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Trike steering geometry


DCN
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I'm hoping to get some trike flying training soon, and in the meantime I'm trying to understand what are good (or bad) design features. This is with a view to eventually buying or building a sub-70 kg trike. 

 

On the subject of steering, I note that most trikes have raked front forks like a bicycle. Why? On a bicycle it's to help the front wheel turn when the bike is leaned into a bend, but on an aircraft it seems inappropriate. Microlight flexwing trikes have this same feature, which is a pain in the bum when pushing the trike around on the ground because the front wheel is forever flopping over to one side. Wouldn't it be better to have negative rake combined with some trail so that the wheel self-centres? I believe the Parajet Falco has this. In fact, if you have this feature, does the pilot need to steer at all? Isn't it better to have fixed foot-pegs so that that the trike just follows the wing without risk of the pilot steering the wrong way and tipping it over?

Thanks!

 

David

 

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Raked front wheel is primarily for stability. In a straight up position the wheel will try and vibrate around its axis. With the castor angle, the touch point of the wheel is vertically below the centre, but behind the imaginary touch point of a line drawn through the hinge point ie it trails. Because the force is applied trailing (behind) the pivot point, it is stable and self-centring.

I think trike steering by pilot is better at take-off because it allows the pilot to drive the trike under the wing centre, thus allowing better recovery and cross-wind take-off.

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Thanks Andy. I agree you need trail (tyre contact patch rearward of steering axis) but you can achieve this with negative rake, which I think is what you typically have on castering tail wheels on 3-axis taildraggers. What I'm suggesting would avoid the dreaded wheel flop, but it adds weight (you need a longer keel tube) and if, as you suggest, it's best to have pilot steering then maybe there's not much point. And I've pushed some very wobbly supermarket trolleys!

 

David

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42 minutes ago, AndyB said:

By "negative rake" are you meaning the vertical axis of the hub leaning forward? 

Yes, the steering axis leaning forward, rather than rearward like a bicycle. Then you need to offset the wheel quite a long way rearward of the steering axis in order to achieve trail at the contact patch. 

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