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Do motor glider pilots ever "cheat" and use their motors to thermal?

I understand there are 2 types of motor gliders:

  1. Self launching: with about 80 hp very similar to an ultralight except a lot more aerodynamic. They can self launch (takeoff) themselves without a tow.

  2. Sustainers: with about 15 hp, they cannot self launch, but can use the motor to sustain flight to get home if they cannot glide all the way home due to lack of thermals, sinking air, etc.

I understand gliders soaring over fields spend about 20% of their time thermalling to get altitude.

Why not just put on the motor to thermal faster and spend more time gliding (assuming you're not a purist)?

Is this ever done?

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Do yachtsmen turn on the motor when under sail to increase their speed a little bit just for fun? Not unless they have to get somewhere in a hurry, or to get somewhere when the wind dies. Kind of defeats the point of sailing. Soaring is simply sailing turned from a 2 dimensional art to a 3 dimensional art, by rotating the lift vector created by moving air to a vertical axis from a horizontal one.

You have to understand that the whole point of soaring is the "game" you play with the atmosphere. You're either into the game or you aren't and a motor pollutes the game if you use it while playing, except in specific circumstances.

Most motor gliders use the motor to launch and hunt for initial lift, then once lift is found, the motor goes off and the game starts (if you're are being towed and your tow pilot is any good, he/she is also hunting for lift for you while on tow and will try to get you near or in a thermal when you get to release altitude).

In general, you will use the motor mainly to launch and find lift, then only to rescue yourself if you get caught low. So the motor is mostly to eliminate the tug, and the need for retrieve crews if you land out, or retrieve tows if you land out on a usable airstrip. High performance motor gliders with retractable motors have to go through a retraction/extension procedure as well, so that is another factor in discouraging motor use until you really need it. Of course, the other problem is you're more likely to need it, because the ballast of the launch motor hurts your sink rate, like having water ballast you can't get rid of, and it's more of a struggle to stay aloft in weak lift. But again, this is normally a late in the day gotta-get-back situation.

On the other hand, there are "touring" motorgliders, with medium gliding performance and that are meant to be usable as long-range cross country aircraft in the travelling sense, with engines and props that facilitate starts and stops quickly at any time, as well as pure soaring, and landing gears that make normal power plane airport operations feasible. You could really call these "hybrid" aircraft.

In these cases you may well indeed use the motor to supplement weak lift if it's to help you get from A to B. But even there, you're making a lot of noise, and it's so much more pleasant with it off, so you'll avoid running the motor when you can. This is probably more akin to "motorsailing" yachts that are a hybrid of power boat/sail boat.

With the newest electric motorgliders, you may use the motor more to help with thermaling, but they still make a bit of a racket compared to motor off (fast turning prop) and have limited endurance, so you still would tend to avoid using it unless necessary.

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    $\begingroup$ Lot's of sailors use their engines much more than necessary. It just depends on what is one's mood, skill, motivation, and interest. $\endgroup$
    – bogl
    Apr 27, 2020 at 17:02
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It would only be cheating if there are rules applied that prohibit using it, this is the case when flying competitions or logging and uploading your flight to competition web portals.

To prevent cheating, motor gliders must have loggers with a microphone, so they also log the sound level to check if the engine is running. Depending on the type of competition the engine running will make the flight invalid or split it in separate flights.

If you are just flying for fun you can use it as it makes sense for you, but engine time is expensive and the noise level is no pleasure. Also limiting is the small amount of fuel and the fact that motor gliders are only effective in a small speed range. So the most time its used to get back home, start early to avoid missing the beginning thermals or reaching areas with better weather conditions.

Touring Motor Gliders are an exception, they are mainly used with engine running

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