Tri-Mower Design

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BBerson

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If each failed motor retracted automatically when thrust is low (or off) it would improve engine out performance.
 

BBerson

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It just sounds heavy somehow to do that, just an impression?
Could be as simple as engines on hinged swing arms attached to the fuselage. Under power (thrust) they swing out. No power and a spring and air drag folds it back against the fuselage. Could be open, or fully enclosed with doors like landing gear or motorgliders with retract engines.
 

blane.c

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We talk about a lot of ideas, but the tenants I think are most relevant for the Tri-Motor are (1) keep it light, it is already going to be heavier with 3 engines vs 1 so pretty much that has to be the only concession to extra weight. (2) keep it simple. It needs more complexity like an extra hole in the head. (3) start over at (1).

The thing is once it is flying it is a excellent test bed for other ideas. One of the things I like about the idea is you can modify one of the engines and see if it makes it better or worse and have back up if things didn't work out like you hoped.
 

Vigilant1

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Engines on swinging pylons sounds complicated and hard. 2 or 3 of em on one plane sounds "challenging".

It's not very hard to streamline an engine (and simple if the engine is at the front or rear of an existing structure). The prop is the major source of drag. A feathered prop has about 1/8th the drag of a non-feathered one. The biggest payoff would be designing an inexpensive variable pitch (or 3 position) prop for these small engines that could go all the way from clmb to cruise to feathered pitch.
1) Feathering drastically reduces drag from a dead engine
2) Allows good remaining engines to produce optimum thrust at climb airspeeds in case of an engine failure.
3) These multiengine airplanes will have excess installed HP to allow safe climb in case of an engine failure. Variable pitch allows all engines to produce optimum thrust at higher airspeeds so this excess power can be turned into knots.
 
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jedi

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............ Take pictures eh?
No photos and he does not like to publish vaperware. I can say Space Claim CAD, 1/4 scale model that is too heavy to fly, membrane wing. two tractors and one pusher, low power cheep 4 stroke engine, direct drive, mono hull foatplane with non standard high wing and tail configuration.

Any more will have to be by PM.
 

Vigilant1

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Isn't one of the major points of a tri-motor (or more) that you can use smaller cheaper engines?
Yes, but there's a limit. The plane still presumably needs to be able to climb with one engine inop,
(and the prop stopped---draggy). So the engines can't be too small. And cheaper industrial/commodity engines typically have more mass per HP, and having even more of them increases aircraft weight and the challenge of climbing with one engine inop.
 

jedi

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Isn't one of the major points of a tri-motor (or more) that you can use smaller cheaper engines?
Yes, and with single or twin cylinder modern engine with wood prop on a slow airframe the engine does not windmill, which makes much less drag and minimizes the need to feather, making the shutdown/restart procedure much less complicated.
 

Vigilant1

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Yes, and with single or twin cylinder modern engine with wood prop on a slow airframe the engine does not windmill, which makes much less drag and minimizes the need to feather, making the shutdown/restart procedure much less complicated.
To put a number on it: A stopped 2 blade 46" diameter prop of "typical" chord (8:1 aspect ratio) has a equivalent flat plate drag area of about .77 sq ft. At 60 knots, that will produce about 9 pounds of drag. At this airspeed and a prop efficiency of 75%, it takes about 2.2 engine HP from the remaining engines to overcome the drag from that stopped prop.

Edited to add: If our TriMower weighs 1100 lbs, the "lost" 2.2 HP from a stopped prop will reduce expected rate of climb by about 50 FPM.
 
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lr27

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Test bed? That reminds me of that B-17 with a turboprop on the nose.
 

blane.c

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You have to account for yaw correction too even a centerline thrust twin will have to account for less "P" factor and engine torque which amounts to something to the rudder. If it is on a asymmetrical engine then the drag and other forces are on one side so much more correction from the rudder to fly straight which equals drag and more hp wasted to account for it.
 

Vigilant1

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You have to account for yaw correction too even a centerline thrust twin will have to account for less "P" factor and engine torque which amounts to something to the rudder. If it is on a asymmetrical engine then the drag and other forces are on one side so much more correction from the rudder to fly straight which equals drag and more hp wasted to account for it.
Yes, good point. And if the dead engine is at the rear of a centerline twin, at least some of the prop will be in the high-speed airflow of the front prop so the drag will be somewhat higher. And, on some designs (Cessna 336/337), loss of the rear engine also causes an increase in form drag at the back of the fuselage (due to greater separation/turbulence in the airflow behind the aft cowling when the engine isn't running).
 

jedi

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To put a number on it: A stopped 2 blade 46" diameter prop of "typical" chord (8:1 aspect ratio) has a equivalent flat plate drag area of about .77 sq ft. At 60 knots, that will produce about 9 pounds of drag. At this airspeed and a prop efficiency of 75%, it takes about 2.2 engine HP from the remaining engines to overcome the drag from that stopped prop.

Edited to add: If our TriMower weighs 1100 lbs, the "lost" 2.2 HP from a stopped prop will reduce expected rate of climb by about 50 FPM.
Thanks for sharing the calculations.
What about a 24” diameter prop at 32 mph?
 
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