Why don't they...?

[Marty_d]

Ok, there's some very smart people reading this forum, so can I just put the question out there.

 

Why don't the wheels on aircraft get spun up prior to landing, so they don't have to go from zero to bl**dy fast in nothing flat? What does a fully loaded 747 come in at... 150 knots or so?

 

I'm thinking of pop-out vanes or cups on the wheel hubs, automatically deployed when the gear is locked down. Don't know how much spin they'd put on the wheels, but given the airspeed on approach and the weight of the wheel would be known factors, someone should be able to figure out the size and shape of the vanes required to spin them up... surely?

 

Obviously this idea would have been considered and rejected by aircraft manufacturers. But why?

 

 

[Eric McCandless]

I've also thought the same thing. Surely it would save tyre wear and thus money.

 

 

[bexrbetter]

Marty, was just considering that yesterday!

 

My thinking was generator brakes, spin 'em up then slow you down.

 

Alternately why not just knobby tyres with spats so the drag on the exposed knobs keeps the wheels spinning permanently.

 

 

[503]

Sounds like more un common sense

 

 

[Ryanm]

Simply put... weight... any motor to spin the wheels (there are a lot of them) would add more weight. I'm guessing the weight penalty would outweigh the cost of replacing worn rubber.

 

 

[geoffreywh]

for airliners the story goes that it has been tried but it was found that the airliner could not stop (in time).. They need the initial braking force of the wheels being spun up at the landing...

 

 

[bexrbetter]

Sounds like more un common sense

Of course it is but active minds occasionally come up with a winner - and it's fun too, you should try it!

 

 

[Deskpilot]

My memory's a bit vague these days but this has been tried before. It caused more failures than successes mainly because the wheels ended up spinning too fast.

 

I can't remember if they were spun mechanically or via vanes on the wheels.

 

 

[Marty_d]

I did a bit of googling on this and found a heap of reasons (most which don't make sense), but 2 in particular did:

 

1:

 

Spinning up of the wheels before landing was tried many years ago. The gyroscopic effects of those large rotating masses apparently caused the airplanes to make some strange gyrations on touchdown causing the plan to be abandoned.

 

 

 

2:

 

 

 

Large aircraft have a system on the aircraft for anti-skid and also part of that is wheel not turning light. If that pre-spin system was there in icing conditions or turbulence it may only spin one wheel or any combination. Setting off the anti-skid system prior to touchdown. This could make the landing tricky at times. The weight and cost of fins or other systems to do this pre-spin might also be heavy or expensive.

 

 

 

Another problem is that the spoiler panels that come out after landing are tied into Main-wheel spinup. I don't know if that would be a huge problem but I think it would be interesting applying fixes to that system too.

 

 

 

I think my main point is that it is just a lot easier to change the tires. Plus a fresh piece of rubber is always nice to have. We dont want them to last too long and I think that the complexity of the pre-spinning wheel will be too much for very little gain. One other point is it would take a little longer to stop the Aircraft due to the initial overcoming of static-friction.

 

 

[Billzilla]

Why don't the wheels on aircraft get spun up prior to landing, so they don't have to go from zero to bl**dy fast in nothing flat? What does a fully loaded 747 come in at... 150 knots or so?I'm thinking of pop-out vanes or cups on the wheel hubs, automatically deployed when the gear is locked down. Don't know how much spin they'd put on the wheels, but given the airspeed on approach and the weight of the wheel would be known factors, someone should be able to figure out the size and shape of the vanes required to spin them up... surely?

The 747 Classic freighter I used to fly would approach at 157 kts at max landing weight and touch down at around 150 kts so that's a very good guess.

 

There's the problems mentioned above though gyroscopic precession isn't a problem because you'd spin the wheels up once they're already down.

 

One of the larger problems, as far as I know, was making sure that all the wheels were spinning at exactly the same speed left & right for touchdown because if they were at different speeds the aeroplane would tend to yaw otherwise. So it was easier to just have them all at zero speed.

 

It wouldn't be all that difficult to have ribs moulded into the side of the tyres to help them spin up, but as mentioned there's no guarantee that they'd all be spinning at the same rate.

 

 

[Guest Howard Hughes]

When my father worked at Qantas 30+ years ago, I believe this was suggested by one of the employees and given serious consideration. Despite all of the very plausible reasons above, I understand the reason they were not implemented, is because if the additional vanes on the tyres to make them spin were damaged, this would cause the wheel to become unbalanced, with potential catastrophic consequences.

 

 

[503]

Of course it is but active minds occasionally come up with a winner - and it's fun too, you should try it!

What I was trying to say is its a good idea (vanes on wheels or tyres) (common sense is not that common so it should be called uncommon sense)

 

 

[Marty_d]

One of the larger problems, as far as I know, was making sure that all the wheels were spinning at exactly the same speed left & right for touchdown

I would have thought that could be got around by an addition to the braking system that automatically applies brakes to whichever wheel is spinning faster than the slowest. Having said that, you wouldn't want any brakes to be applied while touching down, so if it turned itself off at say 50 ft there's still a chance of differential wheel speeds.

When googling this I noticed someone said that the Electric Lightning had pop out vanes to rotate the wheels (http://uk.answers.yahoo.com/question/index?qid=20090501080607AAfumUB), I can't verify that anywhere else though so may be BS. On the same page someone else said some Lear jets were fitted with compressed air pre-rotation for the nose wheel to avoid kicking up debris into the engines when landing on dirt runways (the "drug smuggler" option??)

 

 

[old man emu]

The simple answer is: weight and complexity. Why trade off payload to save on an item that is designed to wear out? Why add a complex mechanism that will incur extra costs in the overall maintenance of the airplane?

 

I doubt if the damage to tyres is done by getting from 0 kts to landing speed. I would think that the damage was done by vertical forces on the tyre caused by dropping the plane onto the ground. Therfore, the better a pilot is at greasing the plane on, the longer then tyres will last. Also, keeping feet off the brake pedals at touchdown is a great tyre saver.

 

I can't see the logic in using the brakes to stop a plane on a long runway. Why not let it roll out at along the runway until normal retarding friction between the tyre and the ground surface slows the airplane to taxying speed? Is it so important on the landing strips we use to vacate at the first taxy-way?

 

 

[M61A1]

I have pondered this a few times, not only for landing, but to decrease take off roll, particularly the initial from start to rolling bit which takes a lot of energy when you are only pushing against air.

 

 

[Marty_d]

I have pondered this a few times, not only for landing, but to decrease take off roll, particularly the initial from start to rolling bit which takes a lot of energy when you are only pushing against air.

When I was hang gliding we used a 4.0 litre motor to reduce takeoff roll... we were towed up by an old XC Falcon. That was fun, but not sure I'd be brave enough to do it these days.

 

 

[Billzilla]

I would have thought that could be got around by an addition to the braking system that automatically applies brakes to whichever wheel is spinning faster than the slowest. Having said that, you wouldn't want any brakes to be applied while touching down, so if it turned itself off at say 50 ft there's still a chance of differential wheel speeds.When googling this I noticed someone said that the Electric Lightning had pop out vanes to rotate the wheels (http://uk.answers.yahoo.com/question/index?qid=20090501080607AAfumUB), I can't verify that anywhere else though so may be BS. On the same page someone else said some Lear jets were fitted with compressed air pre-rotation for the nose wheel to avoid kicking up debris into the engines when landing on dirt runways (the "drug smuggler" option??)

The problem is that the brakes used on the airliners aren't sensitive enough to control low wheel speeds like that when they have no load on them. Just the lightest touch and they'd stop the rotation. You'd need a much smaller independent brake system .... so more weight and complexity.

 

It's not uncommon for the smaller jets to have a bleed air system to spin-up the nosewheel for landing on gravel runways. We had it on the Citation 2 I used to fly in PNG and to make sure the nosewheel got enough speed I'd plan for a short level segment before landing so I could spool the engines up to make enough bleed air to the desired RPM.

 

I doubt if the damage to tyres is done by getting from 0 kts to landing speed. I would think that the damage was done by vertical forces on the tyre caused by dropping the plane onto the ground. Therfore, the better a pilot is at greasing the plane on, the longer then tyres will last. Also, keeping feet off the brake pedals at touchdown is a great tyre saver.I can't see the logic in using the brakes to stop a plane on a long runway. Why not let it roll out at along the runway until normal retarding friction between the tyre and the ground surface slows the airplane to taxying speed? Is it so important on the landing strips we use to vacate at the first taxy-way?

By far most of the damage is done when the tyres go from zero to 150 kts in the blink of an eye and have hundreds of tonnes press down on them to make that happen. There's a bit of extra wear when you turn tight corners as well, due to the nature of the dynamics of the turn and the usual four-wheel truck system - It's why the body gear on the 747 steers in the opposite direction to the nosewheel and also the last two pairs of wheels on the 777 six-wheel trucks.

 

Airliners very definitely require brakes for stopping on the runway. We tired a few times, even at light weight in the simulator, to stop the 747 with reverse alone and even on a 4,000 metre runway we pretty much always went off the far end at about 60 kts. If you have traffic behind you at a controlled airport they sometimes dictate which taxiway to exit from, or it's also sometimes in the Jepps. If you can't make it, you tell them in advance.

 

Airlines are a bit different though, the crew don't have to really look after the brakes as they are an expendable item .... though you have to really watch the brake temps for the next sector.

 

 

[Marty_d]

Hi Bill,

 

I just had a look at your website and love the "scary" flying stories. Have you ever submitted a "near miss" story to Flight Safety magazine?

 

 

[Billzilla]

Hi Bill,I just had a look at your website and love the "scary" flying stories. Have you ever submitted a "near miss" story to Flight Safety magazine?

Yes one of them - http://www.billzilla.org/flying2.html

 

 

[Marty_d]

Thought it was familiar!

 

 

[Guest Pontius Pilot]

I have pondered this a few times, not only for landing, but to decrease take off roll, particularly the initial from start to rolling bit which takes a lot of energy when you are only pushing against air.

I think I might be misunderstanding what you've written. Are you suggesting a system be installed whereby it helps turn the wheels at the beginning of the take-off run? If so, maybe you could look at some simple physics calculations regarding inertia, acceleration and the power required of the motor to shift a 396 tonne aircraft (744) during the initial 'start to rolling bit'. When you come back with a truck motor more usually found in the WA mines and the accountants screaming at you, wondering where their payload has gone, you'll get a pretty good answer to your question.

 

 

[Marty_d]

They devised a system for reducing take off roll for the C130 (and probably others) - RATO (Rocket assisted take off).

 

You'd probably need quite a few for a 747 and I can just see the passenger's faces when they fire up.

 

 

[Gnarly Gnu]

Do they still retread the tyres on the big birds? Maybe the rubber isn't costing that much? Bextyres from China r cheap n r good.

 

 

[Marty_d]

Regardless of the cost of the rubber, it must cost a heap to take the plane out of service for however many hours it takes to change them.

 

 

[Guest Pontius Pilot]

For obvious reasons they change the whole wheel assembly on the line, rather than just the tyre but I've seen them do a wheel change on the main gear of a Jumbo in around 25 minutes. It's the paperwork that takes the time.