Decalage angle

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proppastie

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So, what I have is flying cow and not flying skyboy! Is this situation really can't be improved at all? Problem is in rear part. Front part of this aircraft looks OK.

So, this area is biggest drag generator and not open engine?
you have no way to measure or see the drag of the engine......so you do not know
 

Eugene

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You are barking at the wrong tree- Clean up the fuselage/engine if you want to see a big difference.
Yes, I remember this one!

This is what I am doing = cleaning back side of my fuselage first. After that I do positive wing swept 1°- 3°to move CG forward. Then engine cowling will wrap around the wings.
 

Tornado Flyer

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https://drive.google.com/file/d/11pJ-YuKziNcxifV7tKASLBXa_RrJoICP/view?usp=sharing

https://drive.google.com/file/d/1gGbqsBda2IyikuSIlJZkqvh9PAPpMHpT/view?usp=sharing

Eugene,

The word "cowling" means a nacelle surrounding an engine. When you talk about making the cowling "very long" I assume you are talking about the afterbody you are thinking of adding behind the landing gear. I doubt you will gain anything by making it very long, except maybe weight. I agree with your analysis that the problem are is going to be the one indicated by the green arrows, partly because the propeller is pulling air toward itself, while that surface is sloping away from the propeller. Do you have some tuft photos of flow in that area, preferably taken from directly behind, ie camera on the tail?

It is true that chopping off the back end of a body sometimes does not increase its drag. I'll send you some pages from Hoerner on this subject. Notice, however, that the effect is best for long slender bodies. Let me know if I accidentally send you that same PDF twice. There should be two different ones, pages 3-20 to 3-23.


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Eugene,

Those are horrible looking tufts. I don't know if there is anything you could do to make them good.

You know, some airplanes are just made for the fun of flying around, not for getting anywhere. This seems to me to be one of those. Maybe you should give up trying to make it perform better, and just enjoy flying it as is.

Peter Garrison
I'm with Peter on this. If you do not enjoy flying it move to a different plane. Pushers are less efficient than tractors anyway. If you do enjoy it leave it along and fly it. Kind of like a wife, except you are not married to a plane and made no oath to stick with it.
 

Eugene

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If you do not enjoy flying it move to a different plane.
Yes, unfortunately I hear this all the time!!!:(

And not only from my mother-in-law, but from active EAA members!

Someone at Oshkosh was telling me that I would fit right in to 1976 crowd back then. That was time when so many different people were trying different things and getting different results. They were experimenting, trying different things, not afraid taking the risks. They would kill for all modern tools we have today with computers, internet,softwere.

Most of us assembling 51% kit together knowing that many thousands of them flying already. And this is our comfort zone today? How many new homebuilts we see every year at AirVenture? What ever happened to this country? Were did experimental aviation moved too? Is there different Oshkosh somewhere? Were should I go and who should I talk too? So I don't sound stupid or crazy!!!

And for what? For trying to improve flying characteristics of my aircraft? Why do I get this all the time - "...just sell and get different one"???

Lucky me there is crazy Russians that I still can talk too. They don't see me as crazy, ............I wonder why? They think that is very hard to improve already good design. But if your aircraft is so bad to begin with = there should be all kinds of room for improvement! And if you are willing to try? Why not?!

Recommendation is to make my new rear afterbody longer to get it past propeller.

69296530_2320965134691673_4685260289971585024_n.jpeg
 

akwrencher

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Tornado Flyer is perfectly right, IF, your only goal is to have a nice flying airplane to fly. If your enjoyment of your hobby includes experimenting and trying new things, and seeking to improve, and learning, then you are on the right path already. Different strokes for different folks, as they say.
I have a similar affliction. Constantly want to improve things so they work better and more efficiently, the way they should! :)

It's your hobby, do it the way you want. I'm looking forward to learning what the solution ends up being.
 

plncraze

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60 years ago Peter Garrison started like this. Ask a few questions and play around with problem. Keep going Eugene. This is cool and you accomplish alot.
 

Eugene

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8127C9C1-5A80-4FB4-9BC7-E06A81B9B0E8.jpeg 32D194B8-9DF5-4114-AE81-F37BE5C09C07.jpeg

I did get a chance to fly this airplane a few days ago. I believe this one was just purchased for only $35,000. Definitely 21st century with all carbon fiber construction. Hands-down much better and much more capable machine. You can get it , put in your hanger and don’t do anything at all for a very long time. It is SLSA.

So, what is there for me to do? Absolutely nothing. Convert to ELSA? And then what? Sitting on the couch and wondering where was most drag coming from on my Skyboy? Was engine cowling really that important? Who is right? And who was wrong?
 

Eugene

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fullsizeoutput_17a4.jpeg fullsizeoutput_17a5.jpeg

Ok, here is another question:

Let's say that something sticking out sideways in to the air stream on your aircraft. And you want to build some kind of fearing over it to improve drag. Cross section of this new fearing should look like airfoil, preferably symmetrical. This airfoil does have chord line and you should install this fearing on airplane with chord line parallel to longitudinal axis of your airplane.

So, my plan is to make some kind frame using steel tubing and fill empty space with foam. This structure will have trailing edge and some kind of chord line. You can see it in red on my top picture. It will be made from one tube bend in to U-shape with both ends attached to landing gear. I will suspend it from tail boom with green spacer to make this frame to be parallel to longitudinal axis.

The question is were is longitudinal axis on this airplane??? For no money in the world I couldn't get this information out of my designer. For weight and balance calculation his answer was: - "on wheels, on level ground".That was giving me original angles in green. My angles are different today and you can see them in red.

If I see it correctly, every time you reduce drag or increase engine power = wing AOA is changing, so is position for longitudinal axis. So, I need to know today at what AOA my future low drag Skyboy will be flying at, so I can install my new afterbody creation at correct angle!!! Measuring to to the bottom the wing at 5200 rpm = 75% power I have 4° AOA. Or about 6°AOA to the chord line on 4412 airfoil.

Wow! Don't think I am that smart!
 
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plncraze

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Maybe you mean angle of incidence or angle of attack? And on a symmetrical airfoil it's zero lift line is at no angle of attack.
 

Eugene

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Sorry for confusion . Lets try different approach.

Russians call it horizontal building line or axis. This imaginary line will be level in cruise at 75% power level flight. For weight and balance you supposed to bring aircraft in to this position were horizontal building line is level.

During level flight this line vill be level, but wing AOA to the chord line anywhere from 6° to 0°depending on your design. Horizontal tail will be 1-6°negative.

Problem is, that I don't know were this line is, on my airplane
 

plncraze

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This might not be helpful but can you use a laser level or a bubble level in flight and compare it to the c of g leveling points to get an idea of where your fuselage is now in flight?
 

Eugene

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I know exactly were my fuselage at 75% power in cruise! I am flying with level taped to passenger window for some time now. And at this position my wing has 4° AOA if you hold digital level to the bottom of 4412 airfoil.

Question is were will it all be after I clean up my airplane?

All I really need to know is what is my target AOA should be?
 

ragflyer

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Eugene, It is the wing incidence to fuselage you change as your cruise speed changes not the angle of the streamlining you are building.

AOA is proportional to cl and cl is proportional to square of speed. So if you double the speed you would need about 4 times less angle , i.e. 4 degrees wing AOA becomes 1 degree AOA. This is an extreme case- remember to improve speed by 10% you need to decrease drag by 33% due to cube relationship.

Now for small changes in speed you can keep the math simple and approximate as follows: for every % change in speed you expect double the % in angle change. For example, if you expect to get 10% improvement in speed then your angle will change by 20%, i.e wing AOA change from 4 degrees to 3.2 degrees.Streamlined shapes will not change drag dramatically with these small changes in angles, particularly given the cow like drag currently.
 

Eugene

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Eugene, It is the wing incidence to fuselage you change as your cruise speed changes not the angle of the streamlining you are building.

AOA is proportional to cl and cl is proportional to square of speed. So if you double the speed you would need about 4 times less angle , i.e. 4 degrees wing AOA becomes 1 degree AOA. This is an extreme case- remember to improve speed by 10% you need to decrease drag by 33% due to cube relationship.

Now for small changes in speed you can keep the math simple and approximate as follows: for every % change in speed you expect double the % in angle change. For example, if you expect to get 10% improvement in speed then your angle will change by 20%, i.e wing AOA change from 4 degrees to 3.2 degrees.Streamlined shapes will not change drag dramatically with these small changes in angles, particularly given the cow like drag currently.


Thank you! Thank you! Thank you!

That was all I needed!

My airplane at 75% power during level flight has same position in the air as on level ground with about 4°AOA. If sometime 10 years from now I will get 10% speed increase of 10 MPH position of my aircraft in cruise will change less than 1°.

So if I want to install missiles under my wings today and build brackets to support them. I will just install them level. Same brackets will work for me in the future and will be no need to change angle of incidence.

Screen Shot 2019-08-18 at 08.48.35.png
 

Eugene

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I know you want the best result but this is a classic case of over thinking a problem.

Yes, I hear this from my wife all the time. And I don't know any way to change that, so life can get quick and easy, relaxing and simple. So, I stop thinking about things like angle of incidence.

My ex-wife was trying to change that for 20 years unsuccessfully. Then she gave up and found herself "easy going guy".
 
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