Quick sanity check. The order of magnitude for the circumference is around 1 metre (40 inches, or a 12.5 inch diameter)
1 metre per second = 2 knots (useful thing to know)
so at 50 knots, which is 25 metres per second, it would spin 25 times per second. Or around 1,500 rpm.
So looks you got there in the end…
If you hear anything that is revolution based, it is either a harmonic or a bearing/the brake disk rubbing, the primary frequency of any imbalance (25 Hz) would be too low to hear.
Rah, my numerical literacy is low today, and I blindly trust the results of calculators too much. Scalar (without a unit) values be cursed. The numbers I gave were wrong because I misused the “units” program in a way that confused rotations (a full circle) and radians. The correct values are:
So overall, the numbers I gave were about a factor of 20 too low and the corrected ones (still at 60 kt) are:
So, I found a Cessna 152 POH that said that the main wheel tires were “6.00-6” and the nose wheel “5.00-5”. Then I found https://www.goodyearaviation.com/resources/pdf/databook-6-2018.pdf; all their 6.00-6 tires (page 7 of the PDF) have about 17 inch “inflated outside diameter”. However, I now see they have a 6.9 inch “static loaded radius”. I’m not 100% certain which is the right one to take to compute circumference and thus RPM, but IMO it is the “inflated outside diameter” since that is the actual size of the tire; the fact that it is flattened under load does not change its circumference. But, if we were to take the “static loaded radius”, we would have:
70rpm is very low – too low to be making that sort of noise unless the bearings are knackered or the brakes are binding.
Bingo_Fuel wrote:
How many rpm are the tyres reaching on a Cessna 152 when t/o?
An Internet search suggests the main gear wheels have an outer diameter of about 17 inch, so at 60 knot that’s about 70 rpm. 14 inch and 85 rpm for the nose wheel.
The GA8 that I fly has a non-retractable gear. The fairly big tires sit on a gear strut that is over a meter long and not supported in any way. So similar to a C152/172, it acts as a giant spring. Any slight imbalance in the tires will cause the airframe to vibrate/rattle straight after liftoff, which unnerves the passengers. So standard practice is to dab the brakes straight after lift-off. And yes, you do need a fair amount of right rudder at that time, so it requires some fancy footwork not to induce too much yaw while you do this.
Hi,
Listening to the noise of the tyres-bearings on this video. How many rpm are the tyres reaching on a Cessna 152 when t/o?
I notice the sound stops relatively quickly when airborne. Definitely no need for breaks.
[ vimeo appears to have been removed ]
Looks like your brakes need adjusting….probably not too relevant for a super cub on tundra tyres but that dragging brake pad will probably cost takeoff length to some extent…more relevant for smaller wheels I guess (less lever arm)…
Yes, there’s some friction in the bearings and from pads to disc, but we probably need a 
there.
We can easily calculate that the average net thrust to accelerate 800 kg to 40 knots in 6 seconds is about 2600 N. So if the force needed to turn each main wheel is about 5 N (it’s never more than that) then those ten Newtons increase the take-off roll by about 25 milliseconds. So it’s insignificant – by more than one order of magnitude.
Looking at the video of smaller wheels, they clearly do take longer to stop spinning. I suspect this is because the small wheels are spinning faster for a given take-off speed. Also, the tops of the bushwheels are in or close to the arc of the propeller, so I suspect that they stop due to aerodynamic effects as much as bearing or brake friction. But that’s conjecture – I’m not inclined to try to prove or disprove it.
I do not apply brakes to spinning wheels once airborne, for several reasons: The aircraft is designed and certified to accept a wheel spinning into the landing gear bay, unless it says otherwise. A tire spinning will be slightly expanded by centrifugal forces, at some point, the combination of that, and resultant possible expansion on the rim, compared to the very sudden stop, could slide the tire on the rim, and shear the valve stem of the tube. I don’t want to induce a flat tire. In the winter, if snow or moisture has accumulated around the brake, I want the entire brake assembly to remain as cold as possible (below freezing), and spin off as much of that contamination as possible. I sure don’t want to brake a wheel airborne, warm a brake with accumulated snow/slush, to have it refreeze, and freeze a wheel solid. I have landed with wheels frozen locked, which did not release during the landing nor rollout (fortunately on more slippery runways). It’s not good, and on a dry runway will cost you at least a tire, if not control of the plane.
If an aircraft manufacturer requires a procedure for safe operation of the plane, that procedure will be described in the flight manual. It’s not necessary to invent new procedures.
I don’t bother to brake the wheels before retraction. After take off my heels are on the floor with a lot of right rudder applied. Lifting my feet up to actuate toe brakes without causing a lurch in yaw is awkward. This is a time when I’m pretty busy anyway and can do without the distraction.
In any event by the time I’m well clear of the remaining runway the mains will have stopped rotating (there is always some drag from disc brakes) and the nosewheel has a mudguard. I usually retract at about 200ft.
