. And we can then see how they fly and trim 'em to fly better. Well, "better" if in fact they don't fly good enough.

Changing an ARF or even a kit airplane's AOI's is very seldom needed but almost always a real effort.

If the wing has a cambered profile (Cub models almost always have cambered airfoils), then the line drawn along the flat part of the bottom of that airfoil is probably not the AOI line. That +2degrees is probably a line that goes from the middle of the LE to the middle of the TE. And that's not a horizontal line. These details always make this discussion harder.

RE: Incidence setting ?
 

LOL

I'm just trying to understand the Reference Point which
is the "Aft Deck at 0 Degrees" .

I am following the plans, which the Manual leaves something
to improve on JMO.

Bob



dignlivn

RE: Incidence setting ?
 

When a cambered airfoil like on a Cub is flying along at cruise, it often sort of "flattens out" and flies with what looks to us as a bit LE down. Those airfoils don't need much AOA to generate what little lift is needed to carry a Cub's weight. So they don't fly pitched up. They fly almost pitched down. And what does that do to the angle the stab is flying?

The stab winds up (pun intended) being pitched nose down. And it needs to create some down force to stabilize that cambered wing. But not a lot. And it's already seeing a downwashed airflow. So what does the designer have to do to the AOI of the stab to get the needed AOA? He often has to pitch the stab "funny". Sometimes it winds up with a pitched up AOI.

It takes quite a bit of "school house" aerodynamics to figure out AOIs. It's what AE's get the big bucks for. And they then take the answers to a wind tunnel and find out what really works.

RE: Incidence setting ?
 

That isn't clear to me either. If there are plans, then look for whatever seems to be the datum line or fuselage centerline and look around the stab for any line that is parallel.

BTW, did you know that it's not uncommon for AE's to talk about the "water line" on airplane designs?

RE: Incidence setting ?
 

Thanks for explaining, When the I. meter gets here
maybe that will shine some light on it lol.
I wish Orville and Wilbur where here lol.

Bob

RE: Incidence setting ?
 

I'll re look for the Datum line.

Bob

RE: Incidence setting ?
 

FWIW

Here is the Manuals link,


http://manuals.hobbico.com/gpm/gpma0156-manual.pdf


Bob


I'm new at getting old

dignlivn

RE: Incidence setting ?
 

You have it right stick is wrong; sorry stick.

RE: Incidence setting ?
 

Be carefull with "datum lines"; You want the difference between the stab and the wing, with the wing leading edge higher than the trailing edge.

RE: Incidence setting ?
 

I have not found no Datum lines on the plans.
I find it strange the manual makes no comment
on setting either the Wing or the Stab Incidence.
It is printed om the plans however, and I thought
learning how to Hover a Heli was hard lol.

Thanks for the help, it will come together.

Bob

RE: Incidence setting ?
 

Jeez......... somethings can be made much harder than they are.

The aft deck is the large flat area between the wing and stab. They show a clear picture of it on page 27. It's where you glue formers f4a and f5a. Place a small level on this horizontal sheet (the aft deck). Shim up the tail of the plane until this area is level. When you get the incident meter place it on the horizontal stab (see included instructions on how) and shim the front of the stab up until it reads +3 degrees and glue it into place.

Walaaa!! done. All the hard figuring is done for you. Now get back to work!

RE: Incidence setting ?
 

I just took a better look at the instructions.

It looks like the stab saddle(the part you glue the stab to) will set the proper incident when the horizontal stab is glued down. If you look at the rear portion of the tail you will see the angle of the stab saddle is slight at a upward angle compared to the aft deck. So don't sweat it, just glue it down but make sure it's lined up straight as the instructions say.

Hope this helps. Sometimes the simple solution is the best solution.

RE: Incidence setting ?
 

skeeter_ca

Now that makes since, Thanks. I was hoping that
I was making it harder than it was.


Bob

RE: Incidence setting ?
 

No, it wasn't you I was talking about..........................lol

RE: Incidence setting ?
 

Lol, I didn't think so lol.

Here's a pic , I hope.


Bob

RE: Incidence setting ?
 

The advice back about the 1st or 2nd post.......................

RE: Incidence setting ?
 

That ain't the first time I've gotten the positive/negative incidence explaination wrong.:D

Bill, AMA 4720
WACO Brotherhood #1

RE: Incidence setting ?
 

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

RE: Incidence setting ?
 

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

RE: Incidence setting ?
 

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.

RE: Incidence setting ?
 

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.

RE: Incidence setting ?
 

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.

RE: Incidence setting ?
 

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

RE: Incidence setting ?
 

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.

RE: Incidence setting ?
 

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.

RE: Incidence setting ?
 

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.

RE: Incidence setting ?
 

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

RE: Incidence setting ?
 

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

RE: Incidence setting ?
 

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?

RE: Incidence setting ?
 

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

RE: Incidence setting ?
 

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

RE: Incidence setting ?
 

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