Vertical tail question

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Eugene

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OK, during straight and level flight we need to press right rudder. Or we can bend over trim tab to match our normal cruise power setting, so we don't have to hold it.

To match this situation we really should make our right leg a little longer and left leg a little shorter.
Or more complicated way is to adjust cables so in level flight your pedals will be in line with each other. Is it Ok to do so?

On the way back from Oshkosh last summer I really didn't know what to do with my left foot, was running out of room. But right foot was very comfortable with all kinds of room available. Really thinking of changing that for more comfortable long-distance flights.

Wondering if big distance between pedals (2-3inches) is direct result of too small vertical tail volume for 100 HP engine vs 50 HP originally?

IMG_2856.jpeg
 
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Victor Bravo

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Making one rudder pedal shorter will not solve the problem at all. You will still need to hold pressure with one foot.

$25 or $30 worth of equipment will resolve this problem correctly, using a model airplane servo and a small "servo driver" control box.
 

Victor Bravo

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That's what I meant, a small tab controlled by a servo. A fixed tab usually doesn't solve the trim issue, it just makes the issue not happen at one given speed. Eugene your legs and your rudder pedals are NOT the probllem. If your car doesn't start because the fuel tank is empty, can you fix it by adding water to the radiator?
 

lr27

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Sounds like he's concerned about the position of the pedals, not the,control force required. Trim isn't likely to help with that. Do thrust line adjustments work with that configuration?
 

Eugene

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Sounds like he's concerned about the position of the pedals, not the,control force required. Trim isn't likely to help with that. Do thrust line adjustments work with that configuration?
Yes, problem is pedals position in cruise at 75% power (about 5200 RPM on 912 engine with 100 HP). They are about 3 inches apart.

On the ground when pedals inline = rudder inline with vertical stabilizer.

This airplane simply need about 10° rudder deflection to go straight. Not making any difference how you will be holding this 10° deflection: by pushing with your foot, using fixed trim tab, or electric servo. Need for 10° still will be there.

So, the way I see it this need for 10° is result of undersized vertical tail. Vertical tail volume on Skyboy about Vv = 0.033

3A4B9E07-273B-4E0A-B09B-F4C65930E0AA.png
 

wsimpso1

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Eugene,

Another way of adjusting the amount of rudder needed is possible. I know a pilot that had an airplane with rudder pressure always needed to one side. The vertical stabilizer was built into the fuselage, while the horizontals where hung on simple supports and then wire braced fore and aft. By slightly shortening one top leading wire and slightly lengthening the other top leading wire (always do them together), the vertical stabilizer's leading edge is twisted slightly. Keep records of how much you adjust and then tune this setting. Playing this adjustment to neutralize the rudder in cruise worked wonders in the subject airplane. Oh, and if making such an adjustment made it worse, hey, your adjustment was in the wrong direction - Undo it, then make a similar adjustment in the opposite direction. You may have a similar method available in your airplane to neutralize the rudder.

A fixed tab or a wedge is common and done on many airplanes to neutralize controls in cruise. A flight adjustable tab will allow you to neutralize the rudder in any steady state operation. I still think adjusting vertical stabilizer twist is the best and first adjustment.

You can find the neutral yaw position of the rudder and adjust the pedals to be centered when you have the rudder at that angle, but a few things have to be done along with them. If the rudder has stops setting max travel at the rudder, these stops should be set at factory settings. Then if you have stops at the pedals, they too must be adjusted anew - usually, the control surface hits its stop and then medium level force is added to make the pedal hit its stops - Vaughn Askue cites 0.010" clearance on the cockpit control as the control surface touches its stops. The idea is that no matter where you put the neutral position of a cockpit control, the control surface must go full travel and the cockpit control must be able to put it there. If you have not got stops, you still need to make sure that the pedals can be manipulated to drive full travel both ways on the control surface.

Now lets talk about vertical tail volume.

While your vertical tail coefficient may seem on the low side, it is way more appropriate than your horizontal tail volume. That matters. The vertical and horizontal tail interact significantly if they overlap each other. The flow over the horizontal tail goes up and then down over the top, speeding up over the surface, while similar speed and direction changes occur over the vertical tail. When the tailplanes overlap, the air has to do both while going past them. If one tail plane is too small, the disruption of the flow has to be larger than we would like and that also disrupts the flow effects on the other tail plane. Now let's go further - if one of the two tailplanes induces separation and the tumbling flow that produces, it messes up the flow on not one, but on both surfaces...

Let's go further. The vertical tail is swept. In airplanes that operate at M = 0.15, sweep is mostly bad. Sweeping any of the planes reduces their effectiveness because the lift slope of the foil decreases, making their tail volume seem smaller... and thus are a bad idea. This reduced lift slope is due to increased spanwise flow that occurs on swept surfaces - there is flow from the fuselage toward the top of your fin, also pulling on the air around both the horizontal and the vertical tails.

I think that your substantially undersized horizontal tail is exacerbating issues with the modestly sized vertical tail plane. In concert with that, the modestly sized vertical tail is exacerbating issues with your substantially undersized horizontal tail. If you were to improve the horizontal tail volume situation, I suspect that the vertical tail issue will become significantly less. I have seen this before, where a friend had too small a horizontal tail, increased its size, and not only did he then find he had enough elevator to flare, he found it to hold attitude much more easily in the flare, and he found that his directional control and stability improved too. He did not understand why until I explained how the tailplanes interact. This theory stuff does actually work.

If it were my airplane, and I had decided to keep it, I would:
  1. See if I can neutralize the rudders with vertical stabilizer rigging adjustments;
  2. If the fin can not be adjusted, then I would tab the rudder;
  3. Then I would increase horizontal tail volume as we have been discussing, doing whatever engineering I must to have the tail boom be sturdy with the potentially increased boom bending moments.
Billski
 

BBerson

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Vertical tail area can easily be increased with auxilliary plates on both the horizontal tail tips. These are used on Beaver seaplanes and Champ seaplanes to offset the float bow area. The horizontal tail might benefit from the end plates as well.
These plates are not in the prop swirl. Can be set at any angle.
 

wsimpso1

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That might help both axes. Recall that the auxiliary stabilizers on those cited aircraft are added on somewhat overbuilt tailplanes, and that they are usually symmetric about their attachment to keep the bending forces down. None of those features is by chance.

As for their not being in the prop wash, do not count on it - It would not take many degrees of yaw to lay the prop wash on those auxiliary stabilizers. Stab the rudder to initiate roll during a go around or to level wings after a turbulence induced roll, and you are there.

Billski
 

Eugene

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Vertical tail area can easily be increased with auxilliary plates on both the horizontal tail tips. These are used on Beaver seaplanes and Champ seaplanes to offset the float bow area. The horizontal tail might benefit from the end plates as well.
These plates are not in the prop swirl. Can be set at any angle.
Here is pictures from some of my experiments some time ago. Not sure what it did for directional anymore. i was really after horizontal tail volume

IMG_2464.jpegIMG_2465.jpegIMG_2471.jpegIMG_2467.jpegScreen Shot 2019-05-04 at 16.40.54.jpeg
 

Eugene

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Making one rudder pedal shorter will not solve the problem at all. You will still need to hold pressure with one foot.

$25 or $30 worth of equipment will resolve this problem correctly, using a model airplane servo and a small "servo driver" control box.

So, we can use this motor on airplane for rudder electric trim? Did I get this right?

Screen Shot 2020-04-22 at 08.18.26.png
 

Eugene

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Eugene,

Another way of adjusting the amount of rudder needed is possible.

A fixed tab or a wedge is common and done on many airplanes to neutralize controls in cruise. A flight adjustable tab will allow you to neutralize the rudder in any steady state operation. I still think adjusting vertical stabilizer twist is the best and first adjustment.

If you have not got stops, you still need to make sure that the pedals can be manipulated to drive full travel both ways on the control surface.

Now lets talk about vertical tail volume.

While your vertical tail coefficient may seem on the low side, it is way more appropriate than your horizontal tail volume. if one of the two tailplanes induces separation and the tumbling flow that produces, it messes up the flow on not one, but on both surfaces...



I think that your substantially undersized horizontal tail is exacerbating issues with the modestly sized vertical tail plane.

If it were my airplane, and I had decided to keep it, I would:
  1. See if I can neutralize the rudders with vertical stabilizer rigging adjustments;
  2. If the fin can not be adjusted, then I would tab the rudder;
  3. Then I would increase horizontal tail volume as we have been discussing, doing whatever engineering I must to have the tail boom be sturdy with the potentially increased boom bending moments.
Billski
Thank you for detailed explanation! I think I understand most of it after reading 10 times or so.

IMG_4078.jpeg

Trim tab is there and it works very good. But to convert to electric trim would be pretty cool!! Problem is my pedals 3 inches apart in cruise. Normal rudder deflection is 25° each way. Moment I start playing with cables = I am stealing travel distance from one side and adding to another. And there is no way I can twist vertical tail on this airplane.

IMG_4079.jpeg

This is how I understand that undersized horizontal tail working at large angle reducing vertical tail effectiveness. Hope I got it right.

88361_1461955973.jpegIMG_4072.jpeg

Will tail from SeaMax work for me? If I can find the way to do it.
 

Pops

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So, we can use this motor on airplane for rudder electric trim? Did I get this right?

View attachment 95783

That RC serve has a very low amount of torque for a trim tab at your aircraft speed. The pressure on the tab will back feed to the servo, thru the gear train and the servo motor will be fighting against this pressure and drawing a lot more amps than it should trying to hold position, with a low motor and circuit life and higher drain on your battery. They also run on 6 volts max.
You can build a little simple voltage reg for 12 to 6 volts.
IF you do use these servos use the Gaint Scales servos
But, you can buy these servos that will take a lot of back force. You can buy these linear RC , 6 volts or the linear servo with limit switchs at 12 volts. I use the limit switch linear 12 volt servo for electric trim elevator trim with a rocker switch.
 

lr27

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This is a slow aircraft. People used to use 40 in-oz. servos on 100 mph pattern models. Exactly how much is needed depends on the size and shape of the trim surface. A long, narrow surface will need less torque than a short, wide surface. The long surface might need a little more attention paid to torsional stiffness. Full deflection of the surface shouldn't be more powerful than the pilot's leg, in case something goes wrong. The restricted throw may allow for more mechanical advantage, allowing a less powerful servo. Metal gears are probably worthwhile.

Such a surface may be vulnerable to ground handling, so it will be a crucial pre-flight item.

A sparrow strainer style surface hinged at 23 percent MAC could be smaller and require less servo power, but it wouod be even more vulnerable.

I imagine there can be flutter issues with any of these.

All of these options do nothing for the pedal position issue. It seems to me that, as mentioned above, moving the stops and neutral position might be the way to go.

The above is based on theory, and I could have missed something important.
 

lr27

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45 lbs. of force would probably rip the tab off the tail! Comparable to the pilot's force on the stick when nervous, I should think. Without knowing the size, shape, and hinge line of the surface, it's hard to know how much torque it needs. Without knowing the linkage geometry, it's hard to know how much force is required. Plus, of course, airspeed matters and I forget what that is for your plane. At some point you will need to do some calculations. I'm sure the Magic NACA Archive has pertinent information.
 

Eugene

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Not sure if you can see it on this picture, but there is a lot of pressure on this side of vertical stabilizer. What if I try to build composite 1/2 of symmetrical airfoil and somehow fasten it to this side of the fin? I can start with 6% airfoil and slowly move up to 10% or 12% if needed until I get it perfect for my 75% power setting?

Screen Shot 2019-07-07 at 10.25.20.jpeg
 

lr27

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I suspect reshaping the tail could provide the trim change you want, but I don't know about flutter issues. Maybe your airplane is slow enough that it's not a big deal. However, would this be noticeably better than a simple, fixed tab?

Have you checked to make sure your plane is rigged correctly?
 

Pops

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I have used these servos for Pitch trim since 2010 in the SSSC and now in the JMR. I used the 150 to 1 gear ratio ( also gives about the correct speed along with the most power) and the stoke of 30 mm or 50 mm. 150 to 1 gear ratio gives a max force of 200 N and a max back force of 102 N.

On the elevator trim, I adjust the trim tab for full up trim for my approach speed with engine at idle. Same way Cessna does on the elevator trim in the service manual for the C-150 and C-172.

When I put the homemade autopilot in the JMR, I will be using these servos in the RC mode to operate the tabs on pitch and roll and also have manual trim knobs on the autopilot. I used this autopilot for 2 years in the SSSC with giant scale RC servos , ( without autopilot but with electric pitch trim I used the Linear servos after removing the autopilot), but with a cruise of 80 mph. For the higher speed of the JMR I will using the linear servos with the autopilot.
 
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