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Why does my plane spin out when I try to accelerate?

#1
Hi,

I've just finished constructing my first plane. I went to test-fly it today, but unfortunately it can't seem to take off. Once it gets any amount of speed, it immediately cranks to the left, unable to actually get up to speed and begin lifting off. Here's a video (I have many more, but they're all the same. Plane gets a bit of distance, then cranks left immediately):

Is there any big mistake a newbie might be making that would totally kill the ability to fly?

Thanks!
 

SlingShot

Maneuvering With Purpose
#2
There are 2 things going on here: 1) motor torque. This is perfectly normal and the remedy is to hold some right rudder to compensate at the low speed. 2) very narrow wheelbase. That plane is not designed for good ground handling.

If it were my plane I would just hand launch it. Good luck.
 
#3
There are 2 things going on here: 1) motor torque. This is perfectly normal and the remedy is to hold some right rudder to compensate at the low speed. 2) very narrow wheelbase. That plane is not designed for good ground handling.

If it were my plane I would just hand launch it. Good luck.
Okay yeah, I thought the wheelbase was a little narrow.

Would extending the axle be sufficient to also make the motor torque a non-issue, then?
Also, what degree would you recommend that I extend the wheels by? Currently it's a 15cm axle. I can make it any different length I want (just cut a different piece off my stock-rod).
.
 

SlingShot

Maneuvering With Purpose
#4
Okay yeah, I thought the wheelbase was a little narrow.

Would extending the axle be sufficient to also make the motor torque a non-issue, then?
Also, what degree would you recommend that I extend the wheels by? Currently it's a 15cm axle. I can make it any different length I want (just cut a different piece off my stock-rod).
.
Widening the wheelbase will help with ground handling, but will not eliminate the motor torque. Next time, hold some right rudder and EASE in the throttle. Don't "floor it" right at the start. Ease in the throttle and see how she responds. Use the rudder to keep her straight.
 
#5
Widening the wheelbase will help with ground handling, but will not eliminate the motor torque. Next time, hold some right rudder and EASE in the throttle. Don't "floor it" right at the start. Ease in the throttle and see how she responds. Use the rudder to keep her straight.
Haha yeah, the first couple attempts I eased it in, then on this particular run I was thinking "Hmm, if I throttle up fast enough, I can get it moving enough to get airborne, before it gets a chance to spin out on the ground.

Didn't work...

Thanks though for the suggestion, I'll definitely put in some right-rudder. This is gonna be super awesome to get going with.
 

Hai-Lee

Old and Bold RC PILOT
#6
Motor torque is often confused with a phenomena called "P" factor. "P" factor is the effect upon the vertical fin, rudder and fuselage caused by the spiraling airflow from the propeller and its effect on a standard CCW propeller setup is to push the plane rear to the right, (when viewed from behind), and this causes the plane to want to turn sharply left as you try to take off.

The standard fix is to increase right thrust angle of the motor installation as well as a deliberate and gradual increase in throttle setting during the take off run whilst applying compensating right rudder to keep it straight. The actual lift off should be delayed until sufficient speed has been obtained so that the control surfaces have adequate effect or sufficient force to overcome any residual "P" factor effects.

If coarse throttle is used AND/OR the plane is lifted off of the ground before adequate speed has been obtained then the result is going to be a lift off followed by a rapid turn and roll to the left and a severe ground impact. This turn/roll to the left and crash will NOT be overcome by a wider undercarriage, though a wider undercarriage may allow the plane to lift off more easily and actually increase the risk such a sudden crash.

Just part of my fight testing program for new designs!

have fun!
 

d8veh

Well-known member
#7
Use low throttle at first, then, as the plane speeds up, the rudder becomes more effective to hold the plane straight, so you can go full throttle. Whatever you do, you always need to feed in a bit of right rudder to keep the plane straight. Tricycle undercarriage helps to keep a plane straight, but I think it isn't so easy to take off with it.
 

Hai-Lee

Old and Bold RC PILOT
#8
You can fit a tail wheel and hold it down with full up elevator to help hold it straight as you start the take off run but you MUST return the elevator to neutral and get the tail UP before even thinking of taking off.

@d8veh is quite correct with the trikes being difficult to control on take off but mostly when you attempt to take off too soon and before the control surfaces have sufficient effect to counter the "P" factor. Many Newbs roll and crash their new trikes on maiden because they do not allow for "p" factor effects.

A little further food for thought! When the motor is running the prop wash also flows over the wing, (normally the inboard portion only. This prop wash can and does cause the wing to provide extra lift during take off and so it is possible for the plane to take off with good airflow over the tail surfaces and the inner portion of the wing whereas the Ailerons are still way below the speed they need to operate properly, especially if the rates are turned down for a newb to learn on! Thus the plane, (both trike and taildragger), can lift off quite well but have almost ZERO roll control. When you add the residual turn and roll effects of the prop wash the result is logical. A rapid roll, dive and crash!

Just trying to explain a well know aerodynamic problem caused by propellers from well before the Wright Brothers first flew!

have fun!
 

Merv

Well-known member
#10
We used to call that a ground loop.

I would agree with everything that has been said. I’d like to add, look at the landing gear. Make sure both wheels spin freely and can’t catch on anything. That you don’t have one wheel out of line. In fact you’ll want some toe in, the wheels should point to the same imaginary point about 5-10 feet in front on the plane.

Adding a steerable tail wheel is by far your best option.
 
#12
We used to call that a ground loop.

I would agree with everything that has been said. I’d like to add, look at the landing gear. Make sure both wheels spin freely and can’t catch on anything. That you don’t have one wheel out of line. In fact you’ll want some toe in, the wheels should point to the same imaginary point about 5-10 feet in front on the plane.

Adding a steerable tail wheel is by far your best option.
Why exactly do I want toe-in? Why is that better than perfectly parallel?
 
#13
Forgetting all the aerodynamic analysis, tail draggers are by nature unstable on the ground. Lots of things mentioned above that will help but simply convert it to a trike will fix it completely.
 
#14
Forgetting all the aerodynamic analysis, tail draggers are by nature unstable on the ground. Lots of things mentioned above that will help but simply convert it to a trike will fix it completely.
Okay, guess I just need to track down a wheel then. Hopefully I can figure out something to use as one. Don't want to wait for a parts order to come in :)
 

Merv

Well-known member
#15
Why exactly do I want toe-in? Why is that better than perfectly parallel?
Toe in is self correcting. With to in, when your plane is going straight both wheels will have the same friction. When your plane starts to turn, say to the left. The left wheel will be going straight, less friction. The right wheel will be even more out of line, more friction. The reduced friction on the left and increased friction on the right will tend to pull your plane back to straight. Until the friction on both wheels equalizes.

If your plane has toe out, the opposite will happen. Instead of self correcting, the turning force will increase.
 
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#16
Okay, guess I just need to track down a wheel then. Hopefully I can figure out something to use as one. Don't want to wait for a parts order to come in :)
If you are going to kludge a front wheel, put the mains just a hair and a half behind the CG, that way you can pick up the nose right off on the take-off roll.