Aircraft Question pertaining to wheels.

When an aircraft comes in to land, the wheels have to skid along the ground until they are a ground speed. Seeing a large aircraft coming in to land they seem to rip off quite a bit of rubber on each landing judging by the screech that the tires make on each landing.

I cannot understand why that aircraft manufacturers have not made small air scoops on the wheel rims so that when the landing gear is extended the wind caught in these scoops would rotate the wheel up to wind speed before the tires touch the ground.

there must be a reason that this has not been implemented, but I cannot see why not.

I used to wonder why the wheels didn’t have motors to get them up to speed, but then it just seemed like more weight and parts to break for something that didn’t seem to be a problem. As for air scoops, it seems like a good idea, but my guess would be that the wheels are just far to massive for the wind to bring them up to speed and/or any scoop big enough to bring 16 wheels (can’t find a weight on them) up to speed would cause too much drag on the plane. Also, since that drag is on the bottom, it would bring the nose back down. Of course, the pilot could adjust for this (in fact, I think the computer would do this automatically) but I’m thinking that they’ve decided that replacing the wheels is just easier and cheaper. I’m sure there’s a better answer since I’m sure the aeronautics engineers have gone over this question a thousand times. In fact, I would bet the EPA has pestered them about it too since I’m sure they would prefer that they wouldn’t go though as many tires as well if at all possible.
How often do the tires on a plane get changed? Sure they skid every time they land, but they might only land 5 or 6 times a day, it’s not like they’re peeling out of red lights every 10 minutes and having to replace their tires at 10,000 miles.

Aircraft tires are many multiple layers. A Boeing 737 tire is anywhere from 12 to 30 ply… that is a TON of rubber that can burn off. It also makes an aircraft tire heavy, more than 200lb on a main landing gear tire - and harder to spin up to speed with theoretical air scoops. Wouldn’t want those to go breaking off under strain and possibly damage the actual wheel just prior to landing.

I’d imagine that landing with non-rotating tires is a consistent condition for the pilot. If you attempted to spin up the tires, then to what speed? Matching ground speed would result in a no skid landing, but if the wheels were a little above or below this then the plane would handle differently at each landing. And there are multiple wheels that would need to be brought to the same speed. If some of the tires were at 100% of ground speed and others were at 110% and 90%, then the plane may be a handful for the pilot to handle.

It would be like landing a plane on a treadmill.

I asked this question of an aircraft engineer (who worked on Falcon bizjets) and the answer was that the additional weight, cost and complexity make this idea not worth the trouble.

One detail is that when landing gear is retracted there’s typically very little room to spare. Eating up even a small amount of additional space inside the fuselage or wings is something to be avoided.

I could swear that they actually tried this on the XB-36’s original 9’ main tires, in an attempt to get them to last more than a handful of landings. I believe they found that the landing distance was lengthened noticeably. However, the real problem with the gigantic single main tires was the huge pressure they exerted on the airport surfaces. In the end, the problem was sidestepped when Convair replaced each single main tire with a regular 4-wheel bogey, but not before they tried using rubber tracks.

I’m looking for a cite.

Given that the mass of the wheels is small compared to that of the plane, this should not be an important consideration.

It seems like you could avoid wear on the wheels by having the plane land on a moving runway, perhaps some sort of treadmill.

I don’t know how fast a tire wears out on a large jet, but on a small plane (C-172) I know tires last years, even with a ton of touch-n-go landings in the mix. In a small plane, where weight is a critical consideration, adding even 6 or 7 pounds to the empty weight of the aircraft would be frowned on by the owners/pilots of the planes.

I have heard that they tried this in WWII because rubber was so scarce.
The way I heard it they fitted a B17 with electrical motors on the landing gear.
When they lowered the gear and spun up the motors the tires acted like gyroscopes and resisted any change in heading.
I don’t have a cite but it seems to make sense.

The white treadmill is for take-off only. There is no landing on the white treadmill.

No the red treadmill is for landings only there are no take offs from the red treadmill.

Aircraft tires are indeed tough and expensive - and replaced fairly often since their wear is closely monitored. It is just part of the cost of having to maintain a safe aircraft. The airlines are famous for figuring out expenses and cutting costs, yet even they have not found a way around this basic fact.

I too read about the B-36 experiments, in a reference book about those planes somewhere in my collection. In the end it is cheaper to buy new tires, than to add the weight to a plane of some complex wheel-spinning motor - and the potential for mechanical mishap that goes with it. Keep it simple is still a valid rule in aeronautical engineering.

Like so many other things, it’s being done the most economical way already, and in the case of aircraft that includes the safety factor as well. Any extra weight on an aircraft costs a lot of money in fuel and smaller payload. And a spin up system that could cause a wheel to lock up would be extremely uneconomical for the aviation industry.

If you can find a picture or bit of video I’d love to see it. My google-fu has been failing me all morning, but I just know I saw film of the experiment in a documentary, possibly 15 years or so ago - I think it was probably on a Discovery Wings episode. I can’t find any reference to it whatsoever. I guess it could have been on a different airplane, but I really thought it was on the B-36. Am I going crazy? :confused:

You’d be surprised. A 200g modification to an aircraft in my company requires an evaluation by the weights and balance department and potentially flight mechanics. There really is no trivial amount of weight or complexity when it comes to aircraft design. Adding motors to landing gear would require examining the weight issue but also the electrical and power demands. You’d have to design the wire harness, modify the genetators to power these motors, potentially modify part of the engines. You’d need to fully evaluate failure modes as well. It really is a big project to add to the cost of a plane , and it’s an unnecessary one. I expect it wouldn’t be doable to retrofit existing fleets, and every single type variant would have to be looked at separately.

I think you may have missed the point of the original comment by Dag Otto, which pertained not to weight and balance but to transient forces as the wheels spin up (or possibly down) upon touchdown.

I’d be curious to hear from an aerospace engineer how much rubber actually comes off of the tire due to:

A) tires spinning up at touchdown, and

B) braking from landing speed to taxi speed.

Decel after touchtdown doesn’t produce the same dramatic puff of white smoke as touchdown, but decel represents a LOT of energy being dissipated into the tread itself (not just into the brakes).

Just for fun, here’s a video of an A380 landing gear drop test. Rather than having the test rig move at 130MPH as it touches down, they spin the wheels in reverse up to 130MPH and then drop the rig straight down; the reaction forces at touchdown end up being the same as a real landing.

And then here’s an A380 brake test. This simulates what might happen in an overweight/overspeed landing situation (we had this discussion here a couple of weeks ago regarding a plane over Las Vegas whose crew decided to burn off fuel for five hours rather than land overweight). In this simulation, touchdown speed was about 200 MPH, and landing distance was about 3/4 of a mile. If the brakes got hot enough to catch fire, you can be sure a fair amount of tire tread was also sacrificed.

I do believe that it would be possible to punch scoops into the wheel itself without adding any weight and without sacrificing strength. I am quite sure that even small scoops would spin the heavy wheel eventually up to wind speed. The mere fact that someone has tried to do it with motors shows that they would prefer the tires to be spinning close to ground speed if at all possible to limit tire wear on landing.

You’re right, I did. I was reading on my phone in the car…I completely missed that. :slight_smile: