Vertical tail airfoil for pusher

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Eugene

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Found this article online about Hornet airplane. I have same problem on my Skyboy always need to hold right rudder. Sure simple trim tab can do it for me, but then I need to make right leg longer and left one shorter.

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Vertical tail volume is only 0.32 on my Skyboy and need for rudder deflection is pretty big. If you want to go straight.

IMG_5600 2.jpeg

New tail will get for sure a little longer and bigger, but I am wondering if anybody have good idea of how they did it on Hornet? Did they use 6% airfoil on the right side of the fin, but 10% on left side? Or they changed the angle as well by 1° or 2°?

Thank you.
 

BBerson

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Probably kept the right side of the vertical fin flat as it is. Then put some curve on the left side of the fin to change it from a symmetrical to a cambered airfoil to lift more to the left.
 

Eugene

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OK, I have another philosophical question. Dan Johnson in this article making a statement that this problem is true for all pushers. I was wondering if it is a correct statement or not. Maybe this is only true for pushers with low positioned vertical tail. Propeller is discharging a rotating column of air and if vertical fin position very low then only one side will get the push.
 

TFF

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Your P factor is direct on the tail. Most P factor is on the fuselage. It is still P factor. Off set, rudder trim, strong legs are usually the solutions. You might look at the high end discus launch RC gliders. Not the commercial ones. The bespoke ones that cost a couple of grand. They have an offset fin/ rudder with an airfoil. It’s intent is range of speed trim control. Launch is fast and then it has to fly slow. Essentially a wing airfoil at a negative angle of attack turned vertical.
 

Eugene

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I did some thinking and as a result made some pretty good conclusions. I think..... All I need is confirmation that my thinking is correct.

Horizontal and vertical tails on my Skyboy undersized. It was probably OK for 50 hp ultralight, but not anymore for 100 hp LSA. I did some calculations to prove that and also horizontal tail incidence is 5.7°. Correctly sized horizontal tail should not need such aggressive angle.

Vertical tail is symmetrical, undersized and doesn't have incidents at all. As a result Skyboy for straight flight at 75% power need pretty aggressive rudder deflection. Sure trim tab can do this job, but this situation is driving pedals apart by at least 2 inches or more.

New correctly sized vertical tail, even if made symmetrical, will not need such aggressive rudder deflection for straight flight. And pedals will stay much closer together without driving me crazy.
 

pylon500

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Your P factor is direct on the tail. Most P factor is on the fuselage. It is still P factor. Off set, rudder trim, strong legs are usually the solutions. You might look at the high end discus launch RC gliders. Not the commercial ones. The bespoke ones that cost a couple of grand. They have an offset fin/ rudder with an airfoil. It’s intent is range of speed trim control. Launch is fast and then it has to fly slow. Essentially a wing airfoil at a negative angle of attack turned vertical.
We can see what you're trying to say, it's only the wording is a bit off.
What you are describing is usually referred to as 'Slipstream effect', one of the three effects from propellors;
Torque,
Pitch ('P' effect),
Slipstream.
gvXr4.jpg
(Must admit, the gyroscopic precession one is new to me) 🤔
 

Eugene

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You may get some relief by adjusting the thrust line off center a bit.
Engine is already installed with 1.5° angle at the factory. Original 50 hp airplane didn't have that. They changed this in the process of switching to 100 hp engine. Apparently I am not the first one who discovered this problem. Test pilots did see that as well. To my surprise they didn't address the tail itself or its flexibility on 120 x 2 mm tube.
IMG_1958.jpeg
 

BBerson

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Or an offset cuff on the leading edge of the vertical stabilizer. Drooped cuff adds camber which changes the angle of zero lift, perhaps a degree or two. Simple to experiment.
 

Vigilant1

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You may get some relief by adjusting the thrust line off center a bit.
This has a couple of advantages:
1) Should be fairly easy to try, esp as the engine on this plane is uncowled. Jam some shims or washers under the engine mount anchors and see if things change.
2) If the existing yaw is due to slipstream effect, precession, etc, then it will be somewhat proportional to the power setting. So will any counterforce provided by changing the thrust line, so that is good.
 

Eugene

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OK, can we try to look at this problem from different angle?

This doesn't seems to be a big deal for most guys with most light sport airplanes. This problem is usually compensated by adding small trim tab. This compensation is most likely driving pedals apart by 1 inch or so only.

But it is a big deal for me because I don't know what to do with my left leg on the long flights. At the same time my right leg is nice and relaxed all the way on the floor. So, why is such a big difference for this airplane?

In my simple experiment sometime ago I put hundred pounds on horizontal stabilizer and discovered that tail boom was deflecting and changing decalage by 1.5° about. That was proved to me that I have a very unique and very flexible tailboom.

Can we assume that this tail boom with side load on vertical tail deflecting as well to the point that I have to fly with pedals 3 inches apart?
 

berridos

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More than flexing it looks like the tail boom is subject to torsion.
Measuring those deflections and torsions should be managable.
 
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