# Decalage angle

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#### Eugene

##### Well-Known Member
Peter,

It was explained to me that my new afterbody should extend behind propeller line. This supposedly will prevent airflow get sucked into a propeller from the bottom. As a result flow into horizontal tail should be improved as well. Making it to narrow ,(looking at the top ), you will potentially create side separation, I was told.

I found this picture from Chinese copy of icon A5. This is what I was trying to accomplished as well. Figure they know much more than I do and they know what they doing.

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Eugene,

My experience has been that it is difficult to get air to do what it doesn't naturally want to do. It will be interesting to see the tufts.

#### proppastie

##### Well-Known Member
Log Member
notice that really sleek cowl for the engine????....might want to consider that...... and you do not have to learn Chinese to copy it.

#### Eugene

##### Well-Known Member
Here is another question.
On my old Garmin 296 battery was falling out inflight last summer. So, I removed it from airplane and fixed at home. Updates still available from Garmin to my surprise. So, I did update as well. After fixing battery I was playing with this unit and found that it can also calculate glide ratio for me. If I ever reduce drag on my Skyboy and airplane will fly same 90 MPH at lower RPM = then glide ratio will be improve as well? Maybe from 7:1 will get to 8:1? Can I use my GPS to measure glide ratio after flying upwind and downwind, with engine at idle?

#### proppastie

##### Well-Known Member
Log Member
Can I use my GPS to measure glide ratio after flying upwind and downwind, with engine at idle?
Flying in both directions is supposed to do that and I have read to do it every 90 degree and make an average.

I am not sure what is being displayed or how it is calculated....if it is showing you your radius of movement (landing spot) based on your decent and ground speed, yes the GPS is calculating based on your position and the wind speed will be factored into your position. As to how accurate that might be might be a question because older GPS altitude could be quite a bit off. If it is showing you an actual glide ratio it still would have to calculate it based on position and altitude so again the wind would be factored in. I could calculate on mine either the head wind component or wind speed and direction (can not remember) by in-putting my airspeed, compass heading and temperature (I think).

#### Eugene

##### Well-Known Member

Didn't make any progress with my airplane unfortunately . Way too busy taking care of "more important" things. Hopefully next year will be more productive.
Sometimes when I couldn't sleep, doing some reading. Trying to find an excuse or explanation for those guys, who was telling me that is OK for airplane to have 10° decalage. Everything I can find is telling that designer should stay with 1-3°positive on the wing and same amount but negative on horizontal tail. This way decalage should be only 4-6°. That is a good recipe for successful flying machine.
If you see aircraft with 10°decalage, = that is somebody's mistake. I just can't find any another way to see it, or explain it.

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#### Johan Fleischer

##### Active Member
Air likes to flow aerodynamically. Think of a seagull (or a salmon) I'm afraid, the sharp corners here will make turbulence and drag.

#### Eugene

##### Well-Known Member
Yes, I agree. I left it for now because I was told to try it this way. We trying to make sure that airflow from the bottom not going to be pooled up by propeller. Hope is that turbulent triangle zone will act as ball bearing. Should be easy to change after tuft testing. Salmon doesn't have propeller to worry about

##### Active Member
Seems you may have taught yourself enough to design a plane of your own!

#### Eugene

##### Well-Known Member
Seems you may have taught yourself enough to design a plane of your own!
No, I don't think so. And that wasn't my goal. I needed to find an explanation of why my Skyboy is not very good flying airplane. Even if I choose to do nothing about it, not be able to explain why my aircraft have 10.2° decalage, would kill me over time. Knowledge is priceless to me. Much more important than \$25,000 for an airplane and much more interesting then level flight.
I suspected all along that explanation should be relatively simple and easy to understand for average shoemaker. I can explain what happened to my airplane to someone in five minutes. And I don't need complicated formulas to do it. I would need to learn much much more if I would ever attempt to design an airplane.

#### Eugene

##### Well-Known Member

Here is my version for Skyboy story:

What you see on my picture above is 50 HP - 65 MPH ultralight with 992 lb. MTOW. Wing incidence of 4.5° for this speed and P3 airfoil was correct. Horizontal stabilizer incidence was originally set for about 3°.

I was told by Skyboy designer that horizontal tail incidence was increased to 5.7° during German certification. I was also told that drag in real life was about 2 times bigger then was calculated on paper. So, what went wrong?

Many people agreed that most "surprise" drag on this airplane is generated by 2 areas: #1 - is open engine and #2 - rear abrupt fuselage ending with landing gear combination.

I found more people thinking that something wrong with fuselage afterbody, then people who believe that engine cowling will make big difference at such low speed.

So, I personally believe that rear part of fuselage is started all problems. Here is my logic, looking at my picture:

#1 - drag at point B turns out to be much larger and lower then original calculations.

#2 - as result = pitching moment between A and B much larger and needed larger down pressure on point C.

#3 - aircraft with more than predicted drag was flying now little slower with little larger AoA of about 6°and wing was generated additional drag.

#4 - So, we need more pressure at point C, but so many things working against us at this point. Horizontal tail is flying lower then predicted and working in very turbulent environment behind wide fuselage with all kinds of flow separation. Horizontal tail volume on Skyboy is only 0.34 and it is barely enough for aircraft with very small pitching moment like Aeronca Champ. We normally should go back at this point and redesign horizontal tail to make it larger and higher if possible in to propeller air stream. But decision was made simply increase incidence to 5.7°. And for 65 MPH - 50 HP ultralight world was OK fix.

#5 - Real problem arises when the decision was made to install 2 times larger engine. Moment between A and B was so much greater and our little tail was fighting out there for its life. Everybody was reporting vibration on control stick, some people call it flutter and company in Europe was searching for a solution. Best they can do is to install heavier elevator counter weights to suppress vibrations.

As a result this flying triangle is fighting and destroying itself at 75% power in level flight with 100 HP engine.

That's my story, and I'm sticking to it!

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#### mcrae0104

##### Well-Known Member
HBA Supporter
Log Member
Eugene, for a more detailed discussion of equilibrium and static longitudinal stability, check out chapter 5 of Airplane Performance, Stability, and Control, which includes additional factors such as airfoil pitching moments, downwash effects on the horizontal tail, upwash/downwash on the fuselage, the influence of wing placement, propeller effects, etc. It's a surprisingly complex topic.

#### Eugene

##### Well-Known Member
[QUOTE="mcrae0104, It's a surprisingly complex topic.
View attachment 91638[/QUOTE]

Yes, I purposely was trying to make it into a very simple version, so I can sell this explanation to anybody at my local airport or another Skyboy owners. If you start talking formulas , then everybody will run away from you.

I was trying to understand and explain tension, that I feel during normal level flight in my own primitive way. And I think , I got it somewhat correctly. And yes, there is so much more to learn and it is much more complex, then I am presenting. But it's good enough for now. I will keep working on it!

Thank you everyone for help and patients.

Happy new year!!!

#### Eugene

##### Well-Known Member

Wondering what kind of horizontal tail efficiency on my Skyboy?

#### mcrae0104

##### Well-Known Member
HBA Supporter
Log Member
Perkins & Hage (interestingly) did not give values for eta-t (tail efficiency). Raymer suggests 0.9 for a T-tail and 0.6 for a conventional design. Since your source has something different, I'd like to understand the context of what this author means by tail efficiency. What's the source?

Although the wing sees free stream dynamic pressure, the tail sees a little less bucause some of the energy has been spent as the rest of the aircraft passes ahead of the tail. This is the reason for a tail efficiency factor, and why a T-tail has a higher value (approaching 1). I think Jedi was kidding but I wouldn't be surprised if yours is less than 0.6, especially if you're getting a lot of separation off the back of the fuselage.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
Did you do a glide test to eliminate the thrust factor?
If it stays near hands off trimmed in a glide test then your decalage is correct.
If it requires forward stick pressure in this no thrust glide test, then the decalage is excessive for the glide condition.
You can't see the glide angle in flight to measure it, so that's about all I can suggest.

#### Eugene

##### Well-Known Member
Peter Garrison
11:02 AM (1 hour ago)

to me

Eugene,

I admire the patience and persistence with which you have tried to get to the bottom of the problems with the Skyboy design. You seem to have finally abandoned the idea that the decalage is the big problem. You may be right that there is insufficient horizontal stabilizer area, but that is not a big source of drag. The drag, as you now say, comes from the fuselage and the engine.

This statement, however, bothers me:

"I found more people thinking that something wrong with fuselage afterbody, then people who believe that engine cowling will make big difference at such low speed."

Drag is drag. It may be that the fuselage produces more drag than the uncowled engine, or maybe not, but in either case low speed has nothing to do with it. Power required is going to be a function of the cube of the speed, no matter what the speed is, and that rule will apply equally to the fuselage and to the engine.

Peter Garrison

#### Eugene

##### Well-Known Member
Tail efficiency on very bottom of page #285

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#### Eugene

##### Well-Known Member
Did you do a glide test to eliminate the thrust factor?
I was told that good pilot would pool power back on downwind every time and turn base in to final, and land with no power at all . Very hard to do this in Skyboy, but I tried many times. As soon as power removed, trim lever needs to go to almost all the way forward position. Trimmed 70 MPH final feels like you have 1:1 glide ratio with 100% perfect visibility over the nose. That's all I know.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
I was told that good pilot would pool power back on downwind every time and turn base in to final, and land with no power at all . Very hard to do this in Skyboy, but I tried many times. As soon as power removed, trim lever needs to go to almost all the way forward position. Trimmed 70 MPH final feels like you have 1:1 glide ratio with 100% perfect visibility over the nose. That's all I know.
Yes, the trim forward in the glide is required to offset the excessive decalage making it pitch up. That glide test proves (to me) that the high thrust line is why it needs more decalage than other aircraft (excess power with the big engine). If it had lower drag it would cruise at a higher speed and you wouldn't need so much decalage at the higher speed because the tail would have more force.
Imagine if the drag was so bad you could only do 50mph at full throttle. It would need a very big tail to hold the nose up at full throttle and 50 mph with a high thrust line. But a centerline thrust airplane wouldn't need a bigger tail.

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