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Old Sep 27, 2007, 08:47 PM
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Best landing gear configuration for grass field

hi,
I would like to know your opinions on the best landing gear configuration for use on a bumpy grass field (grass approx 2"-3"long). Having looked through many of the threads here there seems to be no clear answer, tricycle or taildragger?
Some have said that a taildragger is better for grass runways but I am unsure of the reasons why. I assume, other than the obvious weight and drag differences, for take-off its mostly because of having 2 wheels in contact with the ground instead of 3 (less friction, better chance of getting off the ground) and also being less likely to have the nose wheel dig in on take-off. But on the other hand on landing the taildragger is likely to tip nose down into the ground.
I assume those are the main reasons for selecting one configuration over the other but would appreciate any opinions as I have very little experience with model aircraft.
The reason I ask this question is I'm part of a University team in a design, build, fly project loosely based on this years AIAA DBF rules.
Aircraft should be in the region of 15lb fully loaded, 60" span.
Large, narrow wheels approx 4" Diameter seem to be the best solution to cope with the long grass. Also ground handling should not be a major issue after the landing roll.

Thanks in advance.
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Old Sep 27, 2007, 09:11 PM
BD Flyer's Avatar
United States, NJ, Monroe Township
Joined Aug 2006
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If the plane weighs 15 pounds, I don't think it would matter if it has tricycle landing gear, or if it is a tail dragger. As long as the LG is strong, and the wheels are a decent size, I think it should take off easily with any type of landing gear.

Bill
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Old Sep 27, 2007, 09:12 PM
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Ohio
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My humble opinion:

If you are anywhere near 15#, large wheels of any config should be OK. That weight will help your airplane set down and stay down. Even if you go with a taildragger, judicious use of the elevator upon touchdown should keep you from any nasty nose-overs......those sure are embarrassing.
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Old Sep 27, 2007, 09:20 PM
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I personally prefer tail dragger - perhaps only for the same reason I prefer a good red over a white (i.e personal taste).

With a tail dragger you will find it easier to get a wide track / wheelbase (i.e. wheels placed far apart) which will give more roll stability on a bumpy field. Also the cog is behind the pair of wheels (wide stability) rather than in front of them (like my jogger pram). Compare pushing a 4 wheel pram (whose cog is behind a wide wheel base) and a three wheel pram (whose cog is behind a single wheel / narrow wheelbase). Which is less prone to tipping sideways?

Another advantage is you won't have angle of attack problems - with a trike whose front wheel is too low, the wing will be at negative angle of attack so won't want to take off (without a stack of flap). With the tail dragger the wing is at a positive incidence - the tail rises on the roll out and you can control the angle of attack with the elevator up until take off. Of course you can fix it by lengthening the front wheel! I'm just showing my bias to tail draggers.

Friction isn't really an issue - that's why they make wheels spin. The narrow wheels may help go between bumps rather than over them (at least that's what my slow stick does).

Be sure to post photos - this sounds like a fascinating project!

I've heard it said that trikes are easier to land... have never flown one so can't comment. But I do think the tail dragger looks nicer
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Old Sep 27, 2007, 09:23 PM
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Depends on the model...

Trike gear resists nose-over better than tail-dragger.

Some tail-draggers are notorious for wanting to nose over and don't tolerate grass fields well, unless the field is putting-green smooth even if the model is a 40 lb 1/4 scale aircraft... (Gee Bee Z for example)

If you have any control over the field maintenence... see if you can get the field rolled smooth and a grass planted that tolerates being cut short.
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Old Sep 27, 2007, 10:19 PM
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If you got plenty of landing rollout room, then I'd go tail drager. Butch
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Old Sep 27, 2007, 10:20 PM
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rollout room usually isn't the problem...

Its the drag on the wheels while attempting takeoff.
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Old Sep 28, 2007, 05:40 AM
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Thanks for the responses guys, much appreciated.
As the aircraft will probably be underpowered take-off may be the larger problem.
Is there much added difficulty in taking off and landing the two configurations? We will have an experienced pilot to fly the aircraft for us so I assume (and hope) he'll be comfortable with both configurations.

As regards the taildragger nose-over problem a solution that has been suggested is to move the main gear well forward with relation to the Cog. But I can imagine with the gear well forward then lifting the tail could be difficult if there was insufficient tailplane control authority.
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Old Sep 28, 2007, 07:15 AM
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Two vs three wheels is a non-issue because the wheels produce little friction and resistance.

The most important thing is to have something up front to prevent nose-in. On a tricycle, it could be the front wheel, but on a tail-dragger if the two wheels are at the nose, it's going to be very difficult to nose it in anyway.

I don't know which is more stable but am thinking it may be a tail dragger because the two at the front will prevent the plane from tripping over a single wheel and rolling to either side.
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Old Sep 28, 2007, 07:27 AM
The Eh Team
Ottawa, Ontario, Canada
Joined Aug 2000
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Our club flies from a sod farm so we have lots of experience with this. Tail draggers seem to work well, however are prone to nose-over on landing. The planes that do the very best are hand-launched and belly landed; but hand launch of a 15lb plane is obviously not going to happen. One alternative, if you have control over the design of the aircraft, would be to have it take off from a dolly that uses large wheels, and belly land. This would eliminate all in-flight drag and weight associated with the landing gear and make for straightforward landing in tall grass. You'd need to watch your flare at the point you touch down as obviously there's little shock absorption to be had in a belly landing. Again, if you have control over your design you can probably cope with that load and build in landing skids.
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Old Sep 28, 2007, 09:09 AM
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How many bush planes are taildragger vs. trike? That should answer the question. Large balloon tires help keep the plane on top of the grass rather than dig in. Again, a bush plane characteristic for obvious reasons.

mw
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Old Sep 28, 2007, 09:28 AM
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Quote:
Originally Posted by muppeteer
Large, narrow wheels approx 4" Diameter seem to be the best solution to cope with the long grass. Also ground handling should not be a major issue after the landing roll.
I agree with the "large", but not with the "narrow". It depends on your definition of narrow.

I did the SAE Aerodesign contest in 2000, and we machined our own wheels out of some Delrin that were large and narrow, but we were operating from pavement, so rolling friction was minimal.

When I think narrow 4" wheels, I think of wheels that have a thickess of less than 0.5". Narrow wheels like this on a grass field with a 15lb plane will dig in and cause alot of drag on takeoff and landing. Have a look at commericially available giant scale 4" wheels and you'll see that they're probably about 1" thick. I think that supporting the aircraft weight without digging into the grass should trump any reduction in aerodynamic drag you'll have from making the wheels narrow. If aerodynamic drag is a worry you can always put wheel pants on wider wheels.

As has been said, tricycle vs. taildragger is mostly a esthetic preference. Since (presumably)you're designing from scratch, you can locate the main gear to make the nose-over tendancy almost non existent. Good rule of thumb for a taildragger is to place the main gear at approximately the leading edge of the wing.

If you're shooting for payload carrying, the advantage of taildragger is that a tailwheel is lighter than a nose gear (especially if you include the weight of reinforcing the nose area for the nose gear in your consideration). So taildragger leaves you room for payload

Tom
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Old Sep 28, 2007, 10:44 AM
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Quote:
Originally Posted by Mark Wood
How many bush planes are taildragger vs. trike? That should answer the question. Large balloon tires help keep the plane on top of the grass rather than dig in. Again, a bush plane characteristic for obvious reasons.

mw
What the diameter of an average full scale aircraft's wheel? Compare that to the diameter of the model's wheel.

Takeoff and landing of a 1/6 scale Piper cub on 3 inch tall grass is about like takeoff and landing of the full scale in a nearly mature field of wheat.
Either can be done... but neither is going to be an easy task.

The J-3 Cub was designed to be able to operate off of a mowed pasture... Appx 4 inch tall grass. (it gets very "bouncy" on pavement...) The tires are WIDE and the LG has relatively soft "springs" (bungee cord...) to help it "float" on the grass and to allow for potholes.

***********

As for getting the wheels forward on a tail-dragger... You have to be careful about that. Too far forward can make the plane unstable on the ground. The further the wheels are ahead of the CG, the more the plane wants to spin around and roll backward.

***********

Tail dragger configuration gives slightly reduced weight. (one less large wheel and no nose gear strut) and you don't have to worry about the angle the plane sits on the ground as much as with a tricycle gear plane. Its also easier to make the taildragger landing gear "soft" to allow for rough terrain.

Tricycle gear planes generally are easier to handle on the ground, regardless of if the field is grass or paved.

Just about everything about an aircraft design is based on compromise...
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Old Sep 28, 2007, 01:54 PM
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Some old aircraft had a curved skid intended to prevent turnovers with the tail dragger setup. I have flow small models with large diameter thin tires/wheels that take off and land reasonably well in 2" deep grass. Old time freeflight models had large fat air inflated tires mounted quite near the nose to prevent nose overs, but they probably would tend to ground loop unless the tailwheel leg or tail skid was tall and kept the fuselage near to level with the ground.
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Old Sep 28, 2007, 02:27 PM
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Grass Valley California
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I think Mark Wood is on the right track.

advantages of a tail dragger-stronger ,lighter, easier to adjust,less complicated.

By designe a tail drager can have the gear located any where you want.

The closer to the CG the less the chance for bouncing,balooning after landing, the easier it wil be to track for take off and landing

the farther forward less chance for nose over, harder to track for take off more chance for bounce and balooning after the wheels hit.

designe a longer plate to acomidate the landing gear and move it forward and aft to where you need it,at the leading edge of the wing is comon as a compromise

Trike gear more to catch in the grass, and it will need to be strong(heavy) to keep from being torn off or twisted

I think wider wheels would be better than narrow, but again you can easily change them after you test fly. You do get to test fly don't you?

Dennis
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