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-   -   Tricycle gear question. (https://www.rcuniverse.com/forum/questions-answers-154/1361315-tricycle-gear-question.html)

Dr. Pepper 12-18-2003 01:26 PM

Tricycle gear question.
 
1. On a tricycle gear set up should the nose wheel strut be raked forward, raked back or vertical?

2. When at rest on the ground should the airplane be nose down, nose up or level?

Before I ripped the firewall off with inadvertant high speed contact with the earth, the wheel was raked forward and seemed to want to always find its own wandering track.

About finished with a new firewall and want it right this time.

Gravity sucks, but seems to suck a lot harder when I am flying.

Thanks for the advice.

Dr. Pepper

tiggerinmk 12-18-2003 02:06 PM

RE: Tricycle gear question.
 
1. The nose gear is usually mounted straight onto the firewall. As the firewall usually has some downthrust built in, the nose gear end up being raked back slightly. Also, the coil spring should face to the rear as that is the direction that the spring is designed to flex. It shouldn't matter if the nose gear is straight down just as long as it's not raked forward. The backward rake helps the plane ride over bumps on the ground.

An example: push the tip of your finger along the table / desk top angled forward. Then put an obstruction in front of it such as a coffee stirrer or paper clip. What happens when your finger hits the obstruction? Now do the same thing dragging your finger along raked backward...

2.
Plane level: This should give the plane a neutral attitude and will take off readily with a little up elevator when you reach flying speed.

Nose down: This will reduce the incedence of the wing in relation to the ground and may actually keep the plane on the ground during the take off run. This may also make it more difficult to 'rotate' the plane for a smooth take off. It will also reduce the prop to ground clearance.

Nose High: This will increase the wing incedence and may cause the plane to try and take off before it has gained sufficient speed. This also means that the nose wheel will be lower than the mains when you are attempting to land. You will have to land in a nose high atttitude to prevent the nose gear from hitting first. You do not want the nose gear to strike the ground first when landing.


Correction: The incedence of the wing should be measure in relation to the oncoming airflow. If the wing is set at 2 deg positive incedence, then the wing will start to generate lift near flying speed. With the plane set level you'll see it 'get light' and more bouncy (if you are on grass). If the nose gear is set nose down so that the wing has a negative incedence is relation to the oncoming airflow, this will make it more difficult to lift off. You may see it suggested that the plane is set slightly nose down, but keep an eye on the wing incedence.

Dr. Pepper 12-18-2003 02:12 PM

RE: Tricycle gear question.
 
Makes sense. Raked aft it shall be.

Thanks.

Dr. Pepper

LouW 12-19-2003 01:39 PM

RE: Tricycle gear question.
 
Triggerinva’s analysis is correct as far as it goes, but only for one gear geometry, that is where the wheel axle is behind the gear axis of rotation.

To be stable, the wheel must castor, and this can be achieved by several means. All bicycles, motorcycles, old type three wheel ATV’s, and most full scale airplanes utilize a steering axis that is canted forward. As long as the wheel axle is on or ahead of the steering axis, the forward cant is a stable, castoring configuration. There is an added advantage to this forward cant, in that it greatly reduces the bending load on the strut, and on the firewall in a hard landing.

The geometry where the wheel axle is behind the steering axis is like a castor on a rolling chair or tool box, etc. This is used on the Grumman American Yankee (the only full scale aircraft that comes to mind). While this configuration is stable it often has a tendency to shimmy which is why it’s seldom used on any full size vehicles. This configuration results in high bending loads in the strut and at the firewall. That is why a springy “knee action” is usually incorporated along with this geometry to absorb some of this load.

By far the simplest installation is to attach the mount directly to the firewall. If the firewall is canted, the steering axis can be changed by simply adding a plywood wedge beneath it as necessary. The good news is that if the angles are small it really doesn’t make much difference. The airplane is on the ground only briefly during takeoff and control is not usually a problem even with a bent, out of place nosegear.

feihu-RCU 12-19-2003 08:57 PM

RE: Tricycle gear question.
 
DON'T rake it aft!
The second paragraph by LouW sez it all.
Ten degrees of forward rake is all you need.

Dr. Pepper 12-22-2003 01:58 PM

RE: Tricycle gear question.
 
Conflicting advice indeed.

Bottom line, finished the install before the other two posts came in. Mounted flush with the firewall which gave a few degrees aft rake due to built in firewall down thrust.

Flew this weekend and avoided inadvertant vertical contact with the earth.

Works great. When it was raked forward the wheel always seemed to seek its own path. Now it is very stable.

I think LouW is correct in his technical analysis but in this real world issue both the nose wheel and I are very happy.

When I build my third scale P-38 I will rake it forward.

Thanks for the help.

Dr. Pepper

3DFanatic 12-22-2003 11:08 PM

RE: Tricycle gear question.
 
I wouldn't worry about it, a few bad landings and it'll be pointed backwards anyway :D

When you buy them, they come straight up-and-down. easier just to elav them like that.

superflea 12-24-2003 11:34 PM

RE: Tricycle gear question.
 
lou does indeed have a point when he mentions motorcycles, but the problem is that we are not talking about motorcycles here where stability is important at 65 MPH, we are talking about a model airplane and as he said raked back will help with the bumps and that is what is important here, the bumps, because as the plane gains speed the nose gets light and you lose steering authority so as the nose wheel continues to hit bumps as you noted yourself the plane tries to wander all over the place. rake it back some and it helps the spring work its best forward and it hit the bumps more directly in line with the nose gear wire. it was also mentioned that haveing it swept fore or aft will affect the stance of the model however this is only true if you dont adjust it you can raise or lower the strut in the collar in most cases.

GadgetMann 08-25-2004 01:46 AM

RE: Tricycle gear question.
 
(I just love RCU, typed in my search and found this thread!)
I had a follow up question. I have a Midwest Aerobat (high wing/semi symmetrical trainer). I just finished assembling my 2nd (new fuse). I installed a Dubro main gear. I noticed that the new gear is slightly taller than the stock one. I adjusted the nose gear (a little) to compensate, but I still have 1.4 degrees down angle. The plane is spec'd to have 2 degrees down thrust.
Should I make some effort to adjust this (e.g. modify the location of the main gear axles)?

Bax 08-25-2004 04:53 PM

RE: Tricycle gear question.
 
A model with a nose-low stance will present a few difficulties on takeoff. Because the nose is pointed down, most models will stay firmly planted on the runway as they gain speed. You'll gradually apply 'up' elevator until you get enough to rotate. This is usually a bit much, and the model pops off the ground and goes very nose high. A quick punch forward on the stick will be needed, but too much, and the model heads down again. Overcorrect with 'up' stick, and you get the rollercoaster.

Landings, though, are easier because you can have the model touch level, and the nose will immediately go down. The nose-low position firmly plants the model on the runway with little chance of a bounce.

The best stance is with the plane dead level. It won't take much elevator to rotate for takeoff, and you'll be able to do it smoothly, and you'll be able to have nice nose-high landings, but even level landings will tend to not bounce.

bax

feihu-RCU 08-25-2004 04:58 PM

RE: Tricycle gear question.
 
I would rebend the main landing gear (spread it apart so it sits lower and bend the axles in for a tad bit of toein) so that the airplane sits level on the ground. The engine downthrust (if 2degrees downthrust is built in the nose of the airplane) will cause the nose gear to lean back at two degrees.
You can have the nose gear straight down or even slanted or "raked" forward by tilting the STEERING AXIS forward so long as the steering axis, when projected to the ground is at a point AHEAD of where the wheel touches the ground. This is called POSITIVE caster by definition; and it is used that way on motorcycles, cars, airplanes, and vehicles where steering is required.
This was explained in LouW's post earlier, but I think it was not clearly understood.

GadgetMann 08-25-2004 10:35 PM

RE: Tricycle gear question.
 
Bax & Feihu,
Thanks for the the posts. Unfortunately, the main gear I currently have installed is not bendable. It is some sort of plastic/composite designed to resist getting bent out shape. I could modify the holes the axles are in to lower the main gear. I will definitely look into that. I could also use the stock landing gear that came with the 2nd fuse. As long as it doesn't mess up the balance, I may just use the stock gear and keep the plastic Dubro one in reserve.


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