efish

My Feedback: (5)

Hi, I know many think landing gear is not necessary for 1/2A, but somehow, I just love greasing a nice landing with a rollout and taxi. It's also pretty fun to ROG.

I've been wondering about the various landing gear designs out there and what might be the pros and cons attached to each. Specifically, I'm thinking about tail draggers.

For example, the basic configuration just has the landing gear pointed down perpendicular to the fuselage/thrust line (most tail draggers e.g. GWS estarter). Then there is the sort with the wheels canted forward (e.g. GWS Corsair). There are also some I've noticed with the wheels angled behind the point of attachment.

I've been flying a GWS Corsair powered by a .074 norvel and the forward pointing landing gear does seem to work to cushion out the bumps on a hard landing. Typically, the wire gets slightly bent forward and I take it this is preferable to the shock being transferred directly to the fuselage/wing mounting points. I guess designs with the wheels canted forward also help keep some weight to the rear to help with a nose heavy design.

Does anyone have any views?

While on this subject, I've used piano wire and homemade aluminium landing gear the former is quick and simple, the latter looks nice. Just picked up a carbon fibre one meant for an eletric plane. Any thoughts on material choice?

Thanks!

ptulmer

My Feedback: (5)

efish, usually the landing gear will be canted forward (or backward) to mount the gear to a hard point and still get the wheels in the appropriate place. Too far forward or back and you'll have trouble going straight and ground looping. (nosing over, etc)

BMatthews

Exactly. Within reason it doesn't matter how the wire or struts get to the wheels. What IS important is where the wheel axles sit with relation to the CG. This can vary from only a few % of the wing chord forward of the CG to quite a bit depending on the height of the model, wheel size, type of runway, etc, etc.

As mentioned the strut attachment point in the fuselage is more about strength and support for the high stress point loads.

efish

My Feedback: (5)

Hi PT, Bruce. Thanks for that.

Since I started scratch building I find myself having lots of these ningling little questions and a seemingly meaningless need to question everything.

combatpigg

My Feedback: (3)

I asked this question to our local clubs' aircraft design engineer, and he had a formula to figure out how far forward of the CG the main gear should go. Let's just say that the gear need to be somewhat ahead of the CG, with the gear legs coming more or less straight down from the attachment point, but angled slightly forward like a WWII fighter. The tail wheel should have about 10% of the aircrafts' total weight on it, to be effective. The more distance between the main gear and tail wheel, the more stable the plane should be, if you have the wheels alligned. It is a good idea to shock mount the tail wheel linkage so it can't damage the servo. There are good examples on how to do this if you look through various model plane plans.

KidEpoxy

I agree with everything these guys said.

In addition to those points, I'll mention an aspect of my gear.
I'm running off grass, and it can get a little bumpy if you like long hispeed takeoffs & landing rollouts. To make my gear a little softer to give more float over the bumps, I have the legs trailing slightly rather than leading slightly. This 1" rearward change makes the model a little more prone to tipping on the nose while taxiing (elevator takes care of that), a little less prone to groundloop, but also makes the gear swing up sooner as it swings back- leading gear has no upward travel untill it goes back past vertical some, and by then the model is getting the upward shove from the grassbump: the model going fast gitters & shakes about like Keith Richards on 27 cups of coffe.

This of course is on short leg wire type of gear that will torque rather than flex the wire.
Just something I did to smooth out my takeoffs on bumpygrass

skaliwag

My Feedback: (1)

Now you can appreciate what Wright and Wright, Bleriot, DeMont, Farman, Fokker, Uncle Tom Cobly and all the other pioneers had to go to bed with... and.....

Bipe Flyer

You want the wheels as close to the CG as possible. Right on the CG will cause nosing over, but too far forward will cause ground looping. Grass requires the mains to be further forward than on pavement. One way to lessen ground looping is the set the plane's stance so that the tail doesn't sit too much lower than the front. This effectively puts less mass behind the main wheels while allowing you to have them far enough forward to prevent nosing over.

I've attached a couple of pictures to show what I mean. The Pup in the first pic was very prone to ground looping - notice how much lower the tail sits. The Cloud Buster in the second sits much more level and takes off straight as an arrow.

Raymond LeFlyr

My Feedback: (2)

Valliant allies,

I remember a formula from a magazine years ago that was for the main gear on a monoplane taildragger to be 15 - 17 degrees ahead of the TRUE cg - not the balance point. It was probably in Chuck Cunningham's RCM column. He always had something good to share.