Why not spin the wheels before touchdown?

dohouch

To avoid tyre flat-spots.

This could be done either by adding fins to to the wheel hub, or maybe even by an appropriately designed tyre thread.

Davidth88

you know that's a good question , you always see a puff of smoke as a heavier aircraft touches down

shedweller

Never heard of flat spots on plane tyres but then again i've never looked. I'd say any wear is minimal over the life of the tyre. It certainly leaves rubber though. Is there much of a braking effect i wonder? If they were spinning upon touchdown would the brakes have more work to do? I suspect its all minimal and not worth the effort in any case.

Skatedude

imagine how much extra weight and complexity would be needed on a 747 etc. probably better to keep it simple as possible. less to go wrong.

Comhrá

I believe this was researched years ago in the UK but I have no link.

It was found to be counter-productive in the long run (Sorry for that pun!)

One of the reasons was that rolling gear prior to touchdown increased the stopping distance considerably, thus increasing brake-wear and necessitating longer runways. Apparently it is cheaper to replace tyres than replacing brake components. Makes sense really, I suppose.

murphym7

I suppose for this argument you are assuming the same part of the wheel hits the runway each time. I am not sure this is the case, the wheels do rotate when taxing, landing and taking off, I am not sure the tyre would hit in the same place each time. As was said, this "rotating" equipment results in added complexity, more weight, something else to go potentially go wrong - and also something else that needs to be designed, validated, tested etc.......

I am not sure it is a problem that needs fixing. Looking forward to the experts opinions though.

Lurching

The tyres would only flat-spot if the brakes were on as the tyre touched the ground. This is not the case and the tyres begin to turn at first contact.

If you think about it, the tyres on a car or bike only flat spot when you lock-up.

seanyleary

Only ever seen flat spots if the brakes lock up due braking problems. They don't flat spot under general wear and tear.

radar0976

dohouch wrote: »

To avoid tyre flat-spots.

This could be done either by adding fins to to the wheel hub, or maybe even by an appropriately designed tyre thread.

Really interesting question. A similar discussion took plance on airliners.net

http://www.airliners.net/aviation-forums/general_aviation/read.main/376352/

From reading this and a few other sources it seems that it has been considered and tried in the past. It seems that it causes more problems than it solves though with the gyroscopic effect of spinning the wheels while in the air making the aircraft handling characteristics change.

smurfjed

Never heard of flat spots on plane tyres but then again i've never looked

We get them

I'd say any wear is minimal over the life of the tyre

It really depends, we can see all kinds of cuts, tears etc and flat spots, we obviously don't land with the brakes on, but that initial contact does wear the tyre. We can wear them down to zero rubber in one spot before they have to be replaced.

If an aircraft has brakes deactivated on takeoff, they have to leave the wheels down for 2 minutes to allow the wheels to spin down and stop, otherwise the gyroscopic effect when they are retracted is pretty severe. So this would restrict you from attempting to spin the wheels to speed prior to extending them, plus you really dont want a spinning tyre in the wheel well. (Google DC8 crash Jeddah and you will see why). So you would have to get them spinning when extended. The complexity of a motorised system and the additional weight would be one limiting factor. As for vanes on the wheels, considering that the aircraft is flying at 150 mph, is this a sufficient airflow to rotate the tyres to a spinning speed of 150 mph? If it isn't, then what exactly have you achieved?

smurfjed

LeftBase

It would increase braking distance/ground roll. The spinning system would be working against the brakes as they were applied and the cost of new tires would not outweigh the cost of brakepads/brake fires/possible reduced landing runway length limits.

The spin speed would have to vary depending on ground speed on touch down too. That would be far too complicated to sort out. With a 20kt headwind the wheels need to spin 20kts slower than in still wind to have a "seamless" touchdown.

smurfjed

It would increase braking distance/ground roll.

Why?

smurfjed

LeftBase

smurfjed wrote: »

Why?

smurfjed

If you have a vane system on the wheels as suggested above it will act to keep the wheels moving forwards in some way while it has airflow over it...this is countering the brakes. One wants to push the wheels forward and the brakes want to stop that happening. You have 2 systems countering each other.

You would also have the issue in crosswinds or general operation of the wheels spinning at different speeds on touchdown.

ozzy jr

Another "Why don't they" question.

Rather than start a new thread I thought I'd stick it in here.

Why don't they tow airplanes out to the runway?

Yeah it's a stupid question, but at bigger airports I've often been sitting in a queue for 45 minutes to an hour. All this time they're burning fuel. Would it make sense to tow a plane out to the runway to cut costs?

Probably not, but I'll ask it anyway

A320

well ozzy one of the the new big thing being developed by Lufthansa is electric taxi system,minimizing fuel use for taxi

http://www.lufthansa-technik.com/etaxi

Karl Stein

ozzy jr wrote: »

Why don't they tow airplanes out to the runway?

They're looking at it.

Automated NextGen Taxi System

Terrontress

What about aircraft wheels geared to a flywheel, rather than freewheel. The flywheel provides retardation and could even top up batteries.

The only negative I can think of is the weight of the flywheel. Which probably rules the whole idea out!

I have flown with Flybe before, maybe on a Bombardier Dash, and you can see the landing gear retract with the wheels still spinning.

ozzy jr

A320 wrote: »

well ozzy one of the the new big thing being developed by Lufthansa is electric taxi system,minimizing fuel use for taxi

http://www.lufthansa-technik.com/etaxi

Damn, I should have patented it!

A320

Ha there's no flies on the LHT crowd,they've their hand in everything,next up is A320's fitted with LED External lights

andy_g

Terrontress wrote: »

What about aircraft wheels geared to a flywheel, rather than freewheel. The flywheel provides retardation and could even top up batteries.

The only negative I can think of is the weight of the flywheel. Which probably rules the whole idea out!

I have flown with Flybe before, maybe on a Bombardier Dash, and you can see the landing gear retract with the wheels still spinning.

When it retracts the brakes are applied or it hits the brake strip in the wheel well like on the 737

Stovepipe

Andy is right. The wheels vibrate as they slow down and they need to be stopped before they go into the wells as a spinning tyre may throw debris into the well or even jam in the well, so the brakes are automatically applied and the stopping is assisted in some aircraft by a rub strip or bar. In some cases, the act of retraction shortens the undercarriage so that it may fit into a smaller well (BAe-146 and others).

regards
Stovepipe