This will be my first twin engine plane and I want to do it right. I bought a perfectly good Enforcer delta and am modifying it for two Norvel AX-40's. I built a horizontally mounted plate to mount the engines above the wing spaced to use 3 blade Master Airscrew 9 x 7 pusher props. The engines are raised 3" higher than the stock OS .91 four cycle was. I plugged the original prop hole in the wing and the new props clear the wing by 1/4" and the fins and each other by 3/16". I retained the original down thrust.

My questions are (to help the stability if one engine quits);

Should I build engine side thrust, that is, aiming toward the center of the wing (because they're pushers)?

Should I install a rudder behind each prop for control if needed? (The original Enforcer has no rudders). How about each rudder turned out when in neutral so the engine that did not quit is trying to correct the engine-out turn?

I was going to try both methods above unless more experienced flyers have better ideas.

Anyone done this before?

Gill:

Talk about going off the deep end. Pusher, engines on pedestals, and a bash to boot. I don't think anyone has done anything similar, probably why you've gotten no response.

I'll dive into the deep end with you, giving my thoughts with no guarantees at all.

Basically, it should work.

I would not use any down thrust with the engines up in the air, if anything I'd have them angled up. Consider the effective thrust, if it's down it will tend to make the plane nose down more, while the up thrust would decrease this somewhat.

You will without question need vertical fins/rudders if one engine quits, with both running rudders will also help if they aren't quite at the same rpm. Recommended highly.

Side thrust? Your choice. I build all my twins zero side and zero up/down, the enforcer though, with the minimal yaw control of the delta I might well put some side thrust in.

How much up/side thrust, how much rudder/vertical fin? I'd be inclined to make the engine mounting adjustable, and make the fin/rudder set as big as I could without just looking wrong.

But whatever else, keep us informed of your progress.

Bill.

Well, I just got off work or I would have answered earlier. This does not sound like a good first twin project. I have scratch built and flown deltas with twin motors so I have some experience. First, anytime you move center of thrust off the centerline of an aircraft you should compensate. With twins you should also think about the torque reaction two engines will make. Another thing is how will this effect the CG, plumbing the fuel tanks, servo placement, etc. You are starting with a good basic airframe, simple to build and decent if not good flying habits. The one bad thing in the original design you have already identified, no rudders. When I build Enforcers I also enlarge the stabs to give move yaw stability to reduce hunting. Take all that and add an extra motor and away from the center line and away from the fuel tank and you add some problems. Don't be fooled, these are major changes and you very well might end up with a difficult if not impossible to fly airplane. I have some recommendations. First engine placement, keep the engines as centered as possible, using three or four bladed props will help this. This not only reduces thrust issues but will ease the problem of getting fuel to the carbs. I recommend slight upwards thrust on both engines(3-5°) and outward thrust on each(3-5°) if you want to counteract engine torque you can reduce the outward offset in the right engine a little and increase the offset in the left engine. Good luck with throttle linkage but it will be easier to use two seperate servos and a computer radio to set it up. Pumps will help with moving fuel. Have you though of how you'll set up mufflers to exhaust to the rear? You will need rudders. Rudders in the engines airflow will give you more authority. They will be hard to set up on an Enforcer. If you have never flown a delta I would recommend a single engine one be tried so you get used to their handling. Really I recomend that as a first twin you try something more normal and perhaps an ARF so if you crash it you aren't out a big emotional investment. If yo do go ahead with this have some one double check everything and fly it a little nose heavy the first few flights. And yes send us pictures....

With regard to thrust, flying boats with the engine mounted above the wing on a pod usually need a little up thrust, a couple of degrees, to compensate for the drag down below the thrust line. If you are using pushers, that would be down thrust.

As Bill said, you are breaking new ground and the idea of an engine pod with adjustable angles sounds like a good idea.

My buddy and I are currectly getting ready to start a project experimenting with engine out thrust. We are going to use an ARF for quick results and build in 9 degrees of engine out thrust on each side. 9 degrees gives you 99% (cos9=.9877) forward thrust and 15% side thrust. We are going to offset the nacelles 5 degrees and use thrust plates for the additional 4 degrees.

Thank you all for your comments. I will take pictures of the plane next week after I return from a trip and try to post them.

A few observations that I relied on for my approach of thrust angles;

The North Star, a delta seaplane also by Mikulasko has the engine mounted above the wing almost the same distance as my conversion ( 5 1/2" from center of prop shaft to center of wing rib, my conversion is 5"). The North star has the prop located 14" behind the center of gravity, mine are 8 1/2". Very similar setup for a very sucessfull model.
The North Star plans call for 1 1/2 to 2 degrees down thrust. Thats why I thought it would be OK to leave the original down thrust in the Enforcer. (Thats down behind the CG, so maybe has a net positive effect at the front).

With the prop behind the CG I think the side thrust needs to be intoward the center of the plane to offset the single engine turning force. ( Left engine out, right engine pulls the right side of plane to turn left- so some left offset will push the rear of the plane to the left to correct the left turn - ???) I have mounted the engines so they can easily be adjusted so if the plane lasts I can experiment. I would like to have it right the first time though.

I have mounted a single 16 oz. tank ahead of the engine plate with a single pickup of 3/16" diameter tubing that T's as it comes out of the tank. The two equal length 1/8" lines then go to the carbs. Both exhaust taps for pressure will feed the tank with check valves so one engine running will not bleed pressure out the other's exhaust. I hope I don't have to use a pump, but we'll see.

Each engine will have it's own throttle servo and channel and I will be able to do any mixing and control scheme we can think up. I have a Multiplex MC 4000 system.

The two rudders will also have individual channels and can be mixed to suit.

As far as an emotional investment, every plane I launch has some, but the experiment and learning process is the real thing.

Please think about the thrust offset issue. I want to get it as close as I can for the first flight.

I think we are saying the same thing as far as the thrust agles, you're saying down thrust for a forward pointed engine(the THRUST is actually going up to the rear) and I'm saying up thrust for a rear facing engine(the THRUST is actually going up to the rear).

"With the prop behind the CG I think the side thrust needs to be intoward the center of the plane to offset the single engine turning force. ( Left engine out, right engine pulls the right side of plane to turn left- so some left offset will push the rear of the plane to the left to correct the left turn - ???) I have mounted the engines so they can easily be adjusted so if the plane lasts I can experiment. I would like to have it right the first time though. "

Are we doing the same thing here also? When you say left offset above are you talking the back of the motor which is towards the front of the airplane being offset to the left? If so, then yes that's what I mean too.

"I have mounted a single 16 oz. tank ahead of the engine plate with a single pickup of 3/16" diameter tubing that T's as it comes out of the tank. The two equal length 1/8" lines then go to the carbs. Both exhaust taps for pressure will feed the tank with check valves so one engine running will not bleed pressure out the other's exhaust. I hope I don't have to use a pump, but we'll see. "

I think two smaller individual tanks is better but if you use one I would put two seperate clunks into the tank, one for each engine. The pressure check valves sounds good in theory...

"As far as an emotional investment, every plane I launch has some, but the experiment and learning process is the real thing. "

I have a bunch of my designs that have either never flown or I was lucky to fly once or twice and moved on so I see what you're saying. That said, having a proof of concept model or intermediate can help iron out small problems or improve skills that when combined together on a new project could overwhelm you. There have been many times I wish I had done so that I could have fixed a problem before I ever made it.

"Please think about the thrust offset issue. I want to get it as close as I can for the first flight. "

This is pretty critical, do you think we're looking at it the same but from different angles? I do.

Another thought is the horizontal plate set up. Two engines espesially high rpm one's reverberate off each other, I recommend that you bench test the plate set up before you put it on the airplane to make sure you have enough torsional stiffness each each direction. This would also be a great chance to test your fuel delivery setup.

Ed, could we have been looking at the thrust angles the same but from differing directions too?
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I'm continuing to assemble the motor mount. It is "adjustable" in that I can rotate the mount with some surgery. I'm setting the angle now at 4 degrees toward the plane's center line for each engine (back of engine). I don't know if this is enough, probably not, but I'll do fast taxi tests with only one engine at a time to see what it looks like. I think I'll leave the 2 degrees of down thrust in, that is, prop (back of engine) aiming down toward the front of plane. I can change that too by shimming the engines. I think I understand the thrust angles you were all talking about and I believe we agree.

Pictures later.

Everyone thinks 9 degrees out is too much, but no one knows for sure because no one has tried it. Well, I'm going to know soon because I'm going to put in the out thrust, shut 1 engine down and fly at full power. Then I can see the yaw one way or the other. By changing the Ernst thrust plates, I can vary the thrust line from 1 degree in to 11 degrees out. That ought to let me figure out the exact amount.

Normally I would not fly single engine at full power with a rectangular wing and normally spaced engines, but I wanted to see if I could without much, or any, rudder. I will say that I have lost both sides on my Twin Air and I remember that at half power, havng the right engine out is much easier to control than having the left one out. This probably means the out thrust required is not the same on either side. I assume torque, p-factor, etc. Only flight tests will tell. I'll be publishing the results in R/C Report.

Ed, sounds interesting, BTW I went to the Cocoa Bch area, had a great time but took only land planes.

Here's what I put in my first post, above, about counteracting engine torque.

"I recommend slight upwards thrust on both engines(3-5°) and outward thrust on each(3-5°) if you want to counteract engine torque you can reduce the outward offset in the right engine a little and increase the offset in the left engine."

Obviously the best solution would be one normal rotation and one counter rotation engine.

Jerry etall, some pictures...

Didn't mean to put the bipe in, sorry... So these pics have 2 engined, single engined, and no engined airplane!!!!