Wheel brake effectiveness standards

A very nice analysis except for your question:

So why does the tail weight seem important at first glance?

Tail weight is important for center of gravity considerations.Â* But I'll
bet you knew that.

On 10/20/2020 8:29 AM, Kenn Sebesta wrote:
Brakes on gliders were almost an afterthought until the advent of motorgliders, which are heavier and require more braking authority. My DG400 had a Tost drum brake that was marginal. Schleicher introduced disk brakes which are much more effective.
This is an excellent data point.

But one point that hasn't been mentioned is how much tail weight does the glider has. Braking will be limited to the moment arm of the tail; a light glider can't apply as much braking force as a glider with a heavier tail. And the Schleicher MGs have very heavy tails.
I was initially under this assumption as well, but then I gave it a quick analysis and now I'm convinced the tail weight has very little to do with stopping distance.

Just working off the moment required to tip a modern glass glider forward on its main-- as quantified by hard numbers for a few select aircraft and more generally guesstimated by the effort required to lift the tail to get a dolly under it-- we're looking at around 100Nm per 100kg of plane MTOM.

What this means is that for a 30cm-ish tire diameter, each revolution burns 600J per 100kg MTOM per meter rolled. Nicely, when comparing to kinetic energy the mass cancels out and we can roughly determine that the stopping distance for this maximally effective brake is d=v^2/3.

So for a light plane touching down at 30kts, we're looking at 20m stopping distance without tipping forward on the nose. For a heavier plane touching down at 40kts we're at 35m. Interestingly, those are basically good car stopping distances.

I think all agree that these distances are far shorter than anything we're seeing or can even reasonably expect. We can, therefore, conclude that the tail moment is not the limiting factor.

So why does the tail weight seem important at first glance? Because at anything over a few knots of airspeed you can use the elevator to unload the tailwheel. So it's not the tailwheel weight distribution that's allowing the plane to tip forward when braking hard, it's the (lack of) elevator control.

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It's interesting to consider, in light of this thread, which factors are predominant-- right now I'm hewing toward saying surface quality (no alfalfa!), winds, and airspeed and altitude control are the biggest driver of distance between the start of where a plane could feasibly land and where it ultimately stops. If design choices result in weaker brakes but landing 1kt slower and 500fpm steeperÂ*we might find that the actual stopping distance is improved. Very surprising!

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Dan, 5J