bob8619

So the more I read the more I see cubs have a tendency to behave in funny ways on the ground. Do you think having a bigger engine than called for would magnify these issues? I ask because I recently learned that my Cub has a bigger engine than its supposed to and I am wondering if that is why I have having such trouble with it. Every trip I end up doing damage to the landing gear because it starts turning (mostly left) and once it starts, there is no stopping it.

I fly on a 3 lane back road which has curbs and that is what does the damage.

jetmech05

rudder throw enough to stop the turning on the ground?

bob8619

Yeah plenty of throw, it just won't over power its desire to go left. I've given the landing gear a slight "toe-in" and same old song and dance.



This isnt my video but this is what it looks like when it happens. If I get to full throttle fast enough I am able to take off before hitting anything but I tried easing into the throttle today and as it started turning left I was unable to take off and it hit the curb. I've had it happen a lot on landings as well with no power applied so I dont know what is going on.
: http://www.youtube.com/watch?v=VJFflerWUVg

bob8619

I think maybe the take off issue is torque related or something and when it happens on landing its probably just the cheap landing gear out of alignment maybe.

jester_s1

If it's happening on landing then you have a crooked gear. I had one that did the Cub dance all the way across the field until I took the time to get it right. There are two parts to making the gear straight- you have to get the main gear wheels straight (I run mine with no toe in) so the plane will roll straight on just the mains. You can check that by lifting the tail wheel and pushing the tail straight at the spinner. Don't compensate to make it roll straight; just push the vertical stab straight at the spinner. Once you have that right, then set the tail wheel to make the plane roll straight, and lastly set the rudder to be straight so the plane flies straight. When your gear is set up right, you'll get the left yaw effect (caused by P-factor, not torque IMO) just as you advance the throttle that will go away when the tail wheel lifts. Then you'll get a touch of it again when you rotate for takeoff. Both are predictable and consistent, and can be easily handled with right rudder input. Holding up elevator just during that first second of acceleration will help plant the tailwheel during the worst of the yaw effect.

And yes, overpowering the plane will make more p-factor and more torque, adding to the problem.

Rodney

I have found that some toe in greatly aids in the ground handling, especially on take offs. Any toe out will really make takeoff's difficult.

JohnBuckner

My Feedback: (1)

Bob Your video is a classic takeoff stall and spin, I have seen this hundreds of times literally and the cause is very simple you stalled it off the ground pulled up to steeply and too soon without adequate airspeed. This of course was caused by your haste to get off the ground before you ran off the side.

I have spent an entire carreer if full scale flight training but the solution is so simple and there it being only necessary for the instructor to block the stick or column from being pulled back to soon or to far.

OK lets go back to basics and what makes the gnearic "Cub" a lousy ground handling airplane:

First and foremost is the taildragger of course makes the airplane want to swap ends anytime the the main gear is in ground contact. This being said this is more so during deceleration than acceleration, Hmm.

Deck angle is also important and this is the one that so often is ignored and that is simply the angle that the airplane is sitting on the ground at rest and how far it must rotate to the level attitude before takeoff speed is reached. What this means is simply the more the airplane must rotate the more will be the effect of something called gyroscopic precession on that rotating propeller and that effect is a reaction ninety degrees later in the plane of rotation. guess what that results in a swing to, yes you gussed it, the left.

Is that everything that causes this 'left swing' ? Oh heck no there are many more some minor and major. The next is landing gear geometry and you have already touched on that, of course this can work in both directions and a bit of positive castor as well as positive chamber are aid to controllability. One point on geometry I do want to mention though is the fore and aft relationship of the CG to the main gear's footprint. Yup the fellows will often try to move the CG aft when the airplane wants to nose over. Guess what that is a litteraly a killer move, boom. CG is always adjusted for flight and never for ground handling. Always better to learn to use the elevator at all times on the ground than to move the CG aft . Always better to move the gear itself if needed.

Another of the minor effects that causes the left swing Is the heavy footprint. Yup foot print, you see torque from the engine does not cause a "Yaw to the left" it cause a roll to the left big difference. This roll will cause more of the aircrafts weight on the left main gear than the right, guess what this results in? Yup a swing to the left.

Yet another of the left swing contributors is caused by the propellors slipstream, you see it does not flow straight back at low airspeeds instead it follows a geodetic pattern and it will tend to strike the vertical stab since it is only on top of the fuselage on the left side and of course pushing the tail to the right and yes once agine a left swing.

Now saving the best for the last (left swing contributors) and indeed it is what Jester has hit on, P factor. and as he said this is caused by the propeller producing more thrust on the right side of propeller that the left when it is at guess what? A high deck angle This happens at any time the airplane just sitting still or in a climb. P factor is a major force and is not torque.

Oh ya I forgot of course an overpowered airplane particualarly with a long course props that typically are used by four strokes do agrivate all the contriburors I outlined above. So what to do?


I am going to offer a simple solution for "that airplane and that runway" by the way your runway sucks sure makes learning how to handle a Cub on the ground tough.


This solution is called a Race Start and it is commonly practiced by most warbird pylon racers and yes all of their ground handling contributors suck too with typically two to three times the displacement that would be used by most and this procedure will work every time but it requires help (a launcher) . The procedure is for you to position yourself with both hands on the Tx and a helper will position the airplane in the takeoff position and hold it with right rudder already applied and perhaps some up. You will advance the throttle to something over half throttle or even full throttle and when the engine revs are up and stable you will nod your head for the helper to release and do not rush leaving the ground the steering will be responding well with no uncontrollable swing.

John

bob8619

John that isnt my video, I found the video on youtube and used it as an example of how the plane acts on takeoff as far as the left turn it does when applying power. Nothing about the crash at the end. My runway is worse than this as I said it has curbs.
Yesterday as it started going left I cut power to try to regain control of it but it wouldnt stop turning left, so it plowed into the curb. Looking back at some of my own videos there have been times where it flies straight down the runway on takeoff. I'm done with the plane, just curious as to what went wrong all these times.

JohnBuckner

My Feedback: (1)

Sorry I took that to be your vid but it still illustrates many of the problems perfectly. All that I discussed stll applies and the simple races start as I discussed it will work. If anything it will prove to you that you can do it and successful controllability can be achieved as well as the need once the takeoff is started reducing power or indecision for an abort is usually fatal.

On a relook at that video I believe he did momentarily reduce power and that is almost always a mistake.

In regards your runway if you have curbs then that is an inappropriate runway even if one can get past that the fact that using a public road is wrong on many levels.

John

bob8619

Its in the middle of nowhere there is no harm in using this road except for the curbs being a problem for this plane, any other plane I have flown can be kept straight on the ground without a problem therefore the curbs arent a problem anymore. I'll be damned if I pay the $250 a year to fly at the nearest club

JohnBuckner

My Feedback: (1)

I know nothing of your unwillingness to fly at an organized flying site and could care less but I do know curbs are hard on landing gear.

I also know I have taken care in a carefull explination of many of the factors that effect the problem you are having with that airplane and if you try the type of takeoff start that I proposed it will lead to success and understanding as well.

John

flycatch

My Feedback: (26)

This is normal and only you the pilot in command can correct this problem. This is the reason why full scale aircraft for civilian use are trike gear.

bob8619

Its only happened with the cub for me, no other tail dragger has given me this problem.

flycatch

My Feedback: (26)

Tail moment is the key. The longer the moment only complicates the issue. I flew a H9 1/4 scale cub and had the same problem until I mastered the three key ingredients of; rudder, elevator and throttle. It is all a balancing act and like I said the pilot is in command.

bob8619

Well my original question was will a bigger engine magnify the issue, not who's fault it is.

flycatch

My Feedback: (26)

Yes it will however it is still up to the pilot to control the torque effect.

CafeenMan

How big is your Cub? If it's .60 or smaller you can always hand launch unless it's way over overweight. I've hand launched a lot of planes by myself. The only time I wouldn't do my own hand-launch is if there was someone else there to do it for me or if the plane had a low wing.

If you decide to try it and you have to do it yourself it might be best to go to a club someplace and have someone else launch it for you just to give you confidence it isn't a big deal and to show you what to expect immediately upon release.

bob8619

Its a 1/4 scale cub. Down the road I'll buy some better landing gear and give it another shot.

j.duncker

My Feedback: (2)

What about putting a gyro on the rudder? It would need to be switchable of course. On for takes off then off in the air.

RBACONS

My Feedback: (3)

Even with an over-sized engine and or prop, you should be able to get a straight take off roll. Jester_S1 has it right. The first thing you need to do is ensure that the plane will roll straight. A garage floor is a good flat and level place to do this. With the radio on, and the rudder centered, give the plane a gently push. If it won't roll straight for 10 or 20 feet, you need to correct your mains and/or your tail wheel. Second, ensure you have a good +/- 45 deg or so of rudder travel. Then I would try to start the take off roll very slowly and gradually add power only if/when the plane is rolling straight. Yes, the "floor it" method works but only if your left thumb is reasonably skilled, your mains are properly aligned, and you have enough rudder throw to counteract the engine torque. The slow roll is easier to control and to prevent the plane from getting seriously off line. The final piece is fine-tuning the amount of tail wheel authority with the rudder authority. That is, when you get up to speed and the tail wheel lifts, if the rudder and tail wheel are balanced, the plane won't have a tendency to suddenly yaw left or right. Unfortunately, most ARFs have the tail wheel hard-wired to the rudder so its throw is not independently adjustable. If thats the case, your stuck with what you've got unless you're willing to re-engineer the tail wheel set up.

jmpups

My Feedback: (1)

WOW, you will continue to try to fly and end up damaging and repairing planes, but you won't spend $250.00 a year to join a club with an fellow flyers that just might be able to help you out, and maybe you just might learn more about this hobby and how to fly???

on_your_six

My Feedback: (11)

Welcome to the world of flying cubs... you have to learn to fly the tail... you cannot take off from a 3-point stance. The tail has to come up... rotation speed achieved and then lift off. It took me forever to learn it, but it is a lesson you don't forget. I love flying cubs.

Guys have good luck with gyros and you don't need to turn them off in the air.... just fly the rudder like it was meant to be. They did not put it back there for an ornament.

eddieC

My Feedback: (2)

The Cub isn't a horrible plane with ground handling, it just needs attention. In fact, it's one of the easier taildraggers out there. Take notes on what's been said, especially by John Buckner and Rbacons. It's really fun once you get the hang of it. The Cub's 'evil habits' are a myth!
The biggest thing is to have the gear set right (wheels vertical, and slight toe-in) and to add power s-l-o-w-l-y. Start out with about 50% up elevator to plant the tailwheel. As the plane accelerates, reduce the elevator. The tail may rise by itself, which is great. You'll find you will have plenty of rudder authority when this happens.
The worst thing is to rush the takeoff. Too much power too early means you have 100% torque & P-factor, and almost 0% rudder to counteract it. You have to get about a 50% balance of each.
I have taildraggers that have swiveling tailwheels, so they give no directional input. The rudder does that!

Quikturn

My Feedback: (12)

Hi Bob,

To answer your question yes, a bigger engine will make your airplane want to turn left more. Since a bigger engine has more power it needs a bigger prop diameter and/or pitch. Both of which will increase left turning tendencies. A smaller engine or a 2 stroke that uses a smaller pitch prop would help. If you prefer to keep the larger engine, adding more right thrust angle to the engine set up would also help.

I have a Kyosho P-40 that calls for a 40 2-stroke or 52 4-stroke but of course I had to put in a Saito 65. It's a challenge to take off for the same reasons you state. What I do is ease into the power slowly adding more right rudder with more power but releasing right rudder as speed increases. I lift off at about 50-75% power.

Hope this helps.

on_your_six

My Feedback: (11)

The size of the motor maters but not as much as correct flying technique. The cub has a lot of tail surface area and is blown around by any cross wind. You have to hold the tail on the ground, build up speed, release the stick and allow the tail to come up to a flying stance on the two mains. Build up more speed, and pull her off. I have flown many tail draggers, the Cub is a great learning experience...same as the GA version.
Flying off roads is not desirable what so ever.