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Author Topic: Curious about landing gear design  (Read 2453 times)

Offline Bob Zambelli

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Curious about landing gear design
« on: September 13, 2006, 06:22:46 AM »
Why don't we see more tricycle gears in stunt planes?
I have used them in a few designs and have found no inherent problems.

Bob Z.

Offline Ralph Wenzel (d)

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Re: Curious about landing gear design
« Reply #1 on: September 13, 2006, 10:25:43 AM »
Hi Bob,
The primary objections to trike gear are: 1) added drag due to the extra (large) wheel up front, and 2) additional weight due to the large wheel, heavier wire gear, and additional structure reinforcement required. OK,  the heavy wheel doesn't count as much these days with the super-lite wheels available, but the rest still obtains.

One additional drawback I can think of is that it complicates stooge installation. You need a longer hunk of wire to get down to most stooges.
(Too many irons; not enough fire)

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Offline Ted Fancher

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Re: Curious about landing gear design
« Reply #2 on: September 17, 2006, 07:13:17 PM »
Hi Bob,

I'm right now contemplating a twin and it would probably have a trike gear.  My biggest concern about doing so is the problem with "tricycling" on takeoff on grass (we fly a lot of our local meets off grass.  It's usually pretty good but I've seen some ships which try to tip over forward and sideways when the grass drag is too much)

I think the solution to that is finding the right location for the main wheels relative to the CG.  Right now I'm not sure what the best location would be.

That point has to be proper for good takeoffs and landings, too (I'm talking smooth transitions for lift off and touch downs that don't "clunk" the nose down after the main gear touches).  Obviously, it has to be aft of the CG with the fuel tanks empty but how much is something I'm not too sure about.  My tendency is to think they should be only slightly further aft than necessary to keep the nose gear on the ground at rest.

I'm not overly concerned about the weight and drag of the additional gear strut and wheel.  Our thrust to drag situation is pretty darn good and I think we get a little too uptight about additional drag.  Al Rabe's (and now Billy W's) scale like ships with big fuses (and in Al's case, drop tanks and bombs, etc.) don't have any problem flying very well despite their relative bulk.

Are you flying your ships off grass?  Where did you locate the wheels relative to the CG?  Have any problems with the takeoff roll?

Ted Fancher

Offline Ted Fancher

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Re: Curious about landing gear design
« Reply #3 on: September 17, 2006, 07:18:02 PM »
Oops, forgot to mention...

Trike gears need to be set up so that the angle of attack at rest is slightly negative.  Especially with todays powerplants and low pitch props, acceleration can be very rapid.  If the airplane sits at a positive angle of attack it will be very difficult to keep the ship on the ground long enough to satisfy the judges and still keep it from leaping off the ground when you ease up on the down elevator you'll need to keep it from lifting off prematurely.  If the AOA is slightly negative you should be able to control the rate of rotation regardless of airspeed.

In addition, if the airplane sits at a positive angle of attack and you roll around into a strong wind on landing it will be hard to prevent it from lifting off unexpectedly.  With a slight negative angle of attack that probelm should be mitigated.

Ted

Offline phil c

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Re: Curious about landing gear design
« Reply #4 on: September 18, 2006, 10:02:03 PM »
Trike gear can be very sensitive to nose gear height.  One one RC plane I trimmed it took only 1/16 in. change to go from a porpoising monster to slick landings.  He'd set the gear just a bit too long.

If the nose gear is set too short, the plane may rotate faster on takeoff than the pilot can react.  Not a huge deal if the strut is trimmable.
phil Cartier

Offline rob biddle

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Re: Curious about landing gear design
« Reply #5 on: September 19, 2006, 04:41:36 AM »
   G'day all,
 Often wondered why "trikes" aren't more popular myself until I saw one inverted,  are they ugly. They may be an advantage to those of us who fly off  (short) grass all the time.
 (Such as myself.)
I always wanted to build a shark .45 but the undercart put me off. Saw one recently as a  "tail dragger". Spatted wing mounted undercart, man it was dead sexy.   
  Rob.     
Robert Biddle

Online James Lee

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Re: Curious about landing gear design
« Reply #6 on: September 19, 2006, 03:32:12 PM »
Back when Bob Whitely told me that the best starting point for the main gear on a trike was swept back 15 degrees from the CG point.   And then the nose gear was set  to give a very slight nose down stance... 
For what it's worth...  ;D
Personally, I've found a conventional gear to give better results....
Jim

Offline Tom Niebuhr

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Re: Curious about landing gear design
« Reply #7 on: October 01, 2006, 01:14:22 PM »
I agree that two wheels with wheel pants just "look better".

With that said there will probably be a trike design in my future, as a semi scale stunter.

I agree with Jim, that Bob Whitley's 15° is probably a good starting location for the main gear.  The negative AOA controlled by the nose gear length is both necessary and potentially problematic. Too much negative and it will jump off when up is applied. The answer here is to have a removable gear to play with. This can be easily done with a mounting similar to a removable wing gear, except with the wire running fore and aft in the nose, below the tank.

I have looked at many trike designs, including those from C. Mackey, Jack Sheeks, and Don Hutchinson. They seem to vary quite a bit with main gear location. Particularly where it exits the wing. Don's all work very well.

One other question I have is.. Is there an ideal distance between the mains and the nose gear.. or does it really matter?   I would guess that as long as the wheel base is not too short, almost anything goes???
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Offline John Jordan

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Re: Curious about landing gear design
« Reply #8 on: October 03, 2006, 04:10:15 PM »
Hello all  After building my first Tucano and even before I had found out one of the best thing to do is. !st make sure your airplane sits o-o on the ground or a slight nose up 1 to2deg at most. Tail heavy or nose heavy. Do not make it sit on the ground nose down at all it will want to jump off on takeoff as it rotates past o-o. It will do the same if you are too much nose up . So try for o-o to 1-2 deg nose up at most. Hope this helps Thanks John Jordan.
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Online James Lee

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Re: Curious about landing gear design
« Reply #9 on: October 03, 2006, 05:43:37 PM »
I will still suggest the slight nose down attitude for the trike gear....   The mains must be located so it takes minimum force to lift the nose.  Then the plane can be 'flown' with the mains on the ground and the nose wheel off the ground to lift smoothly....  any nose up attitude will cause a launch, especially with the modern power plants.
Just my opinion
thanks
Jim

Offline Randy Powell

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Re: Curious about landing gear design
« Reply #10 on: October 06, 2006, 11:08:50 AM »
I've got a trike gear plane with wheel pants all around on the board at the moment. The gear is set up to be about a 1/2" behind the projected CG with the ability to adjust as much as another 3/4"" either way. The problem is the extra structure needed for the main gear mount. If you attach it to the spar, you have to have a swept back main gear to get the wheels in the proper position. If not, the mount has to span the area between the spar and the trailing edge. Depending on how you attack the problem, this can mean a lot of extra weight. I have some ideas about that, but it's still in the development process.
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Offline Jim Pollock

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Re: Curious about landing gear design
« Reply #11 on: October 18, 2006, 08:34:31 PM »
The Shark series of stunters .35 and .45 have the main gear set abut as perfect as can be gotten.  Just look the angle behind the CG up on the Shark plans.  Use that and you won't be dissapointed.  Mine lands pretty good.  Be careful though, don't hit the nosewheel on the ground first or your in for a pretty good bounce!

Jim Pollock   8)

Offline wwwarbird

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Re: Curious about landing gear design
« Reply #12 on: October 19, 2006, 07:36:08 PM »
When I build my repro Sterling F-94, probably towards spring, it will be redesigned with trike gear. I hear that a good guideline is putting the main gear axle/pivot point 15 degrees aft of the airplanes C/G.

That's my plan and I'm sticking to it! ;D ;D ;D
Narrowly averting disaster since 1964! 

Wayne Willey
Albert Lea, MN U.S.A. IC C/L Aircraft Modeler, Ex AMA member

Offline Ted Fancher

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Re: Curious about landing gear design
« Reply #13 on: October 20, 2006, 10:47:04 AM »
When I build my repro Sterling F-94, probably towards spring, it will be redesigned with trike gear. I hear that a good guideline is putting the main gear axle/pivot point 15 degrees aft of the airplanes C/G.

That's my plan and I'm sticking to it! ;D ;D ;D

That 15degree number actually comes from Bill Netzeband regarding conventional gears.  The magic number refers to the point forward of the cg that the wheel contacts the ground, not the angle of the gear itself.

I think a more important criteria for the trike gear is the angle of attack at which the airplane sits at rest on the gear.  It must be only slightly negative at that point.  If it's positive you'll have little control over the take off roll.  It will simply lift off almost immediately unless some down control is applied.

If too negative the wings lift will vector downward as soon as the ship starts to move.  This will tend to hold the ship on the ground and up control will be required to lift the nose.  Depending on how far aft of the CG the main gear touches down this could require a lot of up control and will result in a jackrabbit take off once the download on the tail is enough to get the nose in the air.  Once the angle of attack is positive it will jump off the ground.

The secret, I would think. is to have a very slightly negative attitude and the main wheel contact only a small distance aft of the cg.  Thus with very little speed the pilot will be in control of the pitch attitude of the airplane and can keep it on the ground or rotate smoothly as desired.

I don't think the 15degree number is particularly relevant in this regard.  I'd have the main wheels touch down so that with the tail depressed to the ground at rest it will rotate lightly to the three point attitude when pressure is released.

Ted

Offline Ron Merrill

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Re: Curious about landing gear design
« Reply #14 on: October 20, 2006, 12:35:54 PM »
On my LA HEAT by Bob Whitely the c/l of the main gear is 1 5/8" behind the CG> Measureing on the plans the plane sets level. The nose wheel wire comes out of the plane at a forward angle then bends back. It uses 2" mains and 1 3/4" nose so it can't set level!!!!!!!! Ron.  AP^

Offline Chris McMillin

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Re: Curious about landing gear design
« Reply #15 on: October 25, 2006, 08:52:56 PM »
Oops, forgot to mention...

Trike gears need to be set up so that the angle of attack at rest is slightly negative.  Especially with todays powerplants and low pitch props, acceleration can be very rapid.  If the airplane sits at a positive angle of attack it will be very difficult to keep the ship on the ground long enough to satisfy the judges and still keep it from leaping off the ground when you ease up on the down elevator you'll need to keep it from lifting off prematurely.  If the AOA is slightly negative you should be able to control the rate of rotation regardless of airspeed.






Chris:  RJ always said and did the opposite. If the AOA is negative, the "break out" elevator input to raise the nose exceeds that pitch necessary to "jump off" the ground. On my trike he suggested a larger diameter nose wheel for a smoother transition on TO. It worked wonders and a little forward pressure, er... down elevator input held the nose on until desired.



In addition, if the airplane sits at a positive angle of attack and you roll around into a strong wind on landing it will be hard to prevent it from lifting off unexpectedly.  With a slight negative angle of attack that probelm should be mitigated.



Chris:    Great theory, but again down elevator is a great extender of the spoilers, and no floating after touchdown.




The less weight on the nose wheel provides the best handling of smooth pitch decrease on rollout too, Ted. LA Heat would rock on it's tail easily, but it made awesome TO and landings. On tailwheel airplanes we put the gear pretty far back too, same idea, get it to balance on the mains without weight overcoming aerodynamic control authority.

Bob's Shark 35 was set-up the same, and was so easy to TO and land this way.


Chris...

Offline Howard Rush

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Re: Curious about landing gear design
« Reply #16 on: October 31, 2006, 09:38:24 PM »
This makes sense to me, although it's been 45 years since I made a plane with tricycle gear.  That one was a Flite Streak.  It had a short nose gear and sat nose-down.  The tiny elevator didn't have enough authority to rotate the airplane.  It rolled about two laps before it hit a bump and bounded into the air. 
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Offline wwwarbird

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Re: Curious about landing gear design
« Reply #17 on: November 01, 2006, 05:48:29 PM »
Ted,

 I'm not sure if I made sense earlier. What I meant was mounting or positioning the main gear so that the axle shaft pivot point ends up being 15 degrees behind the horizontal C.G. of the model. This doesn't have to mean the gear leg itself would have to be angled to achieve that. My plan is to actually have the leg running 90 degrees to the ground, with the L/G mount positioned in the wing so that the axle shaft ends up 15 degrees aft of the C.G. Sound about right? ???
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Offline Dean Pappas

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Re: Curious about landing gear design
« Reply #18 on: November 14, 2006, 10:05:46 AM »
Hi All, Hi Ted,
My experience both CL and RC has been to put the mains quite close to the CG. Usually with an empty tank, the plane will sit flat, but if tipped onto the tail then it sits there. With a full tank, that won't happen. Combine this with a  tiny amount of negative attitude on the ground and you are golden.  The location of mains close to the CG allows you to finesses the rotation, where both aft mains location and overly negative attitude delay the rotation and result in a leap off the ground.
Dean Pappas

Online John Miller

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Re: Curious about landing gear design
« Reply #19 on: November 15, 2006, 01:04:02 PM »
Hi All, Hi Ted,
My experience both CL and RC has been to put the mains quite close to the CG. Usually with an empty tank, the plane will sit flat, but if tipped onto the tail then it sits there. With a full tank, that won't happen. Combine this with a  tiny amount of negative attitude on the ground and you are golden.  The location of mains close to the CG allows you to finesses the rotation, where both aft mains location and overly negative attitude delay the rotation and result in a leap off the ground.

Dean has it right. This is exactly the criteria Gordan and I use with the Pathfinder series, which are usually known for smooth takeoff's and landings. The distance seems to work out to about 1/2 to 1 inch behind the CG. O-O to just barrely negative when sitting on the gear. With the tank empty, if you set the plane on it's tail it will stay there. with the tank full it will return to the nose gear.

On takeoffs, the plane will stay on the ground for as long as you want. It seems to lift off when you think take off, though I suppose that's because of a small amount of up given when forming the thought.

Best of luck with your project, twins are for real masochists ya know. I'm working and a twin Pathfinder that uses a pair of OS .15's for Pro 40 this next year. j1
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Offline Don Hutchinson AMA5402

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Re: Curious about landing gear design
« Reply #20 on: January 17, 2007, 09:04:18 PM »
The F-86-D's I built were set up with the mains very close to the CG, such that the tailpipe would about stay on the ground if you pushed it down. The nose wheel length was such as to set the wings at 0 incidence on the ground. The first flights were done on grass and as it was released, the nose would turn in toward me til the airspeed built up some. The same thing also happens on asphalt. It didn't take me long to figure out that one needs to hold some down elevator early in the takeoff roll. This is the result of having almost all the weight on the mains. Don't take this to imply that I would move the mains if I did it again. I would not. No one has mentioned this but a tricycle gear is like stealing candy from kids when it comes to TO and landing points! Especially an F-86 dragging in, tail low like the real ones did.

Don


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