# Would spinning bush planes' tundra tires in flight be useful?

Short take off and landing bush planes are often fitted with very large diameter and width tires, so called tundra tires so that rough terrain can accomodate to the plane when both meet.

Sometimes those tires' size becomes significant with relation to the whole aircraft's dimensions.

(source)

Those large wheels in flight do only provide drag.

Woud it make sense to spin these tires during flight, so that at least some lift component is created using Magnus force ? The spinning force coming from Savonius shaped hub* or tire profile.

*

• Have you ever seen a clothes washer that has the clothes improperly balanced?
– eps
Apr 6, 2023 at 14:25
• @eps they wouldn't need to spin that fast yet sticking chunks of mud may forbid them to even start spinning in flight.
– user21228
Apr 6, 2023 at 15:01
• Also to consider: once they're spinning fast enough to produce meaningful Magnus forces, would they also be spinning fast enough to create gyroscopic forces that impact maneuverability?
– bta
Apr 6, 2023 at 22:36
• @eps Clothes washers and large tires (particularly for off-road vehicles) are dynamically balanced in the exact same way. Movable weights, in the form of several ball bearings, shot, or a fluid, is introduced into the tire or a ring around the washer tub. They naturally move to a position which minimizes oscillation. You can hear them move in your washer if you manually rotate the tub, particularly those that use a fluid. Apr 6, 2023 at 22:56
• A bush pilot accepts elevated risk, often far away from infrastructure or help. My guess would be minimizing the number of things that can break is more important to a bush pilot than efficiency. Apr 7, 2023 at 16:28

The weight of the drive system would more than cancel out any tiny lift effect you could get from spinning the tires.

• I'm sure this is correct, but I'd be interested in the numbers. The Magnus force on a baseball, for example, may be about 1/3 the weight of the ball itself. A similar effect here would give tens of pounds of lift - probably more than the weight of drive system, but perhaps not so negligible as to dismiss out-of-hand. The net force is surely usually downward in most flight circumstances, but I wonder if there could be net lift in some cases at high speed, low altitude, and with the tires spinning as fast as possible. Apr 5, 2023 at 16:17
• @NuclearHoagie, I was chatting with a fellow mountain biker at a scenic overlook years ago. He was a little on the portly side, but had a really nice bike. As we talked gear he mentioned he had spent around 100 bucks on a titanium seat post and saved a couple ounces, or maybe even grams. I casually squirted about a half cup from my water bottle onto the ground and joked that I had just saved the same amount of weight and it didn't cost me anything. Apr 5, 2023 at 19:03
• @MichaelHall Absolutely agree with all of that, the upfront and maintenance costs, as well as additional points of failure and complexity certainly make it not a worthwhile proposition, especially in niche applications like this. Just pointing out that the Magnus force can be far more than negligible - I'm not convinced that a drivetrain would always outweigh Magnus force by a margin so large that we can dismiss the notion entirely. It's definitely impractical to generate net lift this way, but I'm not convinced it's downright impossible, which I how I read this answer. Apr 5, 2023 at 19:28
• You DO get enough lift from floats to be able to take credit for most of the weight of the floats on a seaplane. Not all, but most. That is, the payload loss is not as much as the floats weigh. But they're quite large and you live with a big speed reduction to be able to operate from water. Apr 5, 2023 at 20:58
• I'm not a pilot, and certainly not a bush pilot — but it seems to me that when the tires maybe-produce lift is also important. If the system produces slight net lift at cruise, while only providing weight on takeoff, then I imagine that's probably a poor tradeoff for a bush planes. As I understand it, they're all about sacrificing cruising efficiency for physical robustness, short takeoff, and short landing. Apr 6, 2023 at 15:15

Sure, you could generate a tiny bit of lift by spinning the tires. The question is, why would you want to?

The wing already generates plenty of lift. By making the tires also generate lift, the pilot will need to slightly lower the nose to keep the airplane at the desired altitude. The slightly lower angle of attack would reduce the drag from the wings very slightly.

However, the drag generated from the tires would increase tremendously. Anything that produces lift produces drag, and that includes Magnus-effect devices. This paper suggests that lift-to-drag ratios of 7.8 are possible. This is already poor compared to a typical airplane (a 747 has an L/D of about 19, according to that same paper). But the device in the question would perform even worse than that.

The efficiency of a Magnus device is dependent on how fast it can rotate*, and (like a wing) its aspect ratio. The slower it rotates, and the lower its aspect ratio, the worse it performs. The L/D of 7.8 in the last paragraph came from a long, thin cylinder (the exact aspect ratio wasn't in the paper) turning at 5.5 times the speed of the air flow. This paper measured an L/D of about 1 for an aspect ratio of 3 and speeds up to 5 times the airspeed. That is an enormous amount of drag for a comparatively small amount of lift. Considering that a tundra tire has an aspect ratio of about 0.5 or less, its performance as a lifting device would be even worse than that.

Of course, the tires are physically much smaller than the wing, and so the albatross you want to hang on your airplane wouldn't be too terribly bad. I don't think that you'd notice too much of a performance drop, but that's just speculation, I don't know that for sure.

* The "speed" of a Magnus cylinder being measured tangentially at its outermost edge.

• this is the whole point; if wheels create lift, the wing can operate at lower angle of attack during cruise, therefore generating less induced drag, total drag being reduced
– user21228
Apr 5, 2023 at 19:29
• @HiddenWindshield I don't think you can guarantee "above the amount of drag they were already generating" part of your claim is true without a lot of very difficult work. Lift requires drag, but modifying how something acts could turn existing drag that does not generate lift into drag that generates lift in theory. Knowing if this happens with a spinning tire seems ... difficult.
– Yakk
Apr 6, 2023 at 15:57
• The Magnus effect wiki article mentions drag a couple times, but none of the occurrences were in the context of lift-to-drag ratio. There is a hint of additional drag in the context of a bullet. But it's plausible that it has a decent lift-to-drag ratio comparable to a wing, and the article even mentioned the idea of cylindrical wings for slow-flying craft, but it'd be great if you could cite a source. Or at least state that this is a result you've seen and are sure of, instead of leaving it as an assumption. Apr 6, 2023 at 19:21
• (Also, as yshavit commented, it matters when you're getting lift. With the wheels rotating forward right after takeoff, they'll generate downforce at the most important time for STOL, so just hitting the wheel brakes after liftoff is a way of getting more lift by taking the Magnus effect into account.) Apr 6, 2023 at 19:24
• I think that L/D ratio of 1 is counting total drag, but we really want incremental drag on top of what a stationary wheel would cause, because that's the point of comparison for non-retractable landing gear. Figure 8 in the paper shows C_d is pretty flat with speed for very low speeds (lambda). C_l is quite low in that regime as well but does have a steep upward slope. So perhaps there's a tiny bit to gain at low rotation rates vs. just form drag and no lift from a stationary wheel. Almost certainly still not worth actually adding an electric motor for it, though. Apr 7, 2023 at 0:10

No, it wouldn't. The way Magnus force is created is that the boundary layer on the spinning wheel would pick up momentum and deliver it too the stagnation bubble behind the wheel. This momentum is neutralized when the stagnation bubble gets pushed around to a higher pressure region, creating an assymetrical stagnation bubble. The flow outside the boundary layer and bubble is distorted and flows around this asymmetry, giving rise to a flow field with circulation that produces a lift force - although I really hate calling it a lift force. Magnus force is related to far-field circulation, but that circulation is not relatable to any of the formula used for computing circulation and lift from rigid bodies. Nothing to do with Magnus forces scales like rigid body lift forces. It doesn't scale with size, it doesn't scale with wind speed, it doesn't obey any rule with rotation rate, it doesn't scale with applied torque or power - it is just hopeless from a design perspective.

Since the apparent wind on a plane is pretty predictable, you could emulate the magnus effect by emulating the deflected stagnation bubble. Just stick a small cowl behind the wheel that deflects the flow in the same way the stagnation bubble would. It would need to be about the size of a mail box, and require no power, and probably reduce drag.

But it still won't produce lift because, as others have mentioned, the aspect ratio is just too low.