Is nosewheel load critical on free-castoring system?

[Brian Jackson]

I am curious how important nosewheel downforce is in a free-castoring system. In my current build I had been planning to rake the mains back slightly to increase nosewheel pressure by recommendation of other pilots/builders. A GyroBee wants ~35 lbs. on the nosewheel, but does this figure become less important if there is no direct steering acting on it, be it soft or hard linked? I want to better understand this dynamic before committing to a gear arrangement I may regret later. Thank you.

 

[Vance]

I doubt if I can give you a better understanding Brian.

A free castering nose wheel can lift off later and touch down earlier in a cross wind without challenges.

The geometry and the dampening controls shimmy.

I would just try something and if it doesn't work I may be able to help you fix it.

In any aircraft there is a variable load on the nose. More weight tends to dampen the shimmy. It is going to get light as you lift off.

With an effective rudder and good brakes a free castering nose wheel works well.

Writing in the most general terms anything over five knots of indicated air speed and I can steer with the rudder in The Predator.

With me sitting in the front seat there is over 100 pounds on the nose in The Predator.

Clients get into trouble if they try to use the brakes at speeds much over ten knots in The Predator.

 

[Brian Jackson]

Thank you Vance. But I think the crux of the question is (in the case of the GyroBee) if the ~35 lb. rule of thumb is assuming it is a steered system as opposed to free. There was a discussion some time back about nosewheel downforce but it never occurred to me if this was in regard to having adequate NW steering control or just in general. I would imagine a passive, free-castoring system isn't as dependent on NW downforce since it's not being forced to turn the ship. Whereas hard linked NW steering is required to have more traction to overcome forces acting against it.

 

[Vance]

Sounds right to me Brian.

 

[Doug Riley]

Brian: My old Dominator (with, of course, a free-castering nosewheel) would go into the most violent shimmy you can imagine. I thought a couple times that the nose leg would break off. Then I discovered the built-in friction collar. Bye, shimmy.

The friction of the tire on the ground is an additional potential damper. I fly off grass, which is frequently so slippery that this source of friction is nil.

My feeling is that a relatively light nosewheel load indicates that the axle is far enough forward to permit early nosewheel lifting -- a good thing if (1) you don't have a 100% RRPM prerotator and (2) you have appropriate aft rotor-ground clearance. Extremely light nosewheel loading will make both steering and nosewheel braking uncertain, especially on grass.

Other than those considerations, IMHO the weight is not that significant, within structural limits, of course.

 

[Brian Jackson]

Thank you, Doug. I'm always left with something valuable to think about after your posts. In the Crescendo Build thread recently you made some important observations concerning the rake-back angle of the mains, and how it could equate to longer takeoff runs as a result. That led me to question the need to increase nosewheel weight at all, and if the advice I was given about doing so was only beneficial for steered and braked systems as opposed to free castering.

 

[Doug Riley]

if you steer with differential braking using a free-castoring nosewheel, and you lock one main brake, the nosewheel will normally swivel to accommodate the resulting turn.

The swivel happens through friction between the tire's contact patch and the ground, assuming that the contact patch is aft of the nosewheel's pivot point (typically 2.5-3" aft). The principle is basic supermarket-cart.

If the nosewheel has extremely low weight on it, or if there's little friction (wet grass), the craft will still turn when you mash a brake, but the nosewheel might be dragged through the turn sideways. Assuming little weight and/or friction, that isn't likely to hurt anything.

P.S. We fly off lake ice up here when conditions are just right. No wheel braking, and no steering except by rudder. Kind of like riding on a hockey puck. On ice, all directions of travel are created equal.