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

I've installed a new engine on my Midwest SuperHots, and SK 80 from
Kangke. This engine swings large props.
The SK 80 replaces a Tower 75--and engine intentionally designed to
have a torturous midrange.
In an effort to accommodate a 15x6 with at least 2" prop clearance,
I've fabricated and installed new mains, which are 6" tall by 13-1/2"
wide. The original mains were 3-5/8" tall by 11-3/4" Wide. I took a
guess at the width of the new mains, the height obviously being
dictated by my prop clearance requirement. I tried to duplicate the
slight toe-in and negative camber of the stock gear.
Dave Brown 3" foam wheels. Old and new gear made with 1/8" Aluminum,
very much conventional construction.
Prior to effecting this mod, I realized a host of ground handling
problems may be introduced, and that has indeed turned out to be the
case. Previously rather well behaved, the plane is now almost
impossible to handle on the ground. Slow taxi, take-off roll, you name
it...it has a mind of its own. Actually taking off was out the
question--absurd.
I'm an ok pilot, mostly experienced with tail draggers.
It was almost comical to observe the lack of response to rudder input.
Crosswinds gusting to 15+ knots helped enormously.
The question, of course, is what is the stinking problem? Well, such a
thing may be impossible to diagnose without first-hand examination and
observation of the hilarity. So instead of asking for specific
answers, let me simply make a inquiry into your general
understanding, philosophy, etc. of the nature of tail dragger gear
configurations, their inherent idiosyncrasies and problems, and your
general approach, angst, and successes.
Toe-in, camber, position of wheel relative to the plane's CG, P-factor
factors, rudder authority--anything and everything in any discipline
you choose: experience, physics, incantations, a 6th sense--anything.
I request rumination.
I have done multi-searches, the bleery-eyed type in the usual venues,
RCU, etc. I value this groups input. Thanks so much once again.
Regards,
Dave
AMA 395450
Reply to
DaveH
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I should note that I fly on grass. Dave
Reply to
DaveH
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
Reply to
Ray Haddad
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!
Reply to
Keith Schiffner
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
Reply to
DaveH
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.
Reply to
Robert Reynolds
Yes, and having too tall a gear is the same as too far forward a gear.
Reply to
Six_O'Clock_High
There's your problem! Lay off the drugs, and the plane will be much easier to control.
Reply to
Tim Wescott
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.
Reply to
Robert Roland
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
Reply to
Dan_Thomas_nospam
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
Reply to
DaveH
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
Reply to
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
Reply to
Dan_Thomas_nospam
Which is what most full-scale manufacturers specify.
Dan
Reply to
Dan_Thomas_nospam
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.
Reply to
Robert Reynolds
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
Reply to
Ed Forsythe
[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
Reply to
Ralph Jones
Sorry, but there's only *one* CG. That is if you're talking about the Center of gravity. :-0
Reply to
Ed Forsythe
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
Reply to
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
Reply to
Ralph Jones

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