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what are the rules of landing gear placement?

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Old 12-17-2013, 04:06 AM
  #1
Kentli22
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Default what are the rules of landing gear placement?

regarding how to place the landing gears, what are the basic rules?

th biggest challenge to me in scratch building is how to position the wheels so that the plane will go in a straight line in order to successfully taking off. Any advice are appreciated.
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Old 12-17-2013, 04:15 AM
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If a trik, the main gear must be aft of the CG, if a tail dragger they must be forward of the CG. The closer to the CG the main gear is the easier it is to rotate the plane with elevator control during takeoff, to close and it becomes overly sensitive to elevator movement. On tail draggers, you will usually benefit if you build in a bit of toe-in. If to far forward on a tail dragger, takeoffs will be difficult and landings will be very bouncy if not flared perfectly.
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Old 12-17-2013, 04:22 AM
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In a trik, do the wheels needed to be toe in or toe out?

In a tail dragger, how to dicide what degrees of toe in is needed?

In both trik and tail dragger, seeing from the front, does verticle axis of the wheel needed to lean inward or outward?
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Old 12-17-2013, 08:13 AM
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Quote:
Originally Posted by Kentli22 View Post
In a trik, do the wheels needed to be toe in or toe out?

In a tail dragger, how to dicide what degrees of toe in is needed?
Varies, usually about 2 to 5 degrees.

In both trik and tail dragger, seeing from the front, does verticle axis of the wheel needed to lean inward or outward?
Usually you do not want any camber but if any, I'd want the wheel to lean outward. I have not really any proven advice on that.

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Old 12-17-2013, 08:20 AM
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thanks for the input. i had many take off failure just because the wheels would suddenly change direction once after picking up speed.
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Old 12-17-2013, 08:25 AM
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lean outward is positive camber?
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Old 12-17-2013, 08:35 AM
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Try to have the axel of the wheels in a tail dragger even with the leading edge of the wing with the tail on the ground. Place the axle of a tricycle system two inches behind the CG for a 50cc model and build in about two degrees positive incidence to a tricycle system by adjusting the gear lengths. For both trike and conventional, I have found the best setup is to have the gear struts perpendicular to the ground and all the wheels parallel to each other. Use the rudder for take off as required. Try programming in 20 to 30 percent of negative exponential to help. Dan.
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Old 12-17-2013, 10:18 AM
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thanks Dan
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Old 12-17-2013, 10:25 AM
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Quote:
Originally Posted by All Day Dan View Post
Try programming in 20 to 30 percent of negative exponential to help. Dan.
That is of course dependent on the radio brand. Some redios benefit from negative exponential, while other radios benefit from POSITIVE exponential.

Rafael
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Old 12-17-2013, 10:43 AM
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Quote:
Originally Posted by Kentli22 View Post
thanks for the input. i had many take off failure just because the wheels would suddenly change direction once after picking up speed.

its more likely that your plane is turning due to engine torque, unless you have very thin landing gears. Is it possible to flex the wheels significantly with your hand?

Quite honestly for all of the planes I've built I've gone with "straight" in terms of camber or toe in/toe out. You don't spend a lot of time on the ground, and taxiing is at such a slow speed that it shouldn't be an issue. For take-offs and landing, the engine torque is going to dominate anything you do wheel wise…. so go for a stable solid platform.
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Old 12-17-2013, 11:00 AM
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Mattnew, how does the torque affect or dominate the taxi progress that makes the plane to suddenly shifts from a straight line?

the width between the wheels and my thumb control are not an issue.
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Old 12-17-2013, 11:24 AM
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I've found that on taildragger landing gears that anywhere from straight to 2 to 3 degrees of toe in helps. The big thing is to avoid any toe out. Also ensure that the legs are stiff enough to avoid flexing in ways that the angle changes. Flexing up and down is fine. You just don't want the gear to flex fore and aft to any great degree. So if you're using a single piece of music wire then it has to be quite large to be stiff enough. A double leg that forms a "V" shape from the fuselage to the wheel axle does a lot for avoiding any fore and aft flexing which alters the angle. Or using aluminium landing gear legs.

In the end though nothing you can do to the landing gear will totally avoid the model going sideways. We simply have to learn to spot when it's starting and be ready with the rudder to catch it before it gets too far. This is especially true on grass strips where clumps of grass can catch at one side or the other.

The other factor is to always take off and land directly into the wind wherever possible. With tail draggers this is especially important. If your flying field does not allow for this sort of 360 degree freedom then you need to sharpen up those rudder skills to allow you to take off and land with cross winds. Which is something you just have to do anyway since, as I said before, it is just simply not possible to just put the power to the model and get a straight takeoff with any model. They will all respond to torque, runway roughness or slipperiness and cross winds.

The only way to provide a rudder free takeoff would be to set up a rail that the model rides along during takeoffs.
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Old 12-17-2013, 11:46 AM
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Quote:
Originally Posted by Kentli22 View Post
Mattnew, how does the torque affect or dominate the taxi progress that makes the plane to suddenly shifts from a straight line?

the width between the wheels and my thumb control are not an issue.
I respectfully disagree, mainly because of your question.

try this
1. Go out and taxi by quickly moving the throttle to full and back to idle again several times in a row
2. go out and taxi by smoothly moving the throttle stick forward until the plane is moving, then cutting back little on the throttle so the plane coasts...

Which plane went straighter?

The torque from a normally rotating engine ( counter clockwise when looked at from the front ) pulls planes to the left due to the rotation. The quicker the throttle is advanced the more extreme the turn due to the engine torque. Typically engines are installed with a few degrees of right thrust to help counteract this rotation. But, by and large it is this pulling to the left that is what initially causes people's taxiing and takeoff issues. Rudders can be sensitive and it requires a smooth hand to feed in the rudder in order to counteract this effect and then to again feed it out smoothly in order to not overcorrect before the plane becomes airborne. Once overcorrecting happens its difficult to get things back on track. Different airplanes with different setups exhibit this behavior, but they pretty much all exhibit this behavior… especially tail draggers.

Thats one of the reasons people say to learn on a trike… b/c trikes help to minimize this… Although my personal opinion is people should learn to deal with this sooner than later, as almost every plane out there is a tail dragger save a few trainers.


and… +1 what BMatthews stated, all good advice.
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Old 12-17-2013, 12:10 PM
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Bmathews and mattnew, appreciate your input.
I thought i am the only one who has trouble getting a scratch build to go straight. sometimes when a scratch build has a tendency to drift from a straight line, i couldnt figure out why because i thought i had the gears built correctly.

Is a trike easier to go straight than a tail dragger?
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Old 12-17-2013, 12:24 PM
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Yes. Dan.
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Old 12-17-2013, 01:26 PM
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Probably the biggest mistake a beginner makes is to have to much throw on a trike nose gear, you need very little. Often just 10 degrees maximum throw is enough.
The location of the rudder can make quite a difference also, especially on a tail dragger. Get the rudder area as low as you can, even a sub rudder can make a big improvement.
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Old 12-17-2013, 01:41 PM
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Tail moment arm is the key to success of any tail dragger airframe. The shorter the moment the harder they are to manage during roll out to lift off. This is all a matter of engine torque and propeller slipstream hitting the left side of the fuselage.
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Old 12-17-2013, 01:57 PM
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Rule # 1
The landing gear goes on the bottom of the wing (or fuselage) so that the pilot is upright during takeoff and landing.
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Old 12-17-2013, 01:58 PM
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In my experience with trike gear on sport models is to have the nose gear slightly lower than the main gear. This helps to eliminate hopping on landing. If the nose gear is higher than the main gear, it is very difficult to land without hopping.
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Old 12-17-2013, 02:13 PM
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Airplanes400, what are rule #2, #3...
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Old 12-17-2013, 04:25 PM
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the other thought is that a wider, further apart main gear is more stable than close together mains.

Personally, while tricycles most people say are easier to learn on, I'd still pick tail dragger almost every time… its simpler to install and once you learn how it handles its not that big a deal.
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Old 12-17-2013, 08:11 PM
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Typically when I design a model aircraft, I put the landing gear on the side of the plane that's supposed to touch the ground first on landing. Usually it works out ok, but I had one plane that needed the gear set up so that one wheel was at the tip of the spinner and another wheel was on a wing tip since that's always what touched down before anything else.
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Old 12-17-2013, 08:16 PM
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cafeenman, were you talking about a tail dragger set up in which one of your front wheels are ahead of the other? which means they are not aligned?
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Old 12-17-2013, 08:19 PM
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No, I'm trying to be funny. You know.. put the gear on the bottom of the plane. I had one plane that couldn't slow down without snap rolling into the ground.
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Old 12-17-2013, 08:23 PM
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i see.
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