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Possibly naive technical question

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Possibly naive technical question

Old 1st Oct 2010, 16:44
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Possibly naive technical question

I expect this has been asked before, but why are aircraft not fitted with a motor or an aerodynamic device to spin the wheels up to speed before touch-down? I would have thought the savings in tyre wear and general wear and tear would be considerable, but the idea is so obvious that there must be a good reason why not.
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Old 1st Oct 2010, 17:06
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The cost in extra fuel burn of carrying the weight of the motors around for the life of the aircraft exceeds the cost of the extra rubber that gets burnt off.

Not spinning up the wheels before landing improves the deceleration of the aircraft on touchdown.

If the wheels were not spun up equally, ie a motor failure on one side, you would have asymmetry and possible contol issues on the landing roll.
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Old 1st Oct 2010, 17:12
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after take-off

After an aircraft takes off how long do the wheels spin for? i.e. are they still spinning when they are retracted into the landing gear compartment?
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Old 1st Oct 2010, 17:57
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I won't presume to speak for all types, but ours either apply a brief spot of braking or have some other means to stop the wheels spinning as part of the retraction sequence.
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Old 1st Oct 2010, 18:05
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Tests have shown that the reduction in wear of the tyres is minimal. Wheel speed is related to groundspeed and aircraft do not have ground speed measurement. Whatever method would be used to spin up the wheels, would be added weight. This means added fuel burn to carry this weight during flight. The system would also require maintenance; cost of ownership. All in all, a non starter.

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Old 1st Oct 2010, 18:44
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This was actually tried in World War Two. Due to the shortage of rubber in the early days of the war, the powers to be thought this to be good idea to decrease tire wear on landings.

A B-24's wheels and tires were modified with vanes attached that would spin the wheels up to the same speed of the aircraft. Sounded good and it was a simple idea with very little weight penalty.

One problem though. When the wheels spun up to the speed of the aircraft, the gyroscopic forces caused by the spinning wheels and tires was so strong, the aircraft could not turn.

The project was dropped.
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Old 1st Oct 2010, 21:12
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Purplehelmet

It was mentioned in the Concorde thread that that aircraft's nosewheels were fitted with an automotive disc brake. This wasn't used during landing, when the nose undercarriage was unbraked, but was solely used to stop the wheels spinning before retraction. Presumably the main brakes did the same for the main undercarriage.
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Old 1st Oct 2010, 21:57
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Thanks guy's, id never given it much thought until a few weeks ago, whilst i was watching a us airways a330 depart egcc i noticed that one of the wheels on the rear under carriage had a white line (painted)on it,as the gear was raised i could still see it spinning until it moved out of my line of sight.
which made me wonder if they stopped before they were fully retracted.


CON-PILOT.
it also got me thinking about how much gyroscopic force must be created by the gear at this point of take-off,and what effect if any it would have on the aircraft..

cheers ph.
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Old 1st Oct 2010, 22:53
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Gyroscopic force would be produced when a still-spinning wheel had its axis of rotation (i.e. angular momentum vector) changed. All the while port and starboard retract simultaneously in a mirror sense, this would not produce any effect.

But a spinning nosewheel retracting sideways, for example, as in the beloved Trident, would cause (by my calculation) a net yawing force on the airframe. Any Groundgripper drivers out there care to comment?
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Old 2nd Oct 2010, 21:27
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By the same token, braking the wheels once in flight would produce a net pitching moment. Would that be significant?
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Old 2nd Oct 2010, 22:53
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A B-24's wheels and tires were modified with vanes attached that would spin the wheels up to the same speed of the aircraft. Sounded good and it was a simple idea with very little weight penalty.

One problem though. When the wheels spun up to the speed of the aircraft, the gyroscopic forces caused by the spinning wheels and tires was so strong, the aircraft could not turn.

The project was dropped.
I used to fly B-24's. Specifically, a variant of them which used the same airframe, save for the vertical and horizontal stab. Rest assured, there is nothing about wheels spinning while the airplane is airborne which prevents turning, and the gyroscopic forces aren't that great. I've stopped the wheels before and during retraction, and simply let them retract while spinning, while maneuvering, and it has no appreciable effect on the control of the airplane.

Standard procedure was to apply gentle braking shortly after takeoff, to stop tire rotation; not because spinning wheels made aircraft control difficult, but because the other method of stopping them was rubber strips in the gear well, and it reduced vibration. If the gear was retracted without stopping the spinning, it would be stopped in the well. A number of modern aircraft use a similar method, or use backpressure in the brake system to stop the wheels during retraction.

it also got me thinking about how much gyroscopic force must be created by the gear at this point of take-off,and what effect if any it would have on the aircraft..
Nothing of significance.

By the same token, braking the wheels once in flight would produce a net pitching moment. Would that be significant?
No. It produces no pitching moment.

I expect this has been asked before, but why are aircraft not fitted with a motor or an aerodynamic device to spin the wheels up to speed before touch-down? I would have thought the savings in tyre wear and general wear and tear would be considerable, but the idea is so obvious that there must be a good reason why not.
Weight and complexity. There's no reason to spin the wheels up. In fact, spin-up logic is used to arm anti-skid systems, and to perform certain functions during landing with systems such as autospoilers, autobrakes, etc.

Most large airplane operations (airlines, etc) use re-tread tires; these are very common. When wheel wear becomes significant, it's not the loss of rubber that's the issue (as in a tire getting thinner), but balance. Tires are resurfaced or "retreaded." Rather than add cost and complexity to an airplane by spinning up wheels, it's less expensive, less complicated, and safer to simply retread the tires.
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Old 3rd Oct 2010, 15:41
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sns3guppy.
Thanks for the info
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Old 3rd Oct 2010, 23:14
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The Lockheed XR60-1 (XR6V-1) Constitution, which first flew in November 1946, used elecrtric motors to spin the main landing gear wheels prior to touch down. The reason for spinning the wheels was not to save wear on the tires but to reduce the stress on the landing gear.
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Old 3rd Oct 2010, 23:31
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You will notice on some rotations a grey mist appear from the undercarriage as the bogeys are retracted. The grey mist is carbon from the 'brake discs' stopping the wheels turning prior to retraction.
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Old 10th Oct 2010, 11:10
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wheel spool up

Hi,

I proposed an idea of having vanes on the wheels, to spin them up, to Handley Page and de Havilland back in 1959.
They both thanked me but said the weight penalty would be too great.

Dave
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Old 12th Oct 2010, 07:50
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I am almost certain I read (years ago) that Handley Page used turning vanes on the wheels of there WWI bombers to try and prevent tail overs.

I could be very wrong of course.
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Old 13th Oct 2010, 19:22
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Some aircraft have brake pads in the top of the wheel well which rub against the tyres to stop them spinning.These are usually on the nose wheels where the retraction is in line with the spinning axis and there are no gyroscopic forces in play. These snubbers can make some interesting noises! Nose wheels are generally,but not always, unbraked. Main gear legs that retract sideways would have a gyroscopic problem if the wheels continue to spin whilst being retracted, so the wheels are automatically braked as part of the retraction sequence. The gyroscopic effect puts a twisting strain on the undercarrage leg fittings and is to be avoided. If I remember correctly, on a 757 for example, if one of the wheel brakes is inop(each of the four wheels on each main leg has an individually powered brake), apart from a T/O LNDG performance penalty,you had to wait for two minutes after takeoff before retracting the gear to allow the wheel to stop spinning.
On light aircraft with retractable gear,it is usual to dab the brake pedals before selecting the gear up for the same reason.
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Old 13th Oct 2010, 20:27
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On the 747, we have no wait time when retracting landing gear if a brake is locked out or inoperative. Perhaps the 757 is fragile.
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