elevator position

[cgoldy]

At the risk of over analyzing things, why is my elevator counterbalance always 1 inch higher than my stabiliser (read elevator slightly down) no matter what the trim setting, speed, weight and balance or spring tension? I thought that this type of trim system was supposed to keep them perfectly in line?

Am I just asking too many questions? She does fly great.

Late model Back country cub with extended wings 0375 engine thrust line built in and nose heavy until I fitted a little lead on the tail spring.

 

[RAC cubs]

very strange, must be nose heavy, whats a read elevator

 

[NimpoCub]

Maybe your horiz.stab is just a bit out of rig.
Is your trim indicator fairly centered? You can "slip" the cable around the jackscrew pulley so it aligns with the elevator IF the trim indicator is slightly "up".
THEN, maybe you can take the lead out.
Or maybe not.

 

[fobjob]

Checked the elevator bungie spring cable length?

--------------------------------------------------------------------

In flight? or on the ground?

 

[cgoldy]

It's not a read elevator. I was saying that the elevator is slightly down and the counterbalance is showing slightly up. Just a funny way us Australians speak.

My understanding is that the spring only effects the feel on the stick. I would have thought that I would keep adjusting the trim and when it is flying straight and level. I should look back and see my elevator and stabilizer in line.

Yes she is nose heavy. I shortened the spring by an inch and the stick is now quite heavy in the hand now..

But just thinking while I am writing.

a) If I made the spring stronger it would keep the elevator in line therefore I would trim more nose down to keep straigth and level. They would then be in line.

b) But maybe it is not the tension on the spring that is the problem. Maybe it is the cable length. When I am full trim down on the ground the cable goes totally slack.


mmmmmmmmm. I had better ring Wayne.

 

[mike mcs repair]

It's not a read elevator. I was saying that the elevator is slightly down and the counterbalance is showing slightly up. Just a funny way us Australians speak.

My understanding is that the spring only effects the feel on the stick. I would have thought that I would keep adjusting the trim and when it is flying straight and level. I should look back and see my elevator and stabilizer in line.

Yes she is nose heavy. I shortened the spring by an inch and the stick is now quite heavy in the hand now..

But just thinking while I am writing.

a) If I made the spring stronger it would keep the elevator in line therefore I would trim more nose down to keep straigth and level. They would then be in line.

b) But maybe it is not the tension on the spring that is the problem. Maybe it is the cable length. When I am full trim down on the ground the cable goes totally slack.


mmmmmmmmm. I had better ring Wayne.

random thoughts...

1. will not the elevator balance portion just be trying to be in line with line of flight(minus control or spring pulling it?)

2. the spring should probably only pull in some stab settings.....?????

3. are your elevators built right? or is balance not on center line of back part???

4. the lead in tail, and the elevator position is counter intuitive... kinda fighting each other in my mind if i understand what you wrote....

 

[fobjob]

The major function of the bungee cable seems to be to rig the elevator so that it follows in trail to whatever angle the horizontal stabilizer is... I've flown a cub that had a cable several inches too long, and it didn't seem to make much difference in actual trim. But it is a starting point, to illuminate other possibilities, like all the ones Mike mentioned...

 

[jgerard]

I'd take it out and go fly to see if that makes any difference. The stab should be what's setting the trim speed not the elevator position

Jason

 

[tempdoug]

Picture is worth a thousand words?

 

[RAC cubs]

Is the yoke in up side down? guy had pa 12 same problem,yoke was in up side down. they fly with
out those springs . towed banner today and looked back they were always flush. well there is
a slight twist or when one is flush other up 1/2 inch, have three pa 18 stock all the same. spring
has nothing to do with it. pa 12 guy could not get nose up trim ,or ran out,in slow flight.

 

[cgoldy]

Yoke must be OK because I have full travel all the way down to the tubing where it bottoms out. It really isn't a problem but I am just really curious. I bet ther are plenty of cubs out there where the elevator does not align with the stabalizer. I am going to play with cable length. I reckon it is cable length not spring tention. But it intigues me that so many people say they fly around without a spring. I will try that just for sh*ts and giggles.

 

[sharp]

It's just the aerodynamics. I would not worry about the cable lengths, that just impacts the stick orientation, nothing to do with flowing air over tail surfaces. Spring tension would have some impact in amount of resistive force. Does it do the same thing with and without a passenger? (ie change of CG and loading). If it is always the same, regardless of load & CG, then I would play with the spring. If it varies with CG, I would stop looking back during flight, look forward and enjoy the flight.

 

[AkPA/18]

It's just the aerodynamics. I would not worry about the cable lengths, that just impacts the stick orientation,

If the stick moves--don't the elevators move also?? I might have misunderstood your post.

thanks

Mark

 

[btracy]

My elevator and Stab are not lined up either. I think this is part of the reason why I am so much slower than everyone else. My plane is extremely nose heavy. I don't know how it compares to anyone elses

 

[sharp]

My point was that the airplane flying hands-off does not know where the stick is point. It is dynamically positioned. Changing the cable will change where the stick points, but if nothing else changes, then the elevator & stabilizer will not move. They will still not be aligned. Mine are not perfect aligned but think I could vary the pressure & trim to make them align, but not hands off. Playing with the spring tension could make it align, but I don't think it will be perfect for all CG's.
Just my guess sitting here bored. Haven't looked at different loads, so just a guess.

 

[cgoldy]

My mate in NZ says that the more aft your C of G is the faster you will fly. But he is from NZ remember!

(within limits of course)

 

[AkPA/18]

Hey Sharp

Unless the Jet's make a game of it---bored here also. I think I see. I guess I just got hung up on the stick changing positions etc...without the stab and elevators changing also when trimming. Obviously I took your post wrong

Thanks

Mark

 

[cgoldy]

Mr Sharp,

You might have misunderstood me a bit. When I said I thought it was the cable, I meant the cable in the bungee system. Not the elevator cable. My way of thinking is that if the cable is shorter, the action of the spring tension on the elevator will continue for longer when trimming nose down. The spring is plenty strong enough, but it has little or no effect when you start to trim nose down. I think it is probable a juggling act to get it all sorted an maybe different for every plane.

 

[AkPA/18]

At the risk of over analyzing things, why is my elevator counterbalance always 1 inch higher than my stabiliser (read elevator slightly down) no matter what the trim setting, speed, weight and balance or spring tension?

The picture of your airplane on the beach shows your elevator in the up position. Sorry I could not figure out how to post it here. The elevator was in the up position when you landed? Are you just talking about cruise settings when it is in the down position??

A couple of these threads are really confusing me lately Some of this stuff is really hard to describe or picture in a post.

Thanks

Mark

 

[cgoldy]

The picture of your airplane on the beach shows your elevator in the up position. Sorry I could not figure out how to post it here. The elevator was in the up position when you landed? Are you just talking about cruise settings when it is in the down position??

A couple of these threads are really confusing me lately Some of this stuff is really hard to describe or picture in a post.

Thanks

Mark

Yes - in cruise. When I trim nose down for cruise - 100 mph ( this is the only time I have ever turned my head to see what is following) , the elevator is slightly down. When on the ground and I trim nose down the spring tension is progressively lost because the bungee cable becomes lose as it is attached to the yoke. So my idea is to shorten the cable (not make the spring stronger) so as it still has some effect when trimmed nose down.

 

[AkPA/18]

Yes - in cruise. When I trim nose down for cruise - 100 mph ( this is the only time I have ever turned my head to see what is following) , the elevator is slightly down. When on the ground and I trim nose down the spring tension is progressively lost because the bungee cable becomes lose as it is attached to the yoke. So my idea is to shorten the cable (not make the spring stronger) so as it still has some effect when trimmed nose down.

A couple of questions----is the spring stretching on you at any time and are your tail feathers larger than stock. Have no answers---just curious.

Thanks

Mark

 

[skywagon8a]

Your mate in NZ is correct. It will be somewhat faster with the CG further aft.

The purpose of the bungee spring is for the elevator to assist the stabilizer when it is being trimmed to the full nose up position. Full nose up trim is only used at low air speeds. At low speeds the stabilizer does not have enough authority to provide the nose up force which is necessary. Thus the bungee cable & spring will pull the elevator up to provide the additional tail down force required. Remember that the bungee/cable is attached to the leading edge of the stabilizer (l.e. down = nose up trim) and to the up elevator horn. The spring is in the system to enable the pilot to still have full down elevator when needed. When in cruise there is little if any tension on the spring.

 

[skywagon8a]

Some further thought. I am assuming that when this condition was observed that there was no pilot input applied to the stick. The stabilizer was trimming the plane and the elevator would be just flying in trail providing no trimming force. This confirms your previous post of being nose heavy. I strongly suggest that you load the plane to Piper's aft c.g. limit of +20". This is a safe loading condition as it is the approved limit that Piper certified for the plane. You can do this by bolting a small amount of lead to the tail or by placing a large amount of baggage in the aft compartment. Either way, do the math and then make a test flight loaded at 20" aft of the datum. Then report back to us here on the position of the elevator in relation to the stabilizer.

 

[btracy]

A couple of questions----is the spring stretching on you at any time and are your tail feathers larger than stock. Have no answers---just curious.

Thanks

Mark

Mark, Mine has stock tail feathers. Please explain what you mean with the stretching of the spring. If you are sitting at the end of the runway and push the stick forward, there is pressure on the stick. this has to come from the stretching of the spring.
I keep thinking that if the stab is trimmed for level flight that the aerodynamic airload should hold the elevator faired with the stab. With the elevator not inline with the stab this would seem to cause unneeded drag.
I thought I has some photos inflight showing this but I wasn't able to come across any

 

[behindpropellers]

Is it both elevators or one?

At some point Univair's jig was incorrect in where the horn was welded on the elevator.

 

[AkPA/18]

Hi Bill

You are right on the stretching of the spring when you stretch it with stick input. The spring is in the system so you can overide the trim. Where I was going on the stretching of the spring-----if the spring is too weak and is stretching by itself with the weight of the elevator or the airload then it is not working as designed. I don't believe this should be an issue in cruise (Skywagon already posted on this). Below is cgoldy's first post on the tail issue.

With no Pasanger and no bagage my nose up trim only gives me 60 knots. (69 mph) I put about 1.2 pounds of lead on the tail wheel spring and now I can trim out to 50 knots (57 MPH) which I am happy with. I know this is all good when I have a load but if I am just dicking around by myself I want to trim for a slow landing. Does any one know if I can fit a trim tab or somthing I can flick down when alone and light so I can throw away the lead??

I don't know why the elevators don't trail or why he doesn't have enough trim. I just get curious on these little mysteries. It just doesn't seem to act like a cub in my mind. I have not done the W and B calcs. Skywagon you might be right and that was a good description of the trim system.

I am also curious about your AC when you say it is extremely nose heavy. Sorry Bill--no answers from me---probably just more questions at this point.

Thanks

Mark

 

[A BELLAND]

At one time there was some pics of the elevators by ?? (Kase??) showing position of elevators as nose up trim was cranked in while sitting on the ground.I seem to recall thinking it was a good baseline for judging the effect of the trim bungee.
I can not find the pics now though!!!
Aurele

 

[SteveE]

I am not really following you guys,, but could this position of the elevator in relation to the horizontal be what you are talking about.

This is the left side, kinda blurry, but even




This is the right side, level flight trimmed out with no stick pressure.. Pictures were taken minutes apart, but the plane flies great.

 

[behindpropellers]

Steve-

Exactly what I was trying to say.

I know one of mine was incorrectly welded.

Tim

 

[cgoldy]

I just spoke to the guru (Wayne Mackey) and he said in a nut shell

With a forward c of g your stabaliser will be trimmed to keep the nose up. Because of this,the airflow will be over the top of the stabalizer and not underneath the the stabaliser the airflow bends down when leaving the trailing edge of the HS. The elevator is following the airflow. The more aft the cg the more in line with the airflow the HS will be and therfore the more in line the elevator will be. It has nothing to do with the bungee.

I am happy with that