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

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

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

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.

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

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

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.

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.

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.

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

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.

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?

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

I'll re look for the Datum line.

Bob

FWIW

Here is the Manuals link,


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


Bob


I'm new at getting old

dignlivn

You have it right stick is wrong; sorry stick.

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.

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

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!

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.

skeeter_ca

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


Bob

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

Lol, I didn't think so lol.

Here's a pic , I hope.


Bob

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

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

Bill, AMA 4720
WACO Brotherhood #1