RCU Forums

RCU Forums (http://www.rcuniverse.com/forum/)
-   Twin & Multi Engine RC Aircraft (http://www.rcuniverse.com/forum/twin-multi-engine-rc-aircraft-192/)
-   -   Engine mounting angles, how to set up a twin (http://www.rcuniverse.com/forum/twin-multi-engine-rc-aircraft-192/10998135-engine-mounting-angles-how-set-up-twin.html)

tenacious101010 03-13-2012 03:08 AM

Engine mounting angles, how to set up a twin
 
I am working installing motors on my first twin and remember some discussion on mounting the engines at angles. Can anyone tell me where to find any information on the subject?
Thanks

chymas 03-13-2012 04:55 AM

RE: Engine mounting angles, how to set up a twin
 
Typically on a twin you mount the right engine with right thrust and the left engine with left thrust.  Most are in the 2-3 deg range.  I remember reading an article where the guy mounted them @ 8 deg, by doing that he said he could fly single engine with no problems.  I am working on a scale project that the engines are mounted with 2 deg thrust to the outboard sides.

evan-RCU 03-13-2012 05:28 AM

RE: Engine mounting angles, how to set up a twin
 
Most of mine are zero zero but I have built some where both are out and down and others where the left is straight and the right is right with the thought that the left's torque is enough for it's out thrust. If I scratch build I make them zero zero as I really don't think it matters that much as long as you realise the seriousness of an engine out and have a bit of experience on one engine.

jaka 03-13-2012 09:12 AM

RE: Engine mounting angles, how to set up a twin
 
1 Attachment(s)
Hi!
Forget any side trust if the original plane did not have it!

tenacious101010 03-15-2012 10:58 PM

RE: Engine mounting angles, how to set up a twin
 
Some variation in advice. It seems that 2 degrees outboard thrust might be the best option for someone new to twins. I doubt 2 deg. would be very noticable visually, but might be a big help for someone new to twins.

jaka 03-16-2012 01:10 PM

RE: Engine mounting angles, how to set up a twin
 
Hi!
The biggest problems for newcomers to twins is too set the engines right! 2,3 or 4 degrees of outward engine thrust would not help much in an engine out situation. But know-how how to set engines is the importance thing!

tenacious101010 03-16-2012 01:29 PM

RE: Engine mounting angles, how to set up a twin
 
But if they were electric motors, tuning wouldnt be an issue it seems.
1. But the offset would help, not hurt all other things equal, right? Or am I mistaken?
2. Would the 2, 3 or 4 degree angle cause any problems for a new to twins person?
3. Which would give them the best chance to survive an engine out situation?
4. I guess it comes down to, "Why not angle the engines outboard a bit, what is the disadvantage?"
Thanks, Denny

tenacious101010 03-16-2012 11:14 PM

RE: Engine mounting angles, how to set up a twin
 
Then theres the question of direction of rotation of the props. I already have a left and a right rotation three blade prop for my F7F project. If they are rotating opposite from each other, which side should turn which direction? It may not make much difference, but the question is, what would be the advantages and disadvantages be of either configuration?
Thanks, Denny

tenacious101010 03-16-2012 11:19 PM

RE: Engine mounting angles, how to set up a twin
 
After some consideration of the subject of rotational direction, I would think that the right motor/prop would be best to turn counterclockwise and the left turn clockwise. The only benifit would the the torque of the engine ever so slightly counteracting the lateral imbalance of the powered engine versus the non powered engine side if one motor quits. Is this correct? LOL< probably more theory than reality with this way of looking at it.
Denny

ps2727 03-17-2012 04:23 AM

RE: Engine mounting angles, how to set up a twin
 
Denny,
For a twin where the props are rotating opposite to each other you want the downward moving blade toward the inside; that is, closer to
the airplane centerline. The reason is that the downward moving blade operates at a slightly higher angle of attack than the upward moving blade
in high power/slow airspeed situations(takeoff). In that scenario if an engine quits there is less tendency for the airplane to move off heading than if the
props were rotating the other way. Think of the prop arc pulling a little more on the downward moving side.....
(if you want to know why send me a PM. don't want to ramble on too much here)

I believe the effect in our models is small and there have been many quarrels on this topic in the modeling world. Not trying to start another here but
the effect is real and can be demonstrated in the full size world. Look at photos of light twins on the market which use
counter rotating props and you'll see
that almost all have the props rotating the downward moving blade to the inside. There have been exceptions of course with the P-38 Lightning being
the prime example. I have read that it was set up normally at first but they switched it around due to other airframe issues.

For a twin where the props go the same way the prop arcs will not be balanced, so that, depending on which engine fails, it presents slightly different levels of
performance; the one that causes the most decrease in performance when it fails is designated the "critical engine".
I have built several twins and a four engine model and have set up all of them with zero offset. Don't know how much help canting them outward would
be but if one quits you will have a difficult, but not impossible, model to fly. The best rule of thumb is decrease the power a bit in order to maintain control
and find a place to land pronto. Putting larger, more powerful engines in a twin to cover an engine out is a recipe for disaster; extra power in an engine out
situation is not your friend...
Twin flying is great, best of luck on your project!

Hope this helps.
Paul, former multi-engine instructor




tenacious101010 03-17-2012 10:46 AM

RE: Engine mounting angles, how to set up a twin
 
Thanks Paul, well explained.

Ed_Moorman 03-17-2012 10:50 AM

RE: Engine mounting angles, how to set up a twin
 
1 Attachment(s)
Denny,

My buddy, Carl "Flaps" Laffert, and I have built 25 twins. We heard so many "old wives' tales" that we decided to test a couple out to make sure. One of the first ones we tested was engine out thrust. I was flying a Twin-Air 45, low wing sport twin powered by 2 Magnum .52 2-strokes with 11-6 props. I was at full power doing a Cuban-8. When I used aileron to roll upright in the first half of the maneuver, the plane snapped. That got my attention. I throttled back, recovered and slowly added power. Sure enough, one engine was out. I then landed safely.

On the way home and later, we talked about what we could do about an unexpected single-engine situation. I remembered from maybe 20-years ago reading that out thrust would cure the problem. I seemed to remember 8 or 9 degrees out was the specified amount. That looked like a lot, but Flaps said he would modify a kit for the test, if I would buy it.

I ordered a Goldberg Tiger 2 ARF (.40-.46 size plane). I also had 2 nearly new OS .46AX engines we could use. Flaps built 2 nacelles from lite-ply and mounted them on the wing. He also closed in the nose and finished the kit with my radio. It was a little heavy at 8 1/2 lbs, giving it a 32 oz/sq ft wing loading, but I felt it would be OK.

The first 2 flights, I trimmed and got the feel of the plane. It did need to be landed a little faster, but nothing outrageous. For the third flight, I filled one tank (I forget which) to half full and the other was filled completely full. I took off and proceeded to stay at full power and do my usual acro. I did have fellow club test pilot, Mark Pfeiffer, standing with me as a witness. There were other fliers up so I couldn't hear the engines that well. After around 7 minutes, Mark said, "Ed, you have lost an engine." I hadn't noticed anything. I flew around a bit more, then landed. Next, I shorted the other tank and repeated the test. This time, I was watching closer and did notice a drop of speed just before Mark informed me of the engine out.

At no time on either flight did I ever feel the plane was out of control or even hard to control. The engine failures were a "non-event." I got so confident flying with out thrust that on a later plane, I had an engine quit on take off. Rather than abort, I just finished the take off, flew around for a couple of minutes, then landed. Granted I had plenty of power, since most of my planes are over powered, but, even so, to take off on one engine does show the benefit of out thrust.

I did take my out thrust twins to several twin meets and the first question I am always asked is, "Don't you lost a lot of forward thrust with that much out thrust?" Actually, no. I'm an engineer and the math tells me that at 8 degrees, I still have slightly greater than 99% of my forward thrust. I know the angle looks extreme, but the math tells me that's true. The cosine of 8 is .99027. My experience in testing confirms this number, as best as I can tell.

Anyway, Denny, I did write for R/C Report magazine for 10 years. You can find my twin notes in back issues from their on-line site. You can also search this Twin Forum for "Ed_Moorman" that's Ed underscore Moorman for my old posts. Drop me a PM with your email and I'll find those old notes and send them to you.

tenacious101010 03-17-2012 11:03 AM

RE: Engine mounting angles, how to set up a twin
 
Thats great information Ed, I really appreciate you taking time to write that. It will be a help to me and others here I am sure.
A while back, I purchased a twinstar ARF. I never flew it, but my son did. We were so curious about twin engine out performance that we purposely put minimal fuel in one tank so the engine would quit shortly after takeoff. We found that with the stock setup of the Twinstar ARF, the plane behaved about the same (except just a little less power) as it did with two engines except when trying to taxi or take off. He actually got so comfortable he would roll and do low inverted passes over the runway with one engine out and it flew great according to him. I have no idea if or how much angled out the engines were.
Denny

ps2727 03-17-2012 11:25 AM

RE: Engine mounting angles, how to set up a twin
 
Thanks for posting that Ed, there is nothing like trying out ideas for yourself. I haven't tried offsetting the thrust angles but your efforts show that it does work.
A while back my brother and I made a twin out of a .60 size trainer. We put two .25's on it and found the single engine handling to be very nice. The angles were
zero-zero and we attribute the good handling to the small engines which, when one is out, doesn't create the wild swing we've seen on other twins with more power.
I suppose it's all in the trade-offs we choose when building our models.
We have flown many times to practice single engine flight and can do slow flight and even go arounds on one engine. I think this kind of practice will come in handy
when we fly our scale twins.

Paul


All times are GMT -8. The time now is 09:46 AM.