Varies with...
Ball park Vx about 90 mph or 78 kts works pretty good, but......... we want in general best penetration speed or distance.
Definitions:
Vmd - min decent, most time in air, about 90% Vbg
Vbg - the best power-off , greatest (air) distance
Don't forget about wind, fly slower with tail wind faster with head winds. Usually take half of the wind and add or subtract as needed. (Obviously there is a practial limit to the max wind correction.)
Don't forget weight, heavy fly faster, light slower. The variation is about +/-7 mph.
Windmilling propeller - Windmilling reduces Vmd and Vbg speed over a stopped prop. Just keep in mind, if you really do have a dead engine your Vbg will be a little slower than what you got testing with an idling engine. I agree with the other poster there is no need to simulate a real emergency with a real one by intentionally shutting the engine down.
Fixed and constant speed props also have different effects on glide . It's subject to debate wheather stopping the prop is worth it? It's academic if the engine dies right after takeoff. Also some recommend going to high pitch or low RPM if constant speed prop equipped. I say it can't hurt; put the throttle full forward and prop full back. (thanks larry and pete for the correction, but I meant prop back, course, low rpm, and throttle fwd wide open. It's debatable if it makes any significant difference. In theory it sounds good. The reason for open throttle, fwd is less engine force needed in the intake (suction) stroke, ie reducing pumping loss. Less force to turn the prop over equals less drag. This is small stuff but some swear by it. I have not tested the PROP in low RPM (lever back) with a dead engine. With it idling it does make a difference. I suspect the low RPM prop trick works when testing it, is because the idling engine is making residual power/thrust, which is enhanced by going to low RPM. I don't know if you even have enough oil pressure with a dead windmilling engine make the prop move? Depends on the constant speed prop, governor and how fast your are windmilling I suspect. This comes under nice to know, it can't hurt, but it is not top priority in an emergency. Like they say fly the plane, aviate, navigate, communicate.)
Attitude is more important than speed?
More important than the speed is the attitude to me. Know what the sight picture is and have it memorized in your head. There always is going to be vertical gust, winds and other factors you can't account for. Best L/D is an angle of attack thing than speed, right.
Taking off we have a significantly high nose up pitch; it's a big push to get from nose up Vx or Vy climb to nose low best glide attitude. It's a foreign idea for most shove the nose down right after take off near the ground, but that is what you need to do if the engine stops. I know of one guy that did not get the nose down fast enough and mushed into the ground with a high sink rate. That is why I probably carry extra speed in a practice guide. I may not go as far, but I have more energy (and lift) to play with at the last part of the flight, near the ground and most important place. It does not help to get to your landing point if you are sinking like a rock.
When debating this BEST of the best glide speeds, the answer is it depends.
I don't think 5 mph will make much difference. I like to fly Vbg with a little extra for energy to control the flare and a little fast is better than a little slow. The drag curve for one is steeper on the back side.
I go right to 100 mph because it's a round number and easy to remember. I also know unless I am super light or have a big tail wind, 90 or high 80's is a min. At least that is what I think. I am not that good to fly a perfect speed and land in an emergency. I am going to be flying looking out the canopy and only glance at the airspeed. Close is good enough. All I want to do is get on the ground, under complete control, preferably at a reasonably slow speed at a place, my chosen touch down point, which I hope will not hurt too much.
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