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Author Topic: Tricycyle landing gear spacing  (Read 1438 times)

Offline RK

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Tricycyle landing gear spacing
« on: November 09, 2018, 09:38:20 PM »
I am thinking  %^@ about changing the landing gear on a plane that I am scratch building from the normal two wheel to a tricycle type. It has a 36" WS and the engine ( Enya 15) is  5"" in front of the leading edge, I will set the front gear just behind it. The problem I am having is deciding where to locate the two wheels in the wing. How wide & how far back should they be? There is plenty of room in the wing.
The plane is called "The Sportwing" from Hal De Bolt 1949. The plans are shown on the Outerzone.

Any ideas ?  :!  Thanks, RK Flyer
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Offline Tim Wescott

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Re: Tricycyle landing gear spacing
« Reply #1 on: November 09, 2018, 11:26:50 PM »
As wide as a conventional gear plane.

About 15 degrees back from the center of gravity.  On a modern stunter I'd tell you to expect the vertical location of the CG to be in the wing; I can't speak to that plane.  Putting the wheels 1 1/2 or 2 inches back is probably OK.  Too far forward and it'll stay tipped back on the tail; too far forward and it won't rotate nicely on takeoff.

It won't be old-time legal unless you can find a picture in a magazine or something to show it flew that way on or before 1952 -- but if you're just flying for fun, go for it.
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Offline Dave Hull

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Re: Tricycyle landing gear spacing
« Reply #2 on: November 10, 2018, 01:29:51 AM »
RK,

You didn’t pick an easy one, did you?

If I have the plane right, the deBolt Sportwing is a forward-swept flying wing. Flying wings are generally more critical on CG location. I don’t think the Sportwing uses wing twist to increase the pitch stability, so I suspect it will be more sensitive, as well. And I doubt it has reflex, which is the other way to get stability—but I wouldn’t use reflex on a plane I expected to fly inverted. Anyway, the fundamental rule of landing gear location regardless of type is that you put where the CG demands that it go.

There are some rules of thumb for tricycle gear that have been borne out by detailed calculations for full scale aircraft:
1.   The fore and aft location of the main wheels is always behind the CG by a bit more than the maximum angle of attack during landing at the minimum airspeed. Just think of this as the stall AOA, plus an additional angle for stability. The stability angle varies, but assume something like 5 to 25 degrees for a model. More on this below.

2.   The gear length matters. Not just that it has to be long enough for prop clearance, but also has to provide the right static inclination, or “stance.”  If the stance is negative, ie. nose down, then the elevator has to work harder to “unstick” the nose gear for rotation. If the elevator effectiveness is low, or low at low speeds, it will be going pretty good before you can rotate. So shoot for level or slightly nose high. The issue with a flying wing would be that if the setup demands a lot of elevator to rotate, it could get real exciting the instant after the nosewheel unsticks.

3.   If you use a larger stability angle on the mains, it will also take more elevator authority to rotate. So using a large angle, which sounds good at first, has consequences. For a wing, I would suggest going with a very small stability angle—say 5-10 degrees—and then incorporating a tail skid. Airliners all have tailskids so there’s no shame it putting one on a Tri-gear Sportwing.

4.   Because it is a flying wing, and CG at takeoff will be critical, I would absolutely do your calculations using a full fuel tank. Takeoff will be the tough requirement, so tank is full.

5.   As far as the lateral separation of the mains, something around 40 degrees relative to the fuselage centerline, with the apex at the contact point of the nosewheel should work fine.

6.   Put the nose wheel as far forward as you can since this is a flying wing and pretty stubby. Normally, the nose wheel should take about 5-15% of the static load. Too much and the plane tends to “wheelbarrow” and lose directional stability. Wider mains helps this a lot. If this concept doesn’t make sense, think about tongue weight on a trailer. It is not really the same problem, but you can see that weight distribution on three point wheeled vehicles matters.

So just take the plan sideview, decide where you are going to put the CG, and draw in the angles to locate your gear.

I had an interesting time with a monowheel flying wing. If the wheel was not in exactly the right spot, it was virtually impossible to take off. This was a plank wing. I don’t recall if it had any washout or not. The plane flew great once airborne and was not squirrelly—implying that the CG was well located, and the elevators were sufficiently effective for flight. But if the wheel was moved back just a bit—say if the gear bent during landing—it was next to impossible to take off.

I hope this helps. You have a fun project going, and I would definitely give the tricycle gear a shot. You might make it adjustable if you can.

Good luck,

Dave

PS—The only trike gear model plans that I have laying around are for the old Jetco Sabre Stunt. It is pretty short-coupled for a stunt plane (going to be similar to your situation), and the gear is only 8-1/4” apart, fore and aft. The mains are 40 degrees behind the CG. This is the contact point for the wheel, not the axle.

PPS--I think your mains are going to be almost directly below the forward edge of the trailing edge spar. I'd beef it up with a full length strip of 1/8 aircraft ply. Probably sew a one-piece wire to it with Spectra fishing line. Give you a torsion gear. The side view layout with the angles will tell you if you need to cant the gear either forward or aft.

Offline RK

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Re: Tricycyle landing gear spacing
« Reply #3 on: November 10, 2018, 03:07:00 AM »
Tim & Dave, Thanks for the fast reply's, I think I may be in for a lot more than than than I thought!

These days I am building for fun & my own enjoyment, I don't really fly anymore, old age has me side lined. I do try to make my planes as though they are flight ready. So I think I will go ahead & try to work something out with all the new info just because of the "Coolness Factor".

Thanks again for your help, R K.


 
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Offline Dave Hull

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Re: Tricycyle landing gear spacing
« Reply #4 on: November 11, 2018, 08:03:02 PM »
You should definitely go for it. Don't worry that it is too challenging. Just measure out the angles and it will work.

Dave

PS--I spent some time today cleaning up a plane for a friend for our 1/2A Fun Fly next Sunday. It is a LIT Special. It has two wheels in tandem, buried in the profile fuselage. And the tailskid function is provided by the bottoms of the twinboom rudders. Pretty unusual gear on that one.....

Offline Dave Hull

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Re: Tricycyle landing gear spacing
« Reply #5 on: November 18, 2018, 09:48:26 PM »
It might give you more confidence in your new gear geometry if you had a few examples. If so, then check out these two.

N9M flying wing:

     3-View     

https://www.bing.com/images/search?view=detailV2&ccid=F4DmbRVk&id=4FDA7898FB502F82C39BE291AA27AB9ED7593EC8&thid=OIP.F4DmbRVkvFCWafjQ0vGRPAHaJn&mediaurl=https%3a%2f%2faerofred.com%2fdata%2fthumbnails%2f54%2fnorthrop_n9m_flying_wing.jpg&exph=750&expw=578&q=n9m+flying+wing&simid=608024701602497578&selectedIndex=1&ajaxhist=0

     Video of it landing and taking off  (you can see the rotation, the landing bounce, and the over-rotation skid with wheels.)

https://www.bing.com/videos/search?q=building+a+model+airplane+wing&&view=detail&mid=E18329A0274F59765C52E18329A0274F59765C52&&FORM=VDRVRV


Tier III-minus (Darkstar)

Another good one to look at would be the Tier III minus aka the Darkstar UAV. Because of the trailing edge sweep, the wing planform center of pressure wrt span is slightly forward swept.

https://www.bing.com/images/search?view=detailV2&ccid=LW2VmitQ&id=3897BBAC5997BEF2AF658C82D74970BCD49B0D5B&thid=OIP.LW2VmitQOg2XSl8DXcl4uAAAAA&mediaurl=http%3a%2f%2fwww.boeingimages.com%2fDocs%2fBOE%2fMedia%2fTR6_WATERMARKED%2f8%2f9%2f1%2f5%2fBI225125.jpg&exph=248&expw=321&q=Darkstar+UAV&simid=608023803956954525&selectedIndex=16&ajaxhist=0

Notice how short-coupled both of these are. I think your project would turn out to be less challenging, and easier to land than either of these. But note that they put the nose gear as far forward as they possibly can.

It is interesting to note that Hi Johnson of modeling fame, engine and kit maker, was one of the few pilots to ever fly the N9M.  There is only one, and was very painfully restored after rescuing it from a target range on Edwards somewhere--where miraculously it never got blown up. But it is substantially of wooden construction, so imagine restoring that after decades of sitting in the desert. And after all that, they had an engine fire in one of the Franklin engines, and had to substantially rebuild that portion again. The irony is that the faction that insisted the flying wing should die permanently and demanded that the tooling for the B-49 be destroyed to forever end the debate thought that putting the N9M on a bombing range would also solve that problem. It is amazing to watch it fly...with no vertical surfaces and no computers to augment stability. Darkstar did not fare quite so well....

Divot McSlow
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Offline john e. holliday

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Re: Tricycyle landing gear spacing
« Reply #6 on: November 19, 2018, 09:36:42 AM »
Just think where aircraft design would have been if some politician had let it continue.  I was lucky to catch the B-2 wing doing a fly over in the KC area.   Very lucky as there was hardly any sound and very low profile. D>K
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Offline Steve Helmick

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Re: Tricycyle landing gear spacing
« Reply #7 on: November 19, 2018, 08:07:31 PM »
Interesting that Hi Johnson flew an N9N...wondering how/why. My Dad also flew an N9N. He said it had some yaw instability that Northrop cured with a "black box", but that otherwise it flew fine. He also said he couldn't recall if the one he flew had the Franklin engines or the Menasco engines, indicating that there were several. Dad was a test pilot at Wright Field before and after WW2. We used to have a picture of a B-17 with the crew in front of it that was autographed "Thanks for the ride, Orville". Alas, that picture got away from us. Dad was the pilot.

One of his projects at Wright Field was the AZON Bomb, which was a radio & fin set that attached to a standard 500 lb bomb, as near as I can tell from the online research I've done. I found one site that claimed the AZON bomb was used on "The Bridge on the River Kwai", but Dad never said anything about AZON bomb being used in the So. Pacific. We went to see the movie, and he was disgusted by the inaccuracies and distortions by Hollweird. Of course!

He did say they used AZON on a dam in Romania, and the story of having a Senator's or Congressman's kid being appointed from above to be the navigator on that mission, and what a fubar that became. Apparently, silver spoons interfere with magnetic compasses?  LL~ Steve     
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Offline Steve Helmick

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Re: Tricycyle landing gear spacing
« Reply #8 on: November 19, 2018, 08:19:23 PM »
Returning to the OP's question...the Pathfinder is well known for good takeoffs and landings IF the LG is setup correctly. You could do worse than to check out the Pathfinder plans or article. The main gear needs to be only slightly behind the CG with the fuel tank empty. The nose gear needs to be made to hold the nose just about 0-0, or slightly nose down. Some early Classics with trike setup (Mackey's Gobbleschwantz, Crusader, etc.)  had the main gear WAY too far aft, IMO. Note that with Classics, the rules allow you to fix stuff like that without fear of DQ. y1 Steve
"The United States has become a place where professional athletes and entertainers are mistaken for people of importance." - Robert Heinlein

In 1944 18-20 year old's stormed beaches, and parachuted behind enemy lines to almost certain death.  In 2015 18-20 year old's need safe zones so people don't hurt their feelings.


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