Fat Tyres -Directional Stability?

[NT5224]

G'Day folks!

 

Hoping somebody can help me with this. Tail draggers handle rather differently to tricycle gear aircraft on the ground , and one thing I've noticed is the response to steering inputs. Never ground-looped myself, but I've seen plenty of clips of it happening.

 

So can anybody tell me whether those big bush tyres would likely improve or degrade directional stability on the run up and roll out on a tail dragger?

 

Thanks

 

Alan

 

 

[facthunter]

What's a run up and roll out?. They are designed for things like fairly large rocks and you use low pressure so they will "wobble" a bit like nay large section soft tyre will. In certain conditions they would be better as they won't sink in soft earth /sand as much. A "flat" tyre is worse but not much more than a strong sudden crosswind. Nev

 

 

[SDQDI]

[MEDIA=vimeo]104490618[/MEDIA]I can't answer for all surfaces but I have found the bush wheels to be very forgiving on the dirt and rough ground I haven't got much time on tar or decent strips to compare them with the smaller wheels.

 

A few things are affected when changing between them,

 

Firstly the brakes behave differently, skidding the small wheels is easy to do and if on tar they are VERY responsive that can cause you some grief whereas the bigger wheels have more leverage on the calipers which makes them in my case impossible to skid unless on loose gravel.

 

Secondly the difference in flex, the standard smaller tyres are relitively rigid and will be affected a LOT by ruts and gutters and bumps whereas the bushwheels will handle moderate bumps and ruts without affecting your direction at all. (I have a video of me landing on a dirt road with some reasonable ruts, I'll try and find the link.)

 

I guess when deciding on bushwheels or standard I would be more worried about the sort of ground I was going to be landing on rather than directional control issues as the difference in handling different surfaces makes a lot more difference. Bushwheels DONT like tar. (Wears them out quick smart)

 

Hmm not sure if the video will work, but if it does take note of the size of the rut at the end when I turn and they were all a similar depth. The big wheels weren't affected by them at all. (Yes yes I was well over to the left but I wanted to be ready to turn so it was deliberate.)

 

[MEDIA=vimeo]104490618[/MEDIA]

 

 

[SDQDI]

Well look at that it worked twice

 

 

[turboplanner]

When you do the calculations to determine startability, gradeability, and maximum speed of a vehicle, there is a figure for rolling resistance which varies by:

 

Surface, e.g. concrete, bitumen, dirt road, soft sand.

 

Tyre, e.g. Crossply, Radial, inflation

 

I would think the rolling resistance on those tyres would be more, adding to the takeoff roll, but also playing a greater part in yaw dynamics due to the even resistance on the ground.

 

Whether this would be miniscule or significant in terms of improving resistance to yaw would be an interesting experiment.

 

 

[dutchroll]

Put simplistically: Better on rough, unsealed strips. Worse on sealed/paved strips.

 

The reason is mainly the amount of rubber in contact with the ground. On a sealed strip, more rubber and a bigger tyre creates more drag and grip, so they're consequently less forgiving of any directional control issues in taildraggers. Not so much an issue if you're landing on grass, dirt etc where you need that extra grip anyway. At least that's according to pilots experienced in their operation in North America.

 

It does also vary from aircraft to aircraft too. Some aircraft change their handling dramatically with the use of fat tyres. Others not so much.

 

 

[Happyflyer]

I agree with dutchroll. I almost always land on grass, dirt and sand and have found no differences between bushwheels and more conventional tyres in regards to directional stability. I have noticed the bushwheels transmit a lot less shock to the airframe over bumps.

 

 

[poteroo]

Certainly agree with comments about operations on sealed surfaces. LP fatties are great on rough and soft ground - but be very careful when braking with them in a taildragger. If you are using a wheeler - make it 'tail low' so that there's less pivot over the mains as you put the anchors out.

 

If you are going to consistently operate larger LP tyres off seal - pump them up several psi higher so that less rubber is gripping. You'll find that there is a pressure at which the aircraft will handle with less drama. Next best step is to ignore the seal and just land on the grass outside the lights. Who'll ever see it, and much less care?

 

happy days,