wsmalley

My Feedback: (6)

Took my Goldberg Cub out to fly, and, being my first takeoff with a taildragger, I was a bit apprehensive. I made several t/o rolls and aborted because it seemed like it was all over the field. Finally I got a decent run and lifted off. Barely 4' off the ground, a gust of wind hit me straight in and I guess I just ran out of airspeed, going in and knocking the LG loose and breaking a prop. It had been a long build, sandwiched in between other planes, and I don't remember bending any toe-in into the gear-it looked straight off the plane. I bent about, I'd guess, a 1/16th into each side and ran it around the yard without the wing and it seemed to track dead on! Are taildraggers more susceptible to toein problems than trikes?

aerowoof

a little toe in will help alot on a tail dragger,toe out is a real problem.make sure the axles of the gear are under the leading edge of the wing,that should help with the handling,also remeber that while the tail wheel is on the ground it is steering the plane and can be sensitive,once the tail lifts,the rudder is steering and can be ineffectice till airspeed is sufficient.

mirwin

My Feedback: (1)

You definitely need a little toe-in on either tricycle gear or taildraggers. The worst one I ever had was a trainer with tricycle gear. Only 1 - 2° is all you need.


Mike

wsmalley

My Feedback: (6)

Thanks, normally I make sure I have some toe-in cranked into all my planes. I guess I was adding some stuff to make the gear more scale like and forgot to bend the axles. Sure seemed to track a lot better running around the yard 'wingless'.

scratchonly

Good advice from all. Hold the tail down (up elevator) longer to get some speed up and aid steering. A headwind does not affect airspeed; only ground speed.

Good luck next try.

da Rock

A buddy just finished a Cub and fought the sucker for about a week. It wouldn't move 10 feet without groundlooping. He couldn't believe that something as simple and easy as toe-in could fix it.

He finally tried it and it did.

lrtx1

I have a hanger 9 p51 pts that is a bear to handle on the ground. How much toe-in should be on a plane like that? I measured between the front of the tires, and then between the back of the tires. Toed-in by 1/4". Does this amount of toe-in seem adequate? Just curious what you guys thought.


Layne

muzzlemaggot

My Feedback: (7)

On the PTS, see what it does with what you have. I've trained a few folks with the PTS, and noticed that several different models of the same type did need quite a bit of right rudder on take-off, but nothing that would indicate a severe problem. However, I didn't particularly notice any nasty ground-loop problem with the gear, assembled out of the box. Not to question anyone's intelligence, but are you sure you're having a ground loop issue, or simply a lack of inducing right rudder on take-off roll?

MinnFlyer

My Feedback: (4)

Actually, 1/4" seems a bit much. Try 1/8"

lrtx1

I probably should have said a little more. On take off the plane has the normal tendency to pull left. Apply the right rudder correction and I end up chasing the plane until it takes a 90 degree turn either left or right. The few times I managed to get her up in the air and needed to land, it was also squirrelly as soon as it touched the ground. I would say no more than 5 to 10 feet of straight and then wild turns left or right. I am relatively new at flying, I usually fly a tower trainer and have no major problems. So, I don't think I am butchering the take offs and landings as bad as it appears. I have also adjusted the travel throws of the rudder more and less, does not seem to make a difference in the ground handling. I don't know if this makes any sense or changes any opinion. I appreciate the help.


Layne




Layne

jaka

Hi!
What you describes isn't the way a good trimmed tail dragger should behave and certainly not a CUB.
What's makes a tail dragger squirrelly at take off is not lack of toe in (which is good to have though) but it is more likely a too far forward landing gear.
It's very vital that the wheels are not too far forward! Check that wheel center is in line with leading edge of wing when viewed from above. And of course ...that the wheels turn freely...and that that they are cambered in...Not out...very important!!

mirwin

My Feedback: (1)

I would say that 1/4" of toe-in is far far too much. 1° - 2° (barely perceptable) is all you should use. Too much toe-in will actually cause a drag on the aircraft. If one wheel drags more than the other, then your plane could wander or even groundloop. Usually, groundloops and wandering suggest toe-out but you say that isn't the case.

I haven't seen one up close, but I imagine the PTS has wire landing gear. It is possible that the wire is a bit too flimsy, and allows toe-in / toe-out to vary based on the rough runway as the plane . I had a Tower Hobbies trainer that did that and it drove me nuts.

Let us know how you fix it. I'm curious.


Mike

lrtx1

I will try to adjust toe by tommorow, and take it to the field. If the results are better I will be sure to post. Thanks for the help and different perspectives.

Layne

aerowoof

have you checked for proper alignment of all three whells with out the engine running?find a flat smooth surface a concrete garage or floor is ideal.just give it a good smooth push with everthing centered.if it does not roll straight till it stops there is a problem somewhere that needs to be fixed.with a running engine the problem will just become aggravated

TestPilot6

Jan,
Can you explain why the too far forward landing gear cause the problem?

Tom Nied

Since I've been trying to research this problem that exists in my tail dragger Avistar, I found this at DJ Aerotech. http://www.djaerotech.com/dj_askjd/d...taggerbee.html

I'm trying to sort my groundlooping problems out. Toe in, thrust, small tail wheel, maybe main gear to far forward (I have the axles right at the leading edge).

This is only my second RC tail dragger, and it's a bear keeping it straight down the runway on takeoff. My Spad Debonair that I converted to a tail dragger, takes off and lands just fine.

HighPlains

My Feedback: (1)

Ground loops happen when the CG gets outside the wheelbase. The further forward the gear is the more likely this is to happen. Also more likely to happen on hard surfaces, as grass allows some sideways skid (lots of fun in full size airplanes to experience this effect)

You want the airplane to be as light as possible on the tail wheel at rest. Which means the main gear is close to the CG point, usually about 10 or 15 degrees from the actual center of mass of the airplane (depending on whether you are operating from a hard surface or grass).
Note, center of mass is not the same thing as the CG shown on the plans, though both have the same horizontal location, not the same vertical location. The CG shown on plans is usually placed for drawing clarity.

Proper use of the elevator to hold the tail down during the initial roll out, is required, but you want to get the tail off the deck and flying as soon as possible. This means releasing the up elevator as soon as possible or even holding a slight amount of down elevator. Once the tail is up, you can accelerate to flying speed and control direction easily with rudder. Any crosswind element should be compensated for by use of ailerons.

bogbeagle

The engineers will tell you that toe-in is de-stabilising and that you need toe-out to tame your model. The article (below) is written by an engineer; I lifted it for your delectation.






Which way should your wheels point? Ever wondered?

We’ve all heard this one from time to time – should my model have toe-in, toe-out or neither in its wheels. Lots of people have said lots of things on probably every model aircraft forum created since Pontius was a pilot. There has even been an oft-quoted chapter of a book by a “retired Boeing engineer� which was so at variance with the basic laws of physics that I have to assume the author was having a bad hair day. I’m rather hoping that the following explanation will put this one to bed by explaining WHY toeing the wheels does things. But I’m not going to hold my breath...

Firstly I should come clean about the title. I put the word “debate� in there to be kind. There is no “great debate� – it’s a matter of basic, fundamental physics and no debate is needed.

Before we start we need to understand is how wheels work. You might think this is obvious – the wheel (of whatever colour) is the single simplest machine man has ever invented, I hear you say (wrongly). But it’s surprising how many people don’t really understand them. When a wheel is rolling along the ground it has a low rolling resistance and very few other forces acting on it. But this is only true PROVIDED the axis of the wheel is at right angles to the direction it’s rolling in. If you point the wheel in a different direction its rolling resistance increases slightly, but it also develops a “side� or “turning� force. This is what actually makes your car start turning when you steer, which is just as well or our roundabouts would be clogged cars that failed to make the corner. This applies to all wheels; aeroplane wheels are no different to those of cars, buses or bicycles in this respect. Actually it doesn’t apply to roulette wheels or the wheel of fortune, so I’d better change that to “it applies to most wheels�. But I digress.

So how much force do the wheels develop? Well this is quite a complex question, but for our purposes we can simplify it by saying that the grip of any two wheels will be identical provided they are carrying the same weight. If they are carrying different weights then the forces they develop will differ in proportion to the difference in the weights they carry. I’m sure that if I really tried I could think up a more long-winded way of saying that, but in the mean time that will have to do.

So what happens when an aircraft starts rolling along, and why do some track dead straight whilst others behave like they have an attack of St Vitus Dance? As a general rule the wheels on an aeroplane are mounted at ground level because experience has shown this can dramatically improve the ground handling. But most of the REST of the aircraft’s mass is mounted at the top of the undercarriage to get better propeller ground clearance and improve the airborne handling. So when the aircraft turns on the ground the tyre grip acts at one end of a long lever (the undercarriage leg) against the aircraft’s inertia at the other end, and this is the nub of the problem.

Let’s start by considering a tail-dragger. It starts of down the runway straight and true, but then a bump, gust or rudder input pushes it one way and it starts turning. At this point the wheels will be pushing it one way, reacting against the inertia of the aircraft which wants to push it another. As we said earlier, the two forces act at opposite ends of a lever, so they generate what we generally call a “torque couple� because we like to have complicated terms to stop people realising how easy engineering is and nicking our jobs. This force couple makes the aeroplane lean harder on the wheel that’s on the outside of the turn, and takes weight off the wheel that’s on the inside.



Now all of this is pretty academic, because the two wheels are pointing in the same direction so it really doesn’t matter HOW they share the workload. But what if they AREN’T pointing in the same direction? What if they are toed-inwards? What do we MEAN by toe-in? The diagram should make it clear so that we don’t talk cross-purposes. If the wheels are toed, then they must each be generating a sideways turning force but in opposite directions, and as the wheels are normally carrying the same weight these forces cancel out.

But with our turning tail-dragger, (with the wheels toed-in) what happens? Well we said earlier that the turning or side force is proportional to the weight on the wheel, and that when turning the outboard wheel has more weight on it. So it should be obvious that the outboard wheel now develops a side-force that is bigger than the one developed by the inboard wheel, so it wins. And as the wheels are toed inwards it can be seen that this force will tend to make the turn tighter, which will increase the weight on the wheel, which will make it tighter still and so on until we have the classical ground-loop or just a model that simply won’t hold a straight line on the ground – it is an unstable situation.

On the other hand, if the model has toe-OUT then the increased turning force tends to REDUCE the turn, which reduces the weight shift and all is normal again – a STABLE situation.

So the next time someone tells you that a tail dragger needs toe-in either ignore him or put salt in his coffee, because he’s not helping you! Now there is a long-standing myth, perpetuated in the American book I mentioned earlier, that toe-in increases stability because the outboard wheel (being presented at an angle to the direction of travel) somehow develops “road drag�, pulling the model straight again. Well as we can see from the above, firstly this isn’t true, and secondly even if it was it would be completely obliterated by the side-force generated by the tyres, and thirdly it isn’t true. So it’s what we engineers technically refer to as “utter bunkum�.

The one remaining aspect to cover is the trike undercarriage layout. Now this behaves in an almost identical way, but now the wheels (being at the back rather than the front) push in the opposite direction. So with a little thought it should be obvious that if the wheels are at the back you need toe-in for stability, whereas if they are at the front you need toe-out for stability.

QED

Tom Nied

So I need Toe out? I tried shimming the axle bolts on my landing gear with .008 aluminum to create a small amount of toe in. But according to that article that only destabilizes tracking. That trim adjustment didn't seem to improve or worsen the groundlooping condition. So I'm inclined to check other possible factors, like thrust, and maybe a larger tail wheel. I'd hate to have to re-locate the landing gear, but will if that's the answer. All I know is that taking off is like having a squirrel directing my plane, to the point of seeming dangerous.

It's an Avistar with a Dubro landing gear placed directly below the leading edge with a Sullivan tail wheel assembly 1" wheel. CG is placed right where the manufacture Hobbico states it should be. In the air, when I do get it up, it seems fine and quite "agile". I feel I can't work on expanding my flying skills until I come up with a resolution for this problem.

longdan

This thread has opened my eyes. I fly taildraggers and I've never even given any thought to toe in or toe out. I just make sure they are parallel and go fly. Although, after reading all the posts, I will probably keep setting my wheels parallel. Just because its easier.

bogbeagle

My personal opinion is that, at model scales, a bit of toe-in or toe-out will make no discernible difference to your model's handling.

An undercarriage which is raked too far forward can most definitely give rise to severe problems with ground handling ... and can make smooth landings very difficult too. Watch the cavorting that is generally associated with vintage models of this design.

It's impossible to "design out" all the idiosyncracies which are inherent in the "conventional undercarriage"; maybe that's why we like 'em. But, you can overcome those problems by use of your piloting skills, which you are in the early stages of developing.

A pound-to-a-penny, you have way too much movement on your Cub's rudder and tailwheel. A tailwheel sweep of 5-10 degrees, either side of neutral, should provide adequate steering. So, too, with rudder movement.

The rest is down to practise and learning the proper techniques. Good luck with it.

Luchnia

My Feedback: (21)

I also fly mostly taildraggers and from my experience the landing gear does make a difference and you have to also combine rudder/tailwheel movement. I changed out landing gear on two planes from the factory setup and I was amazed at just how much difference in tracking and takeoff.

On one plane I mounted narrower gear and further forward mount due to the plane consistantly trying to tip over on the runway. It definately handles much more erratic than the original gear but nothing that cannot be handled.

On another plane, I mounted a wider gear that is forward mounted about an inch or so and it changed the behaviour as well. It is much harder to control on takeoff and has required quite a bit of re-training on my part. The problem was the original gear was just way too weak. I am hoping to make some swept back gear to duplicate the originals just with the strength they need to support.

I fly off grass landing strips. I am not sure how it would do on hard surfaces at all.

zac137

I have flown the P-51 PTS a lot with students and one thing that I have found is if you are using the three blade prop it really makes it harder to handle on the ground try switching to the two blade prop.

Zach

Rodney

Definitely toe in helps greatly, especially on a tail dragger. Also, a to far forward gear will make both take offs and landing more difficult, erratic steering on take off and very bouncy landings with that gear to far forward. One of the comments above state that CG and center of mass is different. Definitely another gross error, they are by definition, identical. I think what the author really meant was center of lateral area, not center of mass.

Charlie P.

Not all tail draggers need it. Many will benefit from a bit. Too much and if you land hard on one wheel it can make matters worse (the wheel digs in instead of free rolling and you flip).

It's easy to visualize the process if you have toe-in and the model twists to the right, the right wheel is going straighter along it's axis than the left wheel (which is turned in instead of rolling along its aligned path) and that left wheel drags more. That drag brings the models nose to the left - presto!, you're straight . . . briefly. You correct because you noticed it and now you've over-corrected and the model is confused and you're confused and "flop". Also, what works on pavement may not work at all on dirt and rough grass.

And when landing it can be even worse with bouncing alternate wheels factored in when you are over-correcting for excessive toe-in.

I only add toe in if it seems to be needed. Then only a degree or two. Some models taxi best when the tail is "flown" with bursts to get rudder authority and proper elevator management to keep the tail behind the model. My .46 size Ultimate will flip if I release the elevator when the wheels are rolling until that elevator is at flying speed. It's a tricky transition to keep it from ballooning on take-off.

NOTE: That's why the 3-D crowd love their models. An aircraft that can accelerate straight up from a hover allows you to take-off and land with no technique required. ;-) (Mostly tongue-in-cheek, but I see guys relying on Hp to save their bacon frequently). Balliatic take-offs and hover landings. Put them behind an underpowered WWI model that needs 300 feet of roll-out to get to speed or drops like a rock at half throttle and see who's hot.