Hi all, this is my first post in this section, I am usually down 1/2A or in the diesels sections. I have a question (or two) about my scratch built 40 inch trainer/sport airplane.
When I first test flew it, it went crazy hanging on its prop and power stalling like mad, so I checked the CG again (it was OK and I applied a large wedge of downthrust.
It flies, but it handles like a pig! so I took to measuring some angles with my *new* Robart guage.I bought yesterday (lovely tool)
Using the tailpane as the datum line to measure from, I found the wing has +4 degrees incidence and -9 degrees engine downthrust. The wing is a 15% section and I am flying 3 channels R/T/E. The plan does not show a great deal of detail except 2-3 degress rightthrust and 4 degrees of down..
As its a "standard" high winger with dihedral, I think that the +4 degrees incidence is way too much, like 4 degrees too much, would you experts agree?

thanks in advance for your opinions

Part of it depends on how far back the CG is on the wing. But yeah, 4 degrees sounds like too much. In particular if it's fairly heavily powered to faster speeds. I don't know about going all the way to zero but perhaps 1 to 1 1/2 is good and then see where the elevator trim ends up.

I have shaved some wing seating tape off the L.E. seat and built up tape on the T.E. and got it down to 1 degree. At 16 OZ/Sq Ft its not heavily loaded, powered by a PAW 19 swinging a 10 X 4 at 9700 RPM, it might stop it "ballooning" so much. Under full power I had to put in a bit of down elevator otherwise it would power stall, even with so much downthrust( 9 degrees), the plan says 2-3 downthrust so I figured there was something wrong. The balance point is where indicated/supposed to be on the plan.

What is the effect of decreasing wing incidence angle and decreasing the downthrust?

Move your CG back about 1/8 inch and see what happens. I think if you move the CG back enough you can take the downthrust out. This sounds counter intuitive, but I went out one day with a free flight model and a roll of solder and played around. I could get it to loop under power with the CG forward and by moving the CG back took it all the way down to flying level with the same power. I did not move the CG back far enough to make it dive into the ground.

Basicaly having too much incidence is like holding up elevator all the time. So now you've taken out some of the "up elevator" so you should be able to reduce the downthrust and loose some of the ballooning at the same time.

Jim, it's only counter intuitive if you don't realize the relationship between the CG and the incidence angles. In your case that little free flight model showed you the light. Andrew, Jim may be onto something as well here. If it's a light sport job the designer may have specified a forward CG in the mistaken thought that a forward CG is a stable CG. In actuality as long as the CG doesn't get back to or behind the aircraft neutral point the model can be set up to be stable and not overly sensitive. However as the CG moves back closer to the NP the amount of elevator throw to keep the same pitch sensitivity becomes less.

aha, this is starting to make a bit of sense now. how do I calculate/find the NP? , True also about the forward CG, in turns it certainly drops its nose sharply, this would indicate nose heavy/forward CG?

I will see how it flys with the change in incidence before I remove the downthrust (bit by bit) see what effect that has and after that looks like startind loading the tail with lead !

There's a link to a neutral point calculator in the Aerodynamics Tools and Calculators FAQ stickied to the top of the listings.

Hi guys. I'm just starting into designing my own planes. One thing that I'd like to know is what reference you use to measure the incidence angle. Take the Clark Y airfoil for example. It's curved on the bottom and the top surfaces. Do you measure the incidence from the chord line? Or from the relatively flat area on the bottom surface starting about 1/3 aft of the LE? Thanks for your help.

Everything from the chord line buddy

Incidence is measured from a level line on the fuselage.
The amount the chord line is tilted is the amount of incidence.

The incidence is measured from an arbitrary line along the long axis of the fuselage. What is level on a fuselage?

It is an arbitary line, but it's drawn, called "the datum" and used as a reference for everything else on the plane.

It doesn't really matter where it is, providing everything else is measured against the same datum.

We are saying the same thing

.
Anything you want it to be.
For consistency, leveling the horizontal is fairly common.
Then incidence and downthrust can be related to each other.

Just for a grin or two..... once upon a time, some aeronautical engineers used to call the datum line, the water line. Matter of fact, some still do.

And btw, if you look at the original post you could easily figure out what those angles would be from the usual datum line if you looked at and figured the designer of the model would have figured that published 4 degrees from the usual datum line. So if we use that 4degrees against all the values that AndrewB measured, we'd get

The engine is 5 degrees downthrust
The wing in ZERO degrees incidence
The horizontal stab is -4 degrees incidence
And we see a quite common arrangement that flies everyday quite well.

When designing an airplane, the first line you lay down with your T-square is the datum line.

Lockheed choses datum zeroes ahead of and below the maximum expected dimension of the plane, so that all measurements are positive..

The "ancient" model ?plan? (which was originally from a kit with ready cut parts and subsequently "rediscovered" at a garage sale) does not show the datum line, in fact it doesn't show much at all!.
It shows a horizontal reference line in front of the motor and below that, another line (in line with the crankshaft of the motor) INDICATING 4 degrees, but when measured the firstmentioned reference line ACTUALLY measures 10 degrees, so I think its exaggerated in the drawing to indicate that some downthrust should be added.




I have gone back to the model with a long steel straight edge, I found that the horizontal stab has no incidence with reference to the centre line of the model, if as you state the horizontal stab had -4 degrees incidencethe datum line would pass to the outside of the fuselage just below the engine crankcase, whereas at the moment the imaginary line formed by extending the horiz stab forward actually meets and joins to the ?datum? line mentioned above (the line metioned in the first paragraph above).
I will try to post a copy of the plan in forum.

Thanks for all the replies everyone, you show a depth of knowledge on aerodynamics

just hopping and see if any more replies [:-]

Looking at the plan..
Looks quite normal for the plane type.