Whenever I see an airplane land, I see a brief but large puff of smoke
emitting from the wheels hitting the runway. Why don’t they spin up
the wheels before hitting the pavement to prevent unnecessary wear and
tear to the wheels and runway? I’m sure this would save the airlines a
lot of money in routine tire replacements.

This question has been asked before, and the usual response, by pilots
at least, is they prefer it that way so that they can actually feel
the aircraft contact the runway on touchdown.  I suppose that makes
sense, though if I were landing, you wouldn't be able to feel it
anyway ... :-)

--Byrd
Commercial Pilot, Instrument Airplane, ASEL  
Google Answers Researcher

aren't most airplane wheels not powered? obviously it wouldn't be that
hard to change it, but I believe that usually the pilots wouldn't have
a way to spin up the wheels. Besides, I've heard arguments both ways:
doing so would increase tire life and doing so would decrease it. Even
so, I doubt the added (or lost) wear is all that noticeable when
compared to what it takes to actually stop the plane. I have also
heard arguments about what some of the other benefits of spinning them
up before landing might be, but I'm not convinced that they're even
worth the trouble.

I don't know if the following is accurate or not but from what I'm
aware of, jets use their engines to brake during the landing. The
brakes on the wheels are only used for parking and slowing down during
already slow speeds. So the brakes alone shouldn't have a significant
affect on the life of the wheels. Please correct me if I'm wrong.

The head of one of the law firms I use had this 
exact some question and an idea to fix it, get
them spinning before they hit the ground.  My guess
is that there is little chance to get them coordinated
as far as landing speed, so you may make things
even worse - remember, when the wheel touches
down and presses down on the tire, it stops very
briefly and deforms.  Not sure this is the answer
but could be.

wainscott, I'm hardly an expert in the area either. Still, I believe
the wheel brakes play a larger role than you think. Planes certainly
do use many devices to slow them down during landing, but I don't
think they completely replace the brakes. I've heard of pilots having
knee problems from braking during landing.

Maybe byrd could clear up this issue for us.

Jets use their thrust reversers to slow down, not brakes.
Usually a translating sleeve on the engine or a cup
that deploys behind the thrust exit of the nacelle.

I'm not an expert either, but neither does it stop me :-) ...

Larger airplanes use both engines and brakes to slow down after
landing.  Soon after touchdown, a jet pilot would engage
thrust-reversers.  They redirect much of the exhaust in the forward
direction rather than backward.  This is the roar you hear soon after
touch down, when thrust-reversers are engaged and engines are cranked
up.  If you can see the engine, you can watch the reversers is in
action.  It doesn't last long, say 10 seconds or so.  I'd guess this
kills 30-50 percent of the speed.  Brakes are probably also engaged at
the same time, but then more forcefully after thrust-reversal is no
longer effectual.  Spoilers are also deployed and ailerons are fully
pushed forward (up) to make the wing surfaces grab as much air as
possible and push the airplane firmly onto the ground.

In a larger prop plane, the pitch of propeller blades may be reversed
to do the same thing as thrust reversers, that is, the angel of the
blades is swiveled such that they push air forward rather than
backward (the engine cannot turn backwards, of course).

On the original question, airplanes wheels are not powered.  A plane
moves via it's engines, being pushed or pulled by a tug on the ground,
or by the wind.

On why wheels aren't spun up prior to touch down, this is a very
difficult problem.  First, ground speed would need to be calculated,
as this is different than airspeed.  Airspeed is the combination of
wind speed and it's direction plus ground speed. Modern airliners
probably calculate ground speed via some sort of radar or maybe GPS
positioning. So, if you had this number, then you'd need motors on the
wheels to spin them.  They would need to be powerful enough to spin
the wheels and able to react quickly to speed changes since that's
constantly changing in the landing approach.  The killer is - and this
why it's not done - this would add a lot of weight to the gear. 
Weight, of course, is money, so if weight can be avoided, it is.  It's
also more complicated, and complicated things tend to break.  Thus,
it's cheaper and safer to burn some rubber on the tires rather than
spin the wheels to ground speed.

Lastly, if you're thinking those wheel motors could move the airplane
around on the ground, think again: an airliner fully loaded (with
passengers and fuel) weighs several hundred thousands of pounds. 
Those motors would need to be very powerful - as powerful as that big
tug you see pushing them back from the gate.  Way too heavy in other
words.

F - certified airplane passenger

The biggest issue with adding the capability to spin up aircraft
wheels is the added weight of such a device. Adding weight to an
aircraft means adding fuel. Adding fuel is adding costs.

Another not so obvious result of adding the new system is that you
would have another system to maintain. Your just adding complexity to
a system that doesn't need it.

krobert-ga
Aerospace Engineer

Ok, instead of a motor, how about a bunch of cups attached to the
wheels (like a weather vain or pinwheel) so the wind would spin up the
wheels. Although this might cause some drag, it has no moving parts
and would never need upkeep.

Pinwheel?  I doubt that would work.  As feaster stated earlier, most
jets use not only wheel brakes to slow them down as well as engine
brakes(in the form of reverse thrusters).  Most operators, however,
tell the pilots to use the wheel brakes as little as possible, because
the engine brakes work just as well, and brakes are very expensive to
repair.

Back to the original question...

Airplanes do not spin the wheels before touchdown simply because of a
safety issue.  The plane flying at a little bit over 100 kts while
landing is fairly unstable in itself, but to add spinning wheels to
the whole picture, would make the landing not only almost impossible
on the pilot, but just as stessfull on the wheels themselves as not
having them spin.

As far as money spent on replacements, a good pilot, as most
Commerical Pilots are, will be able to land the plane very smoothly. 
Normal tires will give you atleast 750 cycles(takeoffs and landings) 
And if you think about it, new tires for planes are alot cheaper than
new tires for cars.  For a average commerical jet, new tires would run
somewhere between 25-50 dollars a piece, maximum.   Which is alot
cheaper than  developing and maintaining a device to spin the wheels
on a airplane during landing.

Hope this helped,
Kenneth Webster
Certified Commerial Pilot

I am an engineer that has been working on this issue for the last
several years.  There are actually two patents on fins that pre-rotate
the landing gear.  Although something is patented, it is not always
embraced by the airlines or the tire companies.  1) The device(s) add
extra weight to an airplane - and nobody likes extra weight 2) The
tire companies have no interest in seeing reduced sales based on
extended tire life.  Therefore, you are left with the status quo.

"Ok, instead of a motor, how about a bunch of cups attached to the
wheels "

Another problem with this design would be the added drag that would
occur.  This would of course be good for landing, but not a good idea
for take off when you want to increase air speed quickly.

I haven't seen the patented ideas, perhaps they solve this?

A few people have commented on how difficult it would be to get the
wheels spinning to the airspeed of the aircraft.  This would not
necessarily be needed, as whatever speed the wheels are spinning, some
wear of the tyres would be saved.

A final thing, just for interest.  The field length of an aircraft
(the length distance it needs to land safely and determeines what
airfields it can use) is based on wheel braking only, not with any
other devices such as thrust reversers.  This is because in a
worsst-case situation the aircraft is gliding in with no engines.  The
brakes are powered by hydraulics, and use accumulators(?) to store
pressure in the event of a loss of pressure elsewhere in the system. 
I don't know more than that, as my area is aircraft structures, not
systems!

Regarding the following posting;

From: nick_s-ga on 25 Dec 2004 17:51 PST 

My son is working on a science project (4th grade) on this very
question.  Accordingly, could you possibly provide the patent numbers
that you refer to.  He has a fairly elegant solution, however, the
idea presented has to be original.  Therefore, I would like to review
the patents to verify that there is no conflict.

Thanks for your help.