On Friday, October 16, 2020 at 10:55:16 PM UTC-4, Kenn Sebesta wrote:
Does anyone have any data, preferably quantitative, about what sort of braking performance is required? On the one hand, it would seem that effective braking is primordial for safe landing in the event of an outlanding, but on the other hand many gliders seem to have inadequate brakes, to put it charitably. And these brakes oftentimes are not easily actuated, for instance in a B-4 or L-23 where squeezing the wheel brake handle requires releasing the air brake. So it's fair to conclude that brake performance is (or was) a very distant thought.

I've looked through CS-22, but there are no given standards for wheel brakes, only a loose admonition that "If the main landing gear consists only of one or more wheels, the sailplane must be equipped with mechanical braking devices, such as wheel brakes."

In particular, I'm trying to calculate how much energy the brakes need to absorb. An easy analysis is simply calculating the kinetic energy of the plane when landing 5kts faster than stall (since it's hard to glue the plane to the ground when going much faster). However, this grossly underestimates the amount of energy dissipated through rolling and air resistance. It also doesn't account for what might occur if brake forces were so high that the plane tips forward and skids on its nose.

Still, since the consequence of underspeccing the brakes is brake fade and glazing, and the consequence of overspeccing is additional weight and cost, it's worth trying to right-size the system.

Does anyone have any domain specific experience they could share?

... for instance in a B-4 or L-23 where squeezing the wheel brake handle requires releasing the air brake.
I flew my club's B4 a bit and as far as I remember, the wheel brake was actuated by a bike-brake type handle on the stick, so one didn't have to release the spoiler handle.
Anyhow, I owned a H301 with a low serial number. It had the hub of a German post-WW-II Zündapp moped wheel, which was somewhat borderline for stopping a mass of 300kg from 80km/h. However, if properly cared for and adjusted, I could lock up the wheel on grass or make the glider lift the tail on hard surfaces.
The old Schleicher gliders Ka6, Ka8, etc. had a 'steel band over the tire' brake, which worked ok in dry conditions but were useless on wet grass, yet they were certified gliders. IIRC, the POH called it an 'emergency' brake, i.e. only use them if you are about to go through a fence.
I would call 'locking up the wheel at touchdown-speed and MTOW' the upper spec limits of any requirement.
Unless your glider is equipped with an ABS system, locking up the wheel (or wheels in my case) doesn't make you stop any faster but only produces flat-spots on the tread.
The old drum brakes are still adequate as built but require some more attention than the modern hydraulic disc brakes and that may be the core of their bad reputation.

Uli
'AS'