Physics of Landing gear: An existential Inquiry into a modded Tail Dragger's petulance

I should note that I fly on grass. Dave

On Thu, 21 Jun 2007 21:10:58 -0400, I said, "Pick a card, any card" and DaveH instead replied:

Hey, I like the occasional beer but . . .

-- Ray

dang Ray! You made me hork Metoulius Damsel BLone Ale out my nose!!! This is a decent blonde ale and too good for sophomoric shenannigans as horking beer out of ones nose! I mean good CTHULLU!! It almost got the keyboard!

Yes, of course--I should have stipulated: I must be able to fly this plane while fully tanked on Mary-Jane, Whilst horking my ale of choice, which is the the much more complex Millwaukee's best Ice.

Error upon error. Can't imagine why!?

I am clarified and cleansed. Thank you gents. Regards Dave

Usually, when your model is squirrelly on takeoff it means you need a bit more toe-in than you have... but too much toe-in can cause the wheels to chatter against the ground as they roll.

Having the main gear too far forward also causes bad handling, but it is good for preventing nose-overs.

Flying from grass will generally make toe-in less critical.

Yes, and having too tall a gear is the same as too far forward a gear.

There's your problem! Lay off the drugs, and the plane will be much easier to control.

The taildragger setup is inherently unstable. The problem is that the wheels have to be in front of the CG. The further forward you place the wheels, the worse the instability gets.

In your case, making the main gear a lot taller will also move the wheels further forward when the plane is "sitting" with the tail down.

Solution 1: Lengthen the tail gear by the same amount.

Solution 2: Move the main gear further back.

As you can see, a taildragger setup will always be a matter of compromise.

You have four factors you cannot change: Gyroscopic force that make the nose swing when you raise the tail; P-factor, which is nothing more than a higher AOA on the descending blade when the tail is on the ground; torque reaction, which places more weight on the wheel on the side of the upgoing blade; and slipstream effect, which is the swirling prop blast the strikes one side of the fin. All four of these will drive the nose in the same direction. And you can't do any about them except anticipate them and use enough rudder to control the thing.

But you can change your mindset about toe-in. All full- scale certified mechanics and longtime taildragger pilots, like me, know that toe-in is a recipe for instability unless the manufacturer has his reasons for specifying it. I wish I knew where modelers got the idea that toe-in was the right thing. It sure makes life miserable (and can get really expensive) in the real airplane.

The axles should be 15=B0 ahead of the CG. You can easily find the logitudinal CG, but the vertical CG is a little harder to locate. You need the coincidental point so you can get you 15=B0 starting point.

Dan

Understood about toe-in Dan. I'll set it to zero. But I am confused about why you specify the position of the axles ahead of the CG (assume centroid, actually) in units of degrees? Degrees implies angular displacement. I must be missing something obvious. Apologies.

I did know that the more forward wheels may be a problem, in which case this new gear will have to be shimmed to put the wheels back where they were, or build another gear of different geometry. Dave

When I was converting a plane over to a tail dragger I found something that said to make sure the wheels were at the intersection of a line that runs 15 degrees from the vertical line through the CG and one that runs straight down from the leading edge of the wing. I've done this several times now and never had a problem with ground handling.

I also found several advocates of toe-out on the wheels. I don't know what the effect on ground handling was, they did it for landing. With toe-out if one wheel hits first when you land it will slightly pull the plane to that side and force the other wheel down. For example, right wheel touches down first and, with a bit of toe-out, moves slightly to the right pulling the left wheel down. With toe-in and the same landing, the plane is going to move to the left and make the problem worse by tending to raise the left wheel (and lower the right wingtip ever closer to the ground).

Since I am unable to see a few degrees of toe-in or -out, and I doubt if it's molded in the landing gear I use, I just go for mounting them what-looks-likes-straight.

Steve

The 15 degree thing moves the axles ahead as the gear gets longer. A taller gear provides more leverage and will nose over sooner if the brakes are applied or if too much friction is encountered (mud or tall grass or a hole) and the extra forward placement limits the noseover tendency. It also aggravates the groundlooping tendency, so everything becomes a tradeoff. You can build an easy-to-steer taildragger if the gear is farther back, but you'll spend a lot of time cleaning dirt off the prop and restarting it.

Dan

Which is what most full-scale manufacturers specify.

Dan

I'll tell you where I got the idea that toe-in is the right thing. In the right amount it helps to prevent ground loops, which has been verified by personal experience over the years. This topic has been discussed at great length in this forum from time to time in the past.

The basic concept is that when a ground loop starts, toe-in creates more drag on the wheel that gets ahead because the angle increases, while the wheel trailing behind straightens out which causes less drag, and the turning tendency is canceled.

I agree that a lot of planes have zero toe-in specified, but a little bit of toe-in really helps if your model is squirrelly. You don't want to have any more than a couple of degrees.

I've never checked a T-6 or P-51 for toe-in but I *do* know that every ground loop inclined RC taildragger I've had (many) was cured by adding about 3° of toe -in. Phil Kraft gave me the advice in the early 70's. It works, especially when taxiing on a hard surface.

You have four factors you cannot change: Gyroscopic force that make the nose swing when you raise the tail; P-factor, which is nothing more than a higher AOA on the descending blade when the tail is on the ground; torque reaction, which places more weight on the wheel on the side of the upgoing blade; and slipstream effect, which is the swirling prop blast the strikes one side of the fin. All four of these will drive the nose in the same direction. And you can't do any about them except anticipate them and use enough rudder to control the thing.

But you can change your mindset about toe-in. All full- scale certified mechanics and longtime taildragger pilots, like me, know that toe-in is a recipe for instability unless the manufacturer has his reasons for specifying it. I wish I knew where modelers got the idea that toe-in was the right thing. It sure makes life miserable (and can get really expensive) in the real airplane.

The axles should be 15° ahead of the CG. You can easily find the logitudinal CG, but the vertical CG is a little harder to locate. You need the coincidental point so you can get you 15° starting point.

Dan

On Fri, 22 Jun 2007 07:23:07 -0700, Dan_Thomas snipped-for-privacy@yahoo.com wrote: [snip]

Easier than you might think. Attach a string to any point on the model, dangle it from the ceiling, whip out the digital camera and photograph it from the side. Change the string attachment point, do it again, and superimpose the pictures. Draw lines extending the string down through the airplane, and they will intersect at the real CG.

rj

As was once explained to me by an engineer (mechanical), we modelers are guilty of using the wrong terminology when we say center of gravity. What we are really looking for is the balance point.

Ed Cregger

That's the one I'm talking about. Non-engineers, in general, tend to think of the CG only in terms of its position along the longitudinal axis. The string technique shows where it is in the vertical axis too.

For that matter, the CG is not necessarily centered on the lateral axis, especially given wood-and-glue construction techniques, but the deviations are rarely enough to worry about.

rj