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What is the minimum glide ratio of a motorized glider, enough to climb in typical summer afternoon thermals over a dark field or similar?

In other words, at what minimum glide ratio would a glider be considered more of an ultralight rather than a glider, at least conceptually, or based solely on glide ratio?

I don't think anyone would consider an aircraft a "motorized glider style", irrespective of the class the aircraft is registered in,if the glide ratio is 4:1.

A glide ratio of 20:1 seems like a more realistic cutoff. Is this correct, at least conceptually?

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  • $\begingroup$ This classification does not depend on the glide ratio. It depends on the "construction rules" under which the plane is build. They differ for "ultra light" and "normal planes" $\endgroup$ – Timothy Truckle Dec 26 '19 at 16:51
  • $\begingroup$ The Space Shuttle was a perfectly good glider with a (subsonic) glide ratio of about 4.5:1, and the X-15 was 4:1. $\endgroup$ – jamesqf Dec 26 '19 at 20:15
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    $\begingroup$ Ability to climb in a thermal isn't dependent only on glide ratio -- it's mainly about sink rate, which is tied to glide ratio by flight speed. $\endgroup$ – Zeiss Ikon Dec 26 '19 at 20:22
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The minimum glide ratio of any aircraft is zero. Just put it into a vertical dive. Of course this is not sustainable; either you overspeed or collide with the ground.

in order to be certified as a powered sailplane (that is bureaucrat talk for motor glider), EASA CS-22.1 demands a ratio of weight to span squared of less than 3. The second limit is a maximum weight of less than 850 kg. That the code gives the maximum weight in Kilogramms and not in Newtons shows that the weight figure limit of 3 needs to be calculated with the mass of the aircraft, not its weight.

Initially, the criterium was a minimum sink rate of less than 1.0 m/s. That number was raised to 1.2 m/s when the first batch of glassfiber models could not meet that target. The mass per span squared rule came into force in 2003.

The defining criterium for an ultralight depends on the local rules and is a maximum take-off mass of 300 kg or less for single-seaters or 450 kg for two-seaters in Europe. Within the ultralight class there is no motorglider category.

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For motorglider soaring you are more concerned with sink rate than L/D, because the engine is thought of as an L/D substitute and you are already penalized with a higher sink rate than you normally would because of the weight of the engine and fuel.

The L/D, and the speed at which it's achieved, will determine your ability to get between thermals without having to start the engine, but basic thermalling climb performance is a function of min sink rate and the thermalling speed. (If you've flown a Schweizer 1-26 which has an L:D of only around 23 but floats like a butterfly and can stay up in very weak lift because it can slow to the low 30s, you know what I mean).

On a good soaring day you will get thermals of up to 600 or 700 fpm and on boomer days over 1000 fpm, so on such a day a glider with a 300+ fpm sink rate can get by quite well. But as the conditions get weaker late in the day, or on a marginal day, you want to be sinking not too much more than 200 fpm. If the L:D is not so great, it just means you will be starting the engine more often to get yourself out of trouble trying to run through sink between thermals.

As far as Ultralight gliders go:

The Sandlin Goat soars fine with an L:D of somewhere around 10 (there is no published figure - also, the basic design does not accommodate an engine so you'd have to engineer it). It's sink rate is well under 200 fpm and can thermal in weak lift and Goats have done pretty long cross country flights if the winds aren't too strong.

The Mitchell Wing had an L:D of 16 with engine and a min sink (I dimly recall of somewhere between 200-250 fpm) good enough for soaring in most conditions except very weak days or late afternoon.

The Marske Monarch was probably one of the best performing ultralight gliders that an average person could afford and could be motorized with an L:D of around 20 and a min sink well under 200 fpm even with an engine.

Then there's the guy who put two Hobby King RC electric motors on a Millenium, which is around 18:1 and I believe sinks under 200 fpm.

There are ultralight gliders with much higher L:Ds but they are super expensive factory builts, way out of my league.

For a homebuilt ultralight motorglider, I'd be happy with an L/D of better than 15, but preferably better than 20, and a min sink of under 220 fpm. I'd power it with electric motors. Instead of depending on L/D and penetration power, you have the motors.

You could scratch build a Mitchell Wing and power it electrically using RC components for probably under 10K (if you can find plans).

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This question isn't answerable in any meaningful way. There are legal Part 103 ultralights (self-launching sailplanes that meet ultralight speed and weight limits under US regulations) that have glide ratio well above 30:1, and there are commercially sold motor gliders that have glide ratio as low as (as I recall) about 26:1.

Soaring isn't a good defining characteristic, because in slope lift, even a basic Rogallo hang glider can soar for hours, and in strong thermals they have climbed high enough for the pilot to need oxygen.

Minimum sink is a bad criterion, because forward speed contributes to sink rate; of two aircraft with the same glide ratio, the faster will sink at a higher rate.

All we can really say is that a motorized glider can be flown like a glider or sailplane, but has an engine that can be used to take off or climb, removing the need for a tow or winch launch.

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  • $\begingroup$ Well, I think you did answer the question. Looks like a realistic minimum glide ratio for a motorized sailplane, independent of classification, would be around 26. $\endgroup$ – Fred Dec 26 '19 at 18:01
  • $\begingroup$ The point is that ordinary airplanes can glide well enough to soar, in thermals or slope lift (I've seen a video on YouTube of a Taylorcraft doing so) -- I'd be very surprised if a Taylorcraft or Cub had a L/D better than about 10. Soaring is about minimum sink as much as about L/D -- and neither one really defines a motor glider vs. a low-loading utility or sport airplane. $\endgroup$ – Zeiss Ikon Dec 26 '19 at 18:56
  • $\begingroup$ Why do you say minimum sink is not a good criterion? If your goal is to stay aloft, it's a perfectly good criterion. Of course, it's not if you actually want to go anywhere else. $\endgroup$ – Sanchises Dec 26 '19 at 20:28
  • $\begingroup$ @Sanchises "Of course, it's not if you actually want to go anywhere else." Exactly. Modern glider tasks all revolve around speed, with the requirement of being able to reduce sink rate enough to stay aloft and gain altitude being assumed. For sport gliding, you still need more than minimum sink, else an oversize parachute would be the best (cheap, too, compared to any sailplane or even ultralight). $\endgroup$ – Zeiss Ikon Dec 27 '19 at 12:12

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