CG / elevator throw / stall
 

I had my first flight with my new GP .20 J-3 Cub on Friday. I set up the high and low rates according to the instruction manual.

It is not possible to do a nice 3-point landing. Even with full up elevator on high rates it touches down on the main wheels (well above stall speed). The tail won't come down before the main wheels are on the ground. I have checked the CG, and it is correct.

I see two solutions to this.
1) More elevator throw
2) Move the CG aft

I had the same problem with my 4*40, and moving the CG aft solved it. The plane became "snappier" and it was possible to go all the way down to stall speed in level flight. I have to keep an eye on approach speed and not let it go too slow, but the plane is not difficult to fly with the CG a little behind the recommended location.

Another thing is that the 4*40 now needs less trim when going from low to high speed and vice versa. How come? I think it might be because, with the aft CG, the tail drops a little at low speed, increasing the wing's angle of attack and producing more lift => no up trim required. Is this the explanation?

I read that one way to see if the CG is right is to roll the plane 90 degrees and see if the nose or tail drops. I tried it, and the nose dropped a little. This would indicate that the CG is a bit too far forward...?

I know that the simple solution is to use more elevator throw, but it seems strange to me that the plane is impossible to stall from level flight. So I want to find out what the problem is and correct it. I don't think the plane flies the way it is supposed to.

Any comments/advice appreciated
Thanks!

-Tore

CG / elevator throw / stall
 

Tore,

I maybe be blasted by the experts, but I think you are landing too fast. Stall occurs at a particular angle of attack. If you are landing on the mains, you have not achieved the angle of attack for a 3-point. This is independent of elevator throw and CG, so long as the wing stalls before the h-stab (if the h-stab stalls first, you will likely get to rebuild the Cub). If you can't achieve the necessary angle of attack without stalling, your only solution is to reduce weight, or decrease the wing incidence. Both may be impractical with your Cub.

Not to worry. I can't 3-point my Cub, but that is because it is over weight. Still a great flier!

Good luck.

Bedford

CG / elevator throw / stall
 

You don't have to be stalled to do a 3 point landing. Your problem may be throttle management. If you get the plane to fly at the angle of attack where the axles of all your wheels are parallel to the ground, then adjusting the throttle will usually make the plane climb or settle.

CG / elevator throw / stall
 

This is how I land the Cub:
Downwind; reduce throttle/speed
Base; throttle to idle, let the plane start sinking
Long final; descend towards the runway. When the airplane is 1-2 feet off the runway, I feed in up elevator to keep it from touching down.

All the other taildraggers I've owned let the tail come down as speed bleeds off. You can stretch the landing until the plane touches down on all three or even the tailwheel first, or you can release some elevator pressure and let it touch down on the main wheels. Works every time.

The Cub never gets to the point where the landing can be stretched. I have full up elevator and the plane is sinking like a brick long before I reach "proper" landing speed.

The plane seems to be very nose heavy on landing, but the CG is set per instructions. That is my reason for asking. I'd like to move the CG aft and see if it helps, but only if it is safe.

-Tore

CG / elevator throw / stall
 

Bedford this is a no blasting zone!! The angle of attack at a given speed that a wing stalls at is independent of elevator and CG, true, but the ability of the airplane to produce that angle of attack is highly dependent on those things. Two different things going on.

To achieve a 3 point or tail down attitude (not necessarily a full stall but it does look nice when it is) I would increase the elevator throw. If you have access to a RealFlight Simulator you can try this out for yourself and you will find there are elevator settings along with CG locations that will prevent getting the full stall landing regardless of airspeed. Sometimes it becomes impossible to get to a min flying speed even with full up elevator.

Moving the CG aft makes the airplane less stable and so makes the elevator more effective. This makes the input from the trim control on the transmitter more effective in trimming the airplane and makes getting the higher angles of attack possible with the same original elevator control.

There are limits to moving the CG aft of course, again RealFlight is a great tool to investigate these types of things. It is the only place to look at aft CG investigations with out breaking an airplane.

Back to the elevator control. There are a series of power off speeds resulting from the various elevator positions. Without enough elevator throw the low speed may not be attainable.

Without enough throw the nose can't be lifted enough and the speed dropped enough to do the 3 point landing or a full stall. As you raise the nose of the airplane the increased angle of attack on the wing creates a down moment and the stabilizing effect of the stabilizer creates a more tail up force (nose down moment) and the elevator is pushing the tail down (actually the net result of the stab and elevator is what is happening since both are connected but for talking purposes its OK to look at the separately, maybe). The total airplane nose down moment is greater with increased angle of attack.

So if there is not enough elevator throw it can't get or maintain the nose up attitude and it can't get or maintain the slow speeds you desire.

CG / elevator throw / stall
 

Tore,

My response above is based on not being too far forward with the CG. If you are per plans, that is not likely, but you never know.

If you are way forward with the CG, the elevator is not able to counter the up-lift at the h-stab. I have had this experience too. A good test of CG position is a spin. If it is too far forward it really doesn't spin, but sprials and exits easily. If it is right on, it spins OK and exits OK. If it is TOO far back ... make sure to have a big plastic bag, because it is not coming out. Sort of like making sure she is not a witch. If she sinks she is not ... oh, what a relief! <G>

I have cured my similar problem with small adjustments and careful testing.

Good luck!

Bedford

CG / elevator throw / stall
 

Ben,

You got your response in before my last one above. On further consideration I agree.

Bedford

CG / elevator throw / stall
 

Bedford - You got it.

I wonder if anyone at the time of the witch thing actually thought through the implications of the float - sink thing and said, "but, but, but...."

The kit probably had a very conservative CG and elevator throw setup.

CG / elevator throw / stall
 

Bedford;
It spins easily and turns about half a turn after I release the sticks. Is that "good"?

Ben;
I'll increase elevator throw and see if that helps.


Thanks!

-Tore

CG / elevator throw / stall
 

How much throw do you have on your elevator now?

CG / elevator throw / stall
 

Geistware;
5/8" on high rates and 3/8" on low rates (according to manual). It doesn't look like much...

CG / elevator throw / stall
 

Is that 5/8 total or + and -? If it's total then I suspect it's not much. If it's +and - then that sounds like it's quite enough.

The spin response sounds good but to not be able to iduce a stall from a stable glide in a low throttle setting sounds a bit too far forward to me. But then I like my pitch control a trifle more sensitive.

One last thought that may be part of the problem.. I wonder if the wing is mushing near the stall but not fully into it and slowing the model overly much. If such was the case then the elevator control authourity would be compromised.

I suggest this as I had a glider that didn't like to stall very much. Instead it would become very mushy and settle at a high rate when I attempted any low speed "floaty" flight. The elevator was less effective in this mode until I'd lower the nose to pick up speed. I fixed it with an upper surface turbulator at about the 10% and a second at 25% or so chord marks. The lower speed range was much more usable and floaty after that and a definite stall was possible from low speed.

These were made from two layers of 1/8 inch wide auto trim striping tape. Or for an experiment try two narrow layers of electrical tape. You will need two layers about 1/8 wide to get enough thickness on most model sized wings.

CG / elevator throw / stall
 

Tore,

It sounds like you can move the CG back a little and I would go ahead and add a bit of elevator. If you have dual rates on your transmitter, you can set the low rate at the previous setting and the high rate at the new. That will give you the chance to test characteristics with altitude before committing near the ground.

... in Norway and you are flying? I am such a weather wimp, I can't stand it. It gets below 50 (10C) here, I stay home!

Bedford

CG / elevator throw / stall
 

BMatthews;
5/8" +/-. I'm not too good at inches, but I think the total is 1 1/4" on high rates and 3/4" on low rates.

What are turbulators?

Bedford;
I have doubled the elevator throw on high rates and programmed the previous high rates as low rates. If that doesn't help I'll add tail weight until the plane behaves the way I want it to...

The weather is not too bad, and the air is dead calm. Yesterday it was -15C (5F) at the field. The engine runs well, but I am having problems with keeping the monokote in one piece. I can put my finger right through the wing if I want to... Is there a type of covering that works better than monokote in cold weather? My limit is -20C (-4F). Any colder than that, and I'm not flying.

-Tore

CG / elevator throw / stall
 

Tore,

Here on the Gulf Coast of Texas we don't learn much about covering behavior in cold weather conditions so I can't be of any help. Ask me what Monokote does when it is 40C out and the sun is blazing ... with that I have some experience.

Be gentle when you first try your controls on high rates. Your elevator will be quite sensitive. Could be a lot of fun, too. Good luck.

Bedford

CG / elevator throw / stall
 

IT WORKS!!! :D

I doubled the elevator throw and added 1 oz (1/4 at a time) to the tail. Now I can make those smooth 3-point landings.

The plane stalls harder and quicker with the extra tail weight, but nothing to worry about. I have stalled the airplane a few times during turns, but I don't think this will be a problem when I get used to it. I just have to be more careful with the elevator at low speed.

(Today I went to the field early, and found out at 1:30pm that I hadn't eaten breakfast yet...)

-Tore.

CG / elevator throw / stall
 

Try any of the ***tex's (colortex, worldtex, solartex etc) for a good covering material. Easier to apply than the films and much stronger. Another choice, if you insist on film, is OZ or Rhinohide.

CG / elevator throw / stall
 

Tore,

Make those low speed turns with rudder to help avoid stalling.

Sounds like you got your problem solved.

Good flying!

Bedford