Ultralight with dual inline motors?

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Tdwalker

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I would like to design my ultralight with dual motors but have them in-line with each other rather than say, one on each wing. The motors would likely be rotax 277 (27 hp). They would be placed either with one on the nose and one at wing height right behind the cockpit, or both behind the wing. In the later case, the part of the fuselage that extends back towards the tail would be made up of two parts (One on each side of the propeller blades), and instead of meeting the middle of the tail, each one would meet at one side of the tail. My reason for wanting two motors is obvious, possibility of engine failure. That paired with a parachute and I would feel really safe. Please, let me know your thoughts!
 

Vigilant1

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Thoughts,
1) To meet your intent (greater safety in the event of an engine failure), your design will need to be able to fly safely (i.e. be able to climb, at least marginally) with just one engine running and the other engine stopped (and with all the drag of a stopped or windmilling prop). That's a tall order for an ultralight. If the plane can't fly safely on just a single engine, you haven't increased safety, you've decreased it (since now you have two critical engines instead of one).
2) Two 25 HP engines will be heavier than one 50 HP engine (if they are of similar design). The smaller engines will also burn more fuel per HP produced at max power settings. Weight is very important on Part 103 ultralights, and weight "invested" in that heavier two-engine layout isn't available for structure, more wing, minimal occupant protection, etc. It's very possible that the second engine (and the tradeoffs made to include it) could decrease safety rather than increase it.
3) Ultralights usually glide well, have low stall speeds, and land just fine with no power. Also, few are used for transportation, they are used for fun flying, and thus it's usually possible to fly in a place proximate to a field, seldom-used road, clearing, airport, etc. So, with some exceptions, I'm not sure that the "engine-out" scenario is as much of a safety issue as it would be in designs with different uses and different design parameters.

In the case of a US Part 103 ultralight, for the reasons above, I'd work to use a reliable single engine instead of designing a twin.
 
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blane.c

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Two successful 2 engine ultralight designs are the Lazair and the Kolb Flyer. Although both had engines side by side and not in-line a two engine ultralight is certainly feasible. At least in the case of the Lazair single engine performance would provide a small amount of climb and or the ability to maintain altitude. Even if a design only allowed an extended gliding distance to an imminent landing (or crash) it would allow additional options for the pilot. Considering reliability issues with engines used for ultralights a second engine option is worth considering.
I think a design consideration should place any rear mounted engine(s) out of a direct path to the pilot. In advent of forced or crashed landing nobody wants to wear any engine(s). An idea is to place the engines above the pilot or above and behind the pilot. Recently I have been to the Double Ender web site and engines mounted similar to these comes to mind as it will keep the center of gravity near were it belongs.. Whether it had an under slung boom or twin side by side would be immaterial, although it may be easier to mount a conventional tail wheel with an under boom.
The pilot should be aware that because of the thrust line aircraft designs with the engines significantly above the center of gravity will tend to nose over (thrust will rotate over c.g. To push nose down) when power is applied, as in go arounds and stall recovery's. The pilot must be aware of this and provide sufficient control force to counter this tendency and add in power judiciously so as not to make matters too difficult. This is not a insurmountable problem for anyone of reasonable intelligence who gives it a little thought. There are a multitude of above centerline thrust aircraft being operated successfully, and the power available for an ultralight is not excessive.
I would look certainly to more modern engines than the 277's. There are many newer engines within the power range necessary that would be more efficient.
 

Tdwalker

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Thank you both for your replies, they are very helpful. I am working on a design right now for the description I gave above and I will take everything into consideration. I will be attempting to find motors to use that will be considerably lighter and that are possibly even made for dual engine configurations. (Not sure if that exists). Will let you know what I come up with! Also, this tutorial:

http://www.instructables.com/id/Introduction-65/


It is really for RC planes, but how helpful would it be for someone attempting to build a small, simple ultralight? I am not worried about optimizing just yet, just want to design something that can theoretically fly, perform OK and be safe.
 
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Aviator168

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The Rotax 277 is too heavy for the power output. If you put two in an UL, you only have about 60kg for the rest of the plane. I don't know how much you pay for those 277s (that might be the reason you want to use that engine). Using 2 MZ-34 can cut 20kg from the power plant, and 20kg get you a much much stronger structure.
 

bmcj

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If safety through redundancy is the goal, why not run one prop with two engines through a dual belt system. Include some type of centrifugal clutch to disengage a non-running engine. Engines fail, but props rarely do, so you don't need redundant props. Plus, a larger prop gives you more thrust, giving you better results from two smaller engines.
 

blane.c

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If safety through redundancy is the goal, why not run one prop with two engines through a dual belt system. Include some type of centrifugal clutch to disengage a non-running engine. Engines fail, but props rarely do, so you don't need redundant props. Plus, a larger prop gives you more thrust, giving you better results from two smaller engines.
This would be hard to do with an inline setup ... prop sandwiched between two engines running in opposite directions?
 

Aviator168

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This would be hard to do with an inline setup ... prop sandwiched between two engines running in opposite directions?

No. 1 prop, 2 engines with sprag bearing belt redrives.

bmcj, I don't think anyone had done setups like that before. A while ago I threw around the idea of using a few 8 hp model engines using a belts to drive a central shaft; I thought almost everyone here said it was a bad idea. I think the challenge in this setup is to control all the engines so the load is distributed evenly. If someone can make this work, it would be the cheapest way to get a part 103 engine. Model engines usually have high power to weight ratio, and some of them are even 4-stroke. Hopefully they are more reliable then a Rotax 2-stroke. :ban:
 

bmcj

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No. 1 prop, 2 engines with sprag bearing belt redrives.

bmcj, I don't think anyone had done setups like that before. A while ago I threw around the idea of using a few 8 hp model engines using a belts to drive a central shaft; I thought almost everyone here said it was a bad idea. I think the challenge in this setup is to control all the engines so the load is distributed evenly. If someone can make this work, it would be the cheapest way to get a part 103 engine. Model engines usually have high power to weight ratio, and some of them are even 4-stroke. Hopefully they are more reliable then a Rotax 2-stroke. :ban:
:gig: I didn't say it was a good idea, but it does meet the OP' requirement for two smaller engines. ;)

For the multiple 8hp engine setup you proposed, what would be ideal (if it exists) is some sort of centrifugal or load controlled variable gear for balancing the loads.
 

Aesquire

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One problem is the Rotax 277 is no longer made. Plenty of used ones out there still, but that means buying & rebuilding 2 engines. You might still save some money over a new engine.

Weight is the Enemy in aircraft. Especially in weight regulated classes like pt. 103. It's hard enough to keep the weight under the legal limit as it is. That's why the guys here are always pushing to save weight.
 

Dana

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I only skimmed the instructables link you posted and follows the general idea (I did not look closely at the calculations), but I wouldn't take anything on instructables seriously enough to build an airplane your butt is sitting in (but see below).

There have been few twin engine ultralights. The Kolb Flyer was one, with two 15HP Solo engines (commonly used on PPGs). It couldn't maintain level flight on one engine, but would have a long shallow glide. Modern PPG engines would be a good choice. Any twin ultralight able to maintain altitude on one engine will almost certainly be too fast on two to meet the Part 103 limits.

Re two engines, one prop, with clutches: Props rarely fail, but clutches do, and things like sprag clutches don't do well with IC engines.

I've posted this before... still not a substitute for a complete aerodynamic analysis, but at least this one was done by a Boeing aeronautical engineer and intended for homebuilt airplanes, not R/C models:

25037d1373157890-rule-thumb-designing-plane-bowers-mcvey-figure-12-5.jpg


Dana
 

AdrianS

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I only skimmed the instructables link you posted and follows the general idea (I did not look closely at the calculations), but I wouldn't take anything on instructables seriously enough to build an airplane your butt is sitting in (but see below).

There have been few twin engine ultralights. The Kolb Flyer was one, with two 15HP Solo engines (commonly used on PPGs). It couldn't maintain level flight on one engine, but would have a long shallow glide. Modern PPG engines would be a good choice. Any twin ultralight able to maintain altitude on one engine will almost certainly be too fast on two to meet the Part 103 limits.

Re two engines, one prop, with clutches: Props rarely fail, but clutches do, and things like sprag clutches don't do well with IC engines.

Dana

I have some experience with sprag clutches : on electric motors, they are good.

We have used them on temporary rigs driven by 4 cyl IC engines, and they are a disposable item in this usage.
Centrifugal clutches are less fragile, but not maintenance free.

I like the idea of a load-sharing transmission, and I think it could be done, but it would probabably weigh as much as a third engine.
 

clanon

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No. 1 prop, 2 engines with sprag bearing belt redrives.

bmcj, I don't think anyone had done setups like that before. A while ago I threw around the idea of using a few 8 hp model engines using a belts to drive a central shaft; I thought almost everyone here said it was a bad idea. I think the challenge in this setup is to control all the engines so the load is distributed evenly. If someone can make this work, it would be the cheapest way to get a part 103 engine. Model engines usually have high power to weight ratio, and some of them are even 4-stroke. Hopefully they are more reliable then a Rotax 2-stroke. :ban:

Wankel Rotary GmbH

Some like that but lighter and less HP and torque...maybe :ponder:
 
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