Downforce on horizontal stabilizer in flight

[flyguy]

Does anyone know what is the typical downforce on a BC12D tail surface while in flight? Or where to find such information, or how to calculate the value. It doesn't have to be a real precise number, just a ball-park figure would do. (closer would be better though).

Edit: I guess, now that I think about it a little (a few minutes later), the location of the center of lift on the wing is the key to the calculation.

DC

[fearofpavement]

Re: Downforce on horizontal stabilizer in flight

Why?

[Hank Jarrett]

Re: Downforce on horizontal stabilizer in flight

NOT a simple calculation and changed depending on pitching moment and loading which change constantly. Theory of Wing Sections by Abbot and Deanhoff (you know darned well that isn't spelled right, but I don't have my copy here) will tell you how to determine it for a STATIC situation. Pull G' or turn or ........and the number changes.

Hank

[Hank Jarrett]

Re: Downforce on horizontal stabilizer in flight

You might get a quice max number by calculating the stall AoA for a flat plate flapped surface at max deflection and using the lift equation to see how much lift a stab/elevator CAN develop.

Hank

[Hank Jarrett]

Re: Downforce on horizontal stabilizer in flight

What are you really trying to find out? Strength of the stab attach? Control force? Bending moment on the fuselage? In what part of the flight? Like I said, iot changes when almost ANYTHING changes.

Hank

Now you have me curious.

[flyguy]

Re: Downforce on horizontal stabilizer in flight

Fair enough Hank. I have noticed that this particular Taylorcraft that I have now (12D-85) does not behave the same as my other two did under one condition that is at least curious.

If I try to make a low speed pass a few feet off the deck the airplane is much less stable vertically than the other two were. This airplane also seems to be more difficult to control during the landing flare, which is much the same flight condition.

When I bought this plane the trim cable was so tight, perhaps along with other problems, that the elevator would not drop down past center or so when released from a held high position. It would lightly bind in about the middle of downward travel. We corrected that.

Later we found that the bottom elevator cable was way loose. It took a while for that as you will only notice it with the control wheel secured all the way back. Myself and 3 different AI's had missed it for some time. We corrected that. And yes, this airplane, previously owned by two "AI's," has been like that all over.

I had at some point checked the alignment and tension on the horizontal stabilizer and made some adjustment for straight and level and tension, but recently I began to wonder about overall drag in the elevator controls; curious if excessive friction might be contributing to the vertical wandering at low speeds.

In the process of tugging on the elevators I noticed that the clearance on the hinges was "loaded," that is the play between the pins and hinges was pressed tight downwards until I pulled up on the stabilizer. With upward force on the stabilizer the elevator would click back and forth slightly on the pins.

As it was pouring rain outside, I had some time to waste (Oh, Oh,) so I tried some tweaks on the tension wires to see what changes might "correct" this condition. Pretty soon it occured to me that this was not the simplest of problems and that at least part of the problem was: under what kind of load conditions would one want the hinge clearance/play to be optimal/ (correct?) ? My judgement being that perhaps by putting some loading weight on the horizontal stabilizer that I could simulate low speed flight and optimize the hinge "loading" and friction at that condition. I was also thinking about elevator to stabilizer alignment thinking that perhaps this was the best final indicator of that.

See, now aren't you sorry you asked? Chuckle

[Hank Jarrett]

Re: Downforce on horizontal stabilizer in flight

Sounds like the cobbler had poorly maintained shoes.
I think the first thing should be to rig the stabs to square and level with the elevators off. Check that the stab and elevator hinge tubes are on size and in line. It almost sounds like you had a bent stab or elevator at one time and the hinges are binding under some conditions from a poor job of straightening. That is NOT GOOD! I am betting you will find that the the hinge bushings are worn and pushing/wearing on one side too.
I like that the cables have all been re-rigged. That could have actually hidden the problem. You also need to look at all of the pulleys and see that they are all turning (I had some that were painted and the paint would hang up once in a while and the cable would slide over the pulley). If you have SS cables they are weaker than Galvanized and could have stretched.
First step is to confirm rigging and be sure the hinge bushings are all good. While you are doing it, push and pull stuff looking for any cracked welds or tubes that could be moving under load. If you push back and forth on a surface it SHOULD NOT make popping noises!

Hank

[Robert Lees]

Re: Downforce on horizontal stabilizer in flight

Also check the security / ovality of the stab attachment bolts & holes. Grab each stabiliser tip and see if it'll move fore-and-aft.

[flyguy]

Re: Downforce on horizontal stabilizer in flight

Does washing dishes improve one's IQ. LOL. That is what I was doing when I think I came up with a solution for the adjustment. The download will not matter if I tweak it taking all the requirements I have thought of into consideration.

As for the bushings being worn out, I should have said that the clearance is quite small, a few thousands at most. Not a problem IMJ.

That will give me a little project for this afternoon. Still can't fly post baffle repair and annual because of the rain and stormy weather/high gusting winds. Can't complain as we finally are getting some rain here in Central California. Temperatures are still going to be in the mid/high sixties. Feel for the folks elsewhere weatherwise.

[Hank Jarrett]

Re: Downforce on horizontal stabilizer in flight

If you have to adjust the tension to get the hinges in line there is something wrong. With no loads, the hinge holes should be in line (holding a stab, elevator or rudder, off the aircraft in your hands). Once the tail wires are rigged they should still be in line. I don't have the wear tolerance for the bushings here, but I am sure someone can post them. The hinge pins and attach bolts should NOT wiggle in the holes, even one hole at a time. I could put a long drill rod through all three hinge lugs on mine and spin it while it was a snug fit in any single lug (pretty much line bored). My trim system did have a lot more slop than I like for the whole system, but no individual part was excessive. There are just too many working pivots and threads in the trim system to get very snug without getting a lot of drag on the system.

Hank

[N96337]

Re: Downforce on horizontal stabilizer in flight

Flyguy, sounds to me like you've got an alignment issue between the elevator and stabilizer. I'd say that the hinge points at one time were bent or maybe replaced, or maybe even welded wrong in the first place. If I read your statement right, it sort of goes "over center" one way or another.... that screams alignment to me. Remember, alignment is 3 dimensional...so it could very well be a fore and aft problem as well as a vertical problem, and not only in one plane of travel of the elevator. Just thinking outloud.........
John

[flyguy]

Re: Downforce on horizontal stabilizer in flight

As I said the excess drag in the middle of travel was corrected some time ago. I'm not concerned with the safety and general function of the elevator but was trying to consider all the forces at work there and perhaps got a little over exuberant in my analysis.

There are so many factors involved in the pitch control at low airspeeds (such as the total mass of the elevator and the control column assembly) that the "problem" could be most anything. There is even the possibility the my piloting skills in that precise a situation have gone to hell. Chuckle.

Anyway I am not up for replacing any bushings at this point as it would be about 37th on my to-do list with this plane.

This has been worthwhile because I have arrived at a quite different approach to adjusting the flying wires compared to the one I used previously. That one worked "pretty good," so this one should be better. We shall see.

Thanks for the input guys.
DC

[1938BF50]

Re: Downforce on horizontal stabilizer in flight

I have to ask.....2 or 3 hinge elevators? I know they should be (2).

The reason I ask is real experience with alignment on the (3)hinge pre-wars. The center hinge 1/8" out of alignment will throw the tip of the horizontal up and down a full 1 1/2" at full deflection. I learned this the hard way after a recover. The fabric shrink pulled the elevators "in" over an 1/8". Did not catch the Ornithopter like flapping at the tips until final assemble. I cut the fabric off the elevators and "prebent the hinge edge" +3/16". Recovered and shrunk to a straightedge bringing alignment to "perfect". At assembly, the hinge pins pushed in by finger. It is easy to see why the factory went to the (2) hinge to ease final assembly issues.

[pete]

Re: Downforce on horizontal stabilizer in flight

Flyguy,
Are the cable linkages pivoting freely? Previouly had a similar condition. The bind would hold under some tension then suddenly give.
I remember seeing old pics of tail load tests at the Tcraft factory. Can't remember the numbers but they were there.

[flyguy]

Re: Downforce on horizontal stabilizer in flight

All good news today. I tweaked the tail feathers the way I had figured out that I thought would make an improvement. I then tried using a thinner lubricant on the control column shafts. That really made a change.

Stormy weather with gusts went away today and was really quiet this AM so we did about 1/2 hour of flying including 3 landings and cruising around over the hills.

What a difference in control response. I was flying with 2 fingers and my thumb on the yoke when cruising at 2000 RPM. Able to make turns that way and pitch control took very little pressure.

Of the three landings, one was good and the other two were excellent 3 pointers. The first landing, after not having flown for over a month, was so smooth that I thought I had done little flat skip after the tire chirp and was still in the air. I have to admit that the air was very, very calm, but the plane really flew great. Everything considered the flight was like flying an airliner.

Don't know what I am going to do the remainder of the day that will compare with that----

DC

EDIT: CAN SOMEONE PLEASE FIX THE %$#&**# SOFTWARE HERE SO IT DOESN'T DUMP THE COMPLETE REPLY IF YOU TAKE OVER 30 SECONDS TO TYPE IT!!!!