What is wrong with high thrust line???

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Eugene

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I am getting all kinds of recommendations from knowledgeable people. And I'm hearing statements that my problem is engine above the wing, or engine has too much power, or occupants sitting in front of the wing and so on.

I would like to understand why thrust line above the wing is such a big problem for everybody? Is it true that every problem has a solution? All you need is correct Tail to balance this problem!

On Cessna 172 thrust line below the wing and nobody seems to concern.

Found old message from Peter Garrison saying that nothing really wrong with arrangement. So what, that engine is 12 inches above the wing? And yes pilot and passenger sitting 8 inches in from the wing. It should still be very possible and very manageable. An ideal airplane everything will be positioned on the CG and trust line go through drag and CG. But how many airplanes arranged like that?

Screen Shot 2021-11-18 at 21.32.34.pngScreen Shot 2021-11-18 at 21.23.30.png
 
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ragflyer

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I am getting all kinds of recommendations from knowledgeable people. And I'm hearing statements that my problem is engine above the wing, or engine has too much power, or occupants sitting in front of the wing and so on.
what is the problem you are trying to solve?
 

bmcj

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Keep in mind that the vertical displacement of the tail relative to the thrust line (where the thrust flows across the horizontal stab) might compensate (counter) or exaggerate thrust line issues.
 

wsimpso1

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Why bring this up again? We have already been through this with you, and recently too. Instead of asking us, why don't you just do the math yourself and see how big it is. Set up climb speed and cruise speed, calculate:
Pitching moment from the wing and from the thrust times the vertical arm;
Fuselage drag times its vertical arm;
Wing drag times its vertical arm (which will be nose up while the others are nose down).

Then divide them by the arm from CG back to tail, and see for yourself what the tail load has to be for each for all of them.just for the wing pitching moment, the thrust, wing drag, and fuselage drag. Add them together to get the total. Do it at climb speed and cruise speed and see for yourself.

Then decide that you will look at this issue NO MORE, you will listen to folks trolling you NO MORE, and you will get on with your rebuild.

Here are the equations.

Mwing = rho/2*v^2*Sw*MAC*Cm

Dwing = rho/2*v^2*Sw*Cdw

Dfuse = rho/2*v^2*Af*Cdf

where rho is density of air. In Brit units, rho is 0.002378, v is in feet/second, S is wing area in ft^2, MAC is in feet, Cm and Cdw comes out of TOWS, Af is frontal area of the fuselage, while Cdf is probably around 0.25.

Thrust = HP*550*eta/v

where HP is horsepower, eta is prop efficiency - somewhere around 0.80 - and v is flight speed in feet/second. 60 mph is 88 ft/s.

Have fun and we can check your work. No dumb pages. Show your work, either in Excel with the equations or as paper and pencil.

Billski
 

BBerson

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If the airplane flies poorly with engine off then it has nothing to do with thrust line.
 

Dan Thomas

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Sorry, I should've given more details. Simple and short description is that my horizontal tail on Skyboy is way too small and tail boom is way too flexible

View attachment 118140
See how that engine is pointing at the stabilizer? That's because the stab needs that prop blast on it to generate plenty of downforce to counter the pitch-down moment of the high thrust line. So right there we have extra drag, created by the need to control pitch against thrust.

Here's another common place we see that:

1637377599531.png

Those airplanes are not noted for their speed, either.

Here's a standard diagram of the four forces and how they couple to get the best controllability:

1637377970981.png

Lift behind weight make the airplane stable in pitch. Thrust below drag makes it nose up with power, nose-down without; a naturally stabilizing arrangement that ensures that a power failure will initiate a glide, not a stall. Now, getting thrust below drag is not always possible, so the stabilizer has to rely on prop blast to artificially create that stability.
 

TFF

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Don’t confuse perfect with mission or necessity. Everything in line makes a great racing airplane or aerobatic one. If these are not what you want, then your different need makes the compromise necessary.

Every design element of your plane has been discussed, but you still don’t accept that it has to be a compromise to be pilot and passenger in the front. Most people don’t put that first, hence most planes are not configured that way. Has nothing to do with good or bad, but need.

You want to sit in the front, where do you put the engine? Not a lot of choices. The designer could have lowered the engine and put the boom on the top. It does put the prop in a more precarious spot closer to the ground. Where else can you put it? What are the pluses and minuses?

Every part on every plane goes through this type of vetting. Sometimes your needs align with the perfect plane, and sometimes it doesn’t. Has nothing to do with personal fun flying the plane. What everyone wants is different, so don’t let everyone else’s opinion of their needs confuse your needs. You can’t have everything. You have to choose the list and accept what you have to give up to get it. If your needs change, the airplane changes.

Trying to morph a completed design into something else that it really can’t be, makes disappointment on both sides. Maybe your overall needs have changed and what is important needs to be reassessed.
 

Eugene

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Why bring this up again? We have already been through this with you, and recently too. Instead of asking us, why don't you just do the math yourself and see how big it is.

Billski
Yes, we talked about this hundred times and at this point we know consequences and price to pay. That was not the question. Question was why is everybody see it as a problem? I see it as a challenge and I'm willing to pay the price with speed and efficiency.

Sort of like customer is always right. If you want to have large house, you will have a large gas bill.

We already made a conclusion that this type of aircraft will be much slower, but it should still be possible to make it into a good flying aircraft!!! Simply refuse to see it as a problem or as a failure!!!

Report from Sonja

Screen Shot 2021-11-19 at 21.20.00.png
 

Eugene

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but you still don’t accept that it has to be a compromise to be pilot and passenger in the front.
Yes, compromise should be slow speed in comparison to different aircraft, and I am OK with that. It should be still very possible to create pleasant to fly machine.

I should be able to talk to my designer and tell him that this is how I wanted to fly. He should respond with a comment that this aircraft will be slower. And as long as I am agree with that he should proceed with a smile on his face and not telling me that this is a problem or impossible to do. Or even worse take my money with smile on his face and build something what I have right now.
 

TFF

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Good flying aircraft for configuration. You don’t get it all. Your plane can not fly like an RV7. It’s been explained in every one of the threads. More people want the RV over yours. Who is right? Neither. Who is wrong? Neither.

You can make it better, but you can’t eliminate what it is.

You have wanted to exercise your ability to change things and you decided to go big. Finish the tail project. Analyze it by flying. You want something that you only dream on paper now. Without building what you have come up with, you are wasting time and away. Too late to worry. No tail, no airplane. You might want a thousand more things but you can’t have most, and if you don’t watch out, you will run out of time, and you won’t get any of them.
 

wsimpso1

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Yes, we talked about this hundred times and at this point we know consequences and price to pay. That was not the question. Question was why is everybody see it as a problem? I see it as a challenge and I'm willing to pay the price with speed and efficiency.
I doubt it was a hundred times, but even three times is wasting your effort on nonsense that could be used to make your parts.

Certainly the number of folks considering it significant is not "everybody". Several of us on here and your consulting engineer all agree that the high thrust line is not your biggest contribution to pitching moment and drag. That it is present must be countered by a horizontal tail of adequate volume. You are doing that. Folks telling you it is awful can be safely ignored...

We already made a conclusion that this type of aircraft will be much slower, but it should still be possible to make it into a good flying aircraft!!! Simply refuse to see it as a problem or as a failure!!!
I disagree with the notion of "much slower". Sonja Englert's report indicates tail load at 11% of main wing vs 3-5% in other airplanes. let's take that apart with a simplified analysis. First understand that at fixed power, speed goes with cube root of inverse of flat plate drag. Here is a good model of the topic. Next understand that there is a lot more drag on the bird - if the tail is 11 and the wing is 100, with the fuselage similar to the wing, the tail is 11 parts out of 211, going to 3 parts out of 203 is just not big. Lets runs some numbers:

Drag = rho/2*v^2*sum(Si*Cdi)

Power = Drag*v = rho/2*v^3*sum(Si*Cdi)

solve for v

v = (2*Power/(rho*sum(Si*Cdi)))^(1/3)

to make this easy, make power 1, rho 1, fuselage S*Cd = 1, wing S*Cd=1, and a low lift tail S*Cd = 0.03

For a low low lift tail;

v = (2/(1*2.03)^(1/3) = 0.995

For a high lift tail, sum of S*Cd becomes 2.11 from 2.03;

v = (2/(1*2.11)^(1/3) = 0.992

Imagine the first is a 100 mph, airplane, the second is then a 99 mph airplane. Your proportion of drag between the other stuff and the tail may be different but larger drag at the tail is just about the smallest drag issue you have. Compared to your fuselage and engine drag, your high lift tail is not costing you much speed.

Next let's remember that we had the vertical and horizontal tails interfereing with each other AND the existing tail was nibbling at stall and so bigger drag than it should have been. You are increasing tail volume so stall is being distanced and getting the two tailplanes apart a little so there is less interference drag between them. You will likely have less drag from the tail than you used to have, which might even produce a speed benefit, but not much of one.

In short, your bird will be better because of your improved tail design, but I doubt it will make much speed change. Forget about it...

What is costing you speed is:
  • The pusher configuration with the prop operating in the immediate wash of the wing, producing poorer prop efficiency than if it were a tractor prop, which you are not going to change, so you live with it;
  • The uncowled engine which is both big drag by itself AND messes up flows over the wing further adding drag, which you are fixing.
Please quit worrying over the naysayers. Better yet, ignore them. Certainly do not waste your time and efforts thinking on them. You are getting good scientifically valid engineering advice. You want to make it into a better behaving nicer flying bird, and you are going that way as a pursuit unto itself. With the engine drag reduced, the flow into the prop cleaned up, and the wings able to work, it will be a little faster too.

Now go work on the new cowling or the new tailplanes or the garden... Something that has pay value for you.

Billski
 

Aesquire

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I think your plane is quite pretty. Elegant, if you prefer. Great visibility, decent, if not racer, speed. Nice package.

A LOT of planes have the wing mounted pusher configuration and accept the propeller efficiency loss as fair trade for the great view out front for the crew. Mostly "ultralights" and amphibians and, like yours, a "Grown up's" ultralight derived 2 seater. The Quicksilver series and the GCS Hawks, and the Kolbs all have big fans.

Even the Very Early Demoiselle has the high mounted engine above the center of drag. It's a tractor engine, but the many later derivatives like Flightstar all acted much the same with power changes.

I admit, I'm still confused by the Angle of the thrust line, and how it affects pitch under power, I suppose I should search for, ( then if needed start a new thread ) information to enlighten me.
 

Dan Thomas

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We already made a conclusion that this type of aircraft will be much slower, but it should still be possible to make it into a good flying aircraft!!! Simply refuse to see it as a problem or as a failure!!!
"Much slower" means a narrower speed range, and flying closer to the stall all the time. That is not making your airplane into a "good flying aircraft."

And "simply refusing" to see something is called stubbornness, and stubbornness has killed many an experimenter. I once warned a guy who had built a rickety homebuilt contraption, with numerous obvious deficiencies, that he would kill himself in it. No registration, no inspections, no compliance whatever. He was stubborn. He later took off, and on downwind the thing stalled and he fell several hundred feet and actually survived.

Warnings from experienced folks should not be ignored. You won't live long enough to get experience if you can't recognize your own limitations.
 

ragflyer

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What is costing you speed is:
  • The pusher configuration with the prop operating in the immediate wash of the wing, producing poorer prop efficiency than if it were a tractor prop, which you are not going to change, so you live with it;
  • The uncowled engine which is both big drag by itself AND messes up flows over the wing further adding drag, which you are fixing.
Billski
+1 Billski! As he says we have all said this before - example see my post from 4 years ago on this that pretty much says the same thing as Billski!

 

Eugene

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Warnings from experienced folks should not be ignored.
Listening to everybody for last five years, it's exactly what I was doing.

I should've phrased my initial question differently. My fault. Happens to me a lot. Even my own wife sometimes doesn't understand me. That should tell you something.

When I hear statements like, "your problem is because you want to sit in front of the wing", or "your problem is that engine above the wing", "problem is that engine is push". I wanna turn around and tell them that I don't have a problem. This is my wish. This is what I like. This is what my wife likes (who signs the checks).

I want to tell those guys with aeronautical degree that they simply don't know how to handle my wish, how to get me there, how to put this package together into good flying aircraft! What I want is not out of line. Just like Peter Garrison said that arrangement is not really all that different and very manageable.
 

TFF

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Your wish can’t be yes we can break physics. Your wish is you want us to love your plane. Only you can tune this out have do what you want. Your problem is you want people to like it. Your problem should be building, not us.
 
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