Horizontal tail construction

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Eugene

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I am still not so sure that your bird was finished yet when they started production.

Billski

Well, in my book aircraft is ready for production when its safe and doing exactly what you ask.
So, mission was to build 50 HP side-by-side ultralight trainer for European market. This way after they got German certification, mission was accomplished. It is finished and someone can take this paperwork and start production.
Big question is - can you make any changes to what is approved by Germans?

And if you install engine with 2 times more power, is it OK?

Do you need or could you, make needed changes to compensate for more power?

You got approval for 50 HP and MTOW 992 LB, but you are shipping to US 100 HP and MTOW 1320 LB.

So, is this aircraft finished? Is it ready for production?
 
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lr27

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Downwash ought to be at a smaller angle at higher speed, but the lift per degree will be more. But this would be trimmed out if you wanted steady flight. The pitching moment of the wing airfoil will be trying to pitch down, more strongly as the airspeed rises. However, I imagine the thrust line may have larger effects.
 

Eugene

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[QUOTE="lr27, The pitching moment of the wing airfoil will be trying to pitch down, more strongly as the airspeed rises. [/QUOTE]

I was thinking that wing pitching moment is getting smaller as AOA getting smaller.
 

cavelamb

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It all depends on the actual CG location, and that can change in flight.
For instance, burning off fuel from forward or aft fuel tanks...

Moving the CG aft will LIGHTEN the load on the tail regardless of speed.
Moving the CG forward will increase the load on the tail.
 

Eugene

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IMG_3631.jpeg

Yes, many things will need bigger balancing pressure on horizontal tail:

- distance from center of drag to the thrust line.

- more power = need more down pressure

- CG more forward = need more down pressure

- bigger AOA = wing will have bigger nose down pitching moment = need more down pressure = that is another reason to design aircraft this way, so it has small AOA in cruise. And drag will be lowest of coarse.

Hope, I got this right.
 

lr27

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[QUOTE="lr27, The pitching moment of the wing airfoil will be trying to pitch down, more strongly as the airspeed rises.

I was thinking that wing pitching moment is getting smaller as AOA getting smaller.
[/QUOTE]
Well, I suppose you could look at the graph of the Cm. Some vary a bit with angle of attack. However, the actual torque the pitching moment produces is also related to the dynamic pressure.
 

wsimpso1

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Well, in my book aircraft is ready for production when its safe and doing exactly what you ask.

Given your issues with its inability to behave well when both seats are filled, I am inclined to believe that they stopped designing early and started shipping product. You can call that what you want, but this design and product engineer considers it an incomplete design.
 

wsimpso1

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I need to make sure that this is correct statement. Load on horizontal tail is higher at higher speed, but not because downwash bigger. It is smaller at higher speeds, right?

View attachment 94190

Tough to tell which question to answer - you asked a compound question with contradictory elements. Yeah, I am being a grammar gripe, but it helps to be clear. I will just get into the science...

Wing Lift = rho/2*V^2*Cl*S, where rho is air density, V is free stream air speed, S is wing area, and Cl is lift coefficient= m*alpha + b. So let's follow it through - For a given airplane, S, m, and b are all fixed, and for a given g and airplane weight, lift is close to fixed (the tail load may vary some), so L/(rho*S) = V^2*Cl/2. Since Cl is changed roughly linearly with angle of attack, Cl can go from stall at negative g to stall at positive g by changing alpha. Takeaway - Cl can vary a lot, but in level flight lift is pretty close to constant.
.
Wing Pitching Moment = rho/2*V^2*Cm*cmac*S, where rho is air density, V is free stream air speed, S is wing area, cmac is effective chord, and Cm is moment coefficient. Looks pretty similar to the lift equation, but in application, it has a twist. Cm does not change much as we change alpha and is usually a negative number, which means the nose pitches down. Follow this through and M from wing pitching moment starts small at low speed and quadruples each time the speed doubles. Actual pitching moment goes up a lot with speed.

Pitching Moment due to Weight = Weight of the airplane * (CG - Neutral Point), where Weight is weight of the ship, CG is the fuselage station of the CG, and Neutral Point is the fuselage station of the spot where dM/dalpha of the airplane is ZERO. CG aft of here by even small fractions of an inch renders airplanes close to uncontrollable, while CG forward of here the airplane is stable in pitch. The Wright Brothers figured out during their flying at Huffman Prairie that shifting the CG a little forward made airplanes stable. Anyway, the Weight of the airplane times the distance the CG is ahead of the neutral point adds to the pitching moment from the wing, but is essentially constant while on a flight.

There are other possible pitching moments, but can be ignored for purposes of this discussion.

There is one more part to the fuss. As you go faster in 1 g flight, Alpha of the wing must decrease to keep the lift constant, pitching moment from the wing increases with the square of speed, pitching moment form weight and stability is constant, so the total moment grows with speed. To counter that total moment, we put a tail on our airplanes, and adjust the elevator to balance the total moments to ZERO while holding the alpha we need for our speed and g. The extra fuss is just that whatever download is produced by the tail is extra lift the wing has to carry. Anyway, from following this analysis, the pitching moment that is nulled out by the tail increases with airspeed, so the tail down forces go up a lot with speed.

But how do we get that tail download? Lift from the tail uses the same equation as the lift for the wing, but the tail is usually smaller. Cl of a horizontal tail is usually one constant times the stabilizer alpha plus another constant times the elevator angle. At the tail, alpha is the wing's alpha plus any difference in incidence minus alpha from wing downwash. And downwash is essentially linear - the bigger the CL, the bigger the downwash. It is about 3.5 degrees when the foil making it is at Cl =1, so it is not huge, but it is very real. So for slow flight, alpha is big, downwash is substantial, so the elevator trailing edge has to go way up to make downward forces. Speed up the airplane and alpha and downwash diminish, V^2 gets a lot bigger, and so up elevator angle becomes less and less to be able to trim it all out. Yes, it looks like the elevator is making less lift - it is making less Cl in the downward direction. But we already reviewed how wing pitching moment goes up dramatically with speed squared so we know the tail must make more and more downforce as you go faster to balance the airplane. And we have a lot more V^2 to do so, lowering the Cl needed to get that downforce.

One last point in all of this pertient to the Flyboy's landing behaviour. When you are landing an airplane, the system goes into ground effect - the downwash can not proceed through the ground, and so downwash flattens out as we attempt to land. The tail looses some effectiveness that it had in slow flight at altitude - yep, it takes more elevator to hold an attitude in ground effect than it does aloft. If the tail is just barely big enough at altitude, you can expect that you may have trouble flaring and holding the nose off for landing...

So, to cover your area of discussion, if not the compound and contradictory question, the tail typically must make more and more lift (in pounds or Newtons) as you fly faster, but it does so with less and less Cl. And in landing, it has only to make the same Cl, but it needs more up elevator angle to do it.

Billski
 
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BBerson

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And the skinny tailboom deflects more with increased load, which requires more up elevator.

It just wasn't designed to fly fast. Personally, I would be concerned with flutter and would not be interested in flying it fast.
 

Eugene

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And the skinny tailboom deflects more with increased load, which requires more up elevator.

It just wasn't designed to fly fast. Personally, I would be concerned with flutter and would not be interested in flying it fast.

Yes, I agree. It was approve for max cruise speed 103 MPH and I am not going to go faster. It's not about speed. Trying to create good flying machine. I didn't know what to do at first. Have now pretty good idea what is going on. I hope.

One EAA member asked me another day - " at what point you going to give up?".
I didn't know how to answer. Is what I am doing really look so crazy from the side? Why do I have to ever give up?
 

Eugene

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The extra fuss is just that whatever download is produced by the tail is extra lift the wing has to carry.

Thank you! I was reading your post 10 times. Hope I got most of it. For sure will be reading it again and again before I get it all.

So, extra download on the tail = is extra load for wing to carry = larger AOA needed = more drag

Extra tail download = more drag. Drag positioned 3 feet below thrust line will generate bigger load for wing to carry. T-tail should be used for this configuration.
 

lr27

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A t-tail should not be used unless you want to do a complete redesign of the rear fuselage.
 

wsimpso1

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All of that is just intended to help understand

The only place prop driven airplane really benefits from a T-tail is in high mount engined seaplanes, where you need the prop blast on the elevator to keep the nose up... and that benefit comes at a price of the prop blanking the tail in flare...
 

Dana

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If you increase the size of the stabilizer and leave everything else unchanged, the load on the tailboom won't increase since the amount of downforce it has to exert is equal to the nose down pitching moment produced by the wing and the high mounted engine. What will increase is the amount of downforce the tail can produce when you pull the stick back, for example when flaring. And the increased tail volume will improve the dynamic pitch stability.
 

wsimpso1

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Extra tail download = more drag. Drag positioned 3 feet below thrust line will generate bigger load for wing to carry. T-tail should be used for this configuration.

How did you get to that conclusion? Vertical position of the tail only influences whether it is in or out of the wash off of the prop, and it is always in the downwash off the wing.

There are three ways to make your bird fly better when you fill both seats:

Bigger tail area - Allows you to make enough download at lower elevator angles, shifts the neutral point aft allowing more aft CG, will modestly improve stability at modestly increased drag, may overload the tail boom (may force a structural upgrade);

Longer tail arm - Allows you to make enough download at lower elevator angles, shifts the neutral point aft allowing more aft CG, will improve stability and damping while making tiny increase in drag, may overload the tail boom (may force a structural upgrade);

Shift the CG aft - Allows the existing tail to be adequate for stability, may threaten aft CG limits.

If it were my airplane, I would look into all three, with a strong lean toward what it would take to extend the boom a foot or two. That path is particularly desireable - stability will improve, pitch damping will increase, and you can shift the empty CG aft some to reduce the CG changes with and without a passenger.
All of this is dependant upon doing an analysis of the tail and the boom, to see how much margin and deflection the current boom has. Then you can know if the boom can support a bigger tail or one further out. Right now we do not know. Given my composite capabilities, if I needed a beefier boom to extend the tail aft, I would build a tapered composite tail boom, save some weight, shift the neutral point aft, and scoot the engine aft a bit on its mounts...

Billski
 

poormansairforce

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If you increase the size of the stabilizer and leave everything else unchanged, the load on the tailboom won't increase since the amount of downforce it has to exert is equal to the nose down pitching moment produced by the wing and the high mounted engine. What will increase is the amount of downforce the tail can produce when you pull the stick back, for example when flaring. And the increased tail volume will improve the dynamic pitch stability.
I already pointed this out but it didn't seem to go over well. If he left the elevator itself the same size how much more down force would it create? My concern was gust loads.
 

Eugene

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How did you get to that conclusion?

Perfect, rocket-looking aircraft maybe doesn't need T-tail. But airplanes like Skyboy have 90% of total drag positioned below thrust line. This kind of aircraft will need larger tail with larger tail drag. This additional drag can be moved above thrust line. As result center of total drag will move closer to thrust line. Pitching moment will be reduced = smaller tail can be used = with less drag.

Additional benefits:
Tail will be working in clean air inside of propeller air stream = tail effectiveness will be improved = smaller tail can be used = or smaller AOA = smaller drag.

My wife agree with me on this, so I must be right!IMG_1090.jpeg IMG_1445.jpeg IMG_1446.jpeg IMG_2852.jpeg
 

Eugene

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I What will increase is the amount of downforce the tail can produce when you pull the stick back, for example when flaring.

Agree with everything but,

- when flaring only elevator is working. So elevator needs to get larger as well. You would normally do that if you increasing tail volume. I have right now only stabilizer increased.

What is correct stab/elevator ratio ? Skyboy original elevator is only 37% from total tail.

Russians, I talked to not recommending to make tail bigger then 20-25% for gust load reasons. That is using old tail pipe. If I can find composite way to make it stronger....... Maybe whole new carbon tail ???
 
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