handyman

My Feedback: (2)

going to be bashing a tiger 60 kit, need help on motor set up as to off set of any and any info you can help with

Ed_Moorman

My Feedback: (1)

I did testing on thrust offset for my column with a kit bashed Tiger 2 twin. 8 degrees will just about negate all the bad stuff from an engine out. On my Twin Stick, I did acro on one engine without holding the rudder. For testing, I filled one tank partially full and the other all the way. Then I would fly, mostly full power, doing acro. When the partial tank ran dry, I got a good feel for how the plane handled in a surprise shutdown. That's not like these guys who set their plane up so they can idle down, then shut one down and slowly come back in with the power, holding it when the yaw gets too great. I'd be flying along, and I did both tanks this way, and bam! an engine would quit. The plane sounds different and flies a little slower. My buddy did the conversion and I think one engine had more out thrust than the other, but it didn't seem to make any difference.

8 degrees does look like a lot, but, if you do the math (and I'm an engineer), you lose less than 1% of your forward thrust. That's not too shabby for what you get.

I kept hearing these guys whining about synchronizing their engines so I ran another test. I yanked the OS AXs and installed a TT Pro .46 with a Tower muffler and 11-6 prop on one side and a TT .42GP, stock muffler, 10-6 prop on the other and tested it. Still flew fine. I even flew at the Multis Over McDonough, twin meet in 2005 and no one noticed I had different engines and props.

I'll assume you are going to use 2 .46 or bigger engines. The best thing I can tell you is to use very reliable engines, break them in before flying them on a twin and set them slightly rich. Twins tend to lean out more or somethng because an engine that runs perfectly good on a single will quit on a twin. I tried Magnum .52XLS, I tried GMS .47s and ended up using OS .46AXs on 2 twins. Two Mag .28s wouldn't keep running, but 2 OS .25FXs do, at least on my planes. We have had good luck with 2 Magnum .52 4-strokes. Thunder Tigers might be good on a twin, but I haven't tried them. I did have 2 Evo .46s on a Dual Ace and they ran OK.

It is my opinion that having both engines run reliably is much, much more important than synching them up. I keep telling people that 10,000 rpm out of sync is a lot worse than 500 rpm out. When I test a new twin, I crank up one engine, tun it, hold the nose up-do all the things you do for a single. Then I shut down and do the other engine. Then I top off the fuel, crank both and fly.

I also use a filter on each engine as well as in the fuel jug.

I used a JR 8103 radio and used both mixing and a Matchbox. Now I use a 9303 which has a twin engine routine that is really slick. I know guys who use 1 servo and belcranks, but I consider that practice bleeding. It is so much easier to mix, I mean blip, blip, blip and you're running. If you want to spend the bucks for a Matchbox or Equalizer, you can have both engines set in 2 minutes or less.


Anything more you need to know, drop me a PM or e-mail. And send me a picture when you're done to go in R/C Report.

simhatus

Hi Ed

Very interesting, what degrees out thrust would you suggest for a twin with plain bearing .25 engines with a 9*6 propellor?
and a twin with 2 os10 plain bearing engines?.

Simon

Ed_Moorman

My Feedback: (1)

The out thrust is the same no matter what the size. When a buddy and I started building twins a few years back, and we are about to begin our 23rd one soon, we were talking and I remembered reading about out thrust 20 years or so before. I did some simple trig calculations and 8 degrees seemed to be the best number. You have just over 99% of your forward thrust (cos 8 = .9903), but you get about 14% side thrust (sin 8 = .1392). Losing less than 1% of my forward thrust seemed to me a small price for being able to practically ignore the stress of losing an engine.

Your buddies will tell you that you'll be losing half your thrust. As an engineer who did the math, I have learned to disregard many comments, especially when I am doing an experiment to test out if it really works. Once I flew the plane and saw that there didn't seem to be any difference in performance and my engine out tests showed how well the plane handled on one engine, I was convinced.

In most of my planes, I used Ernst Thrust Plates bought from Tower. [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXE951&P=RH]Thrust Plates[/link] They come in 3 sizes, 25, 40, 61. It takes 2 sets to do a twin.

evan-RCU

I used to know it all by heart... You put out thrust in only one engine (assuming you are using engines that spin the same direction) the torque from the other simulates the out thrust for that side. The left is left straight and the right engine has the offset. I used about 4-5 degrees. The easier way is to use one normal and one contra-rotating with the contra on the right.

jrf

My Feedback: (551)

Ed: Could you explain to me how out-thrust makes more power? I mean 13% more thrust just by angling the engines 8 degrees seems a lot easier than buying and installing bigger engines. (99% plus 14% equals 113%. Or is it different for engineers?)

Jim

evan-RCU

It's the old math....