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Model Forum / Radio Controlled / Air Models / September 2006Torque on twin engine planes?
This weekend I overheard a rather interesting conversation at an airfield here, but did not participate myself. Two guys with a large silver oldtimer that looked like a "Tante JU" (JU 52, had to look it up) with three props were discussing torque issues. I know that the common way to counteract torque in twin prop (i.e. the banana heli config) HELICOPTERS is to just counterrotate the blades. On my own planes I've never bothered, since I never consciously thought about it. Google searches showed that counterrotating seems to be the common way to fight it... but why even bother? I'd imagine this here (imagine that's a plane seen from the front or the rear... with the lines being the wings...): -----(engine)-----(body)-----(engine)----- Then if the engines counterrotate, it'd be a force distribution similar to this (depending on which engine goes which way around, I know): -----UP-(e)-DOWN--(body)--DOWN-(e)-UP----- Nice and symmetric. Would there be any difference in flight if the torque was exactly reversed? I should think not? But what (roughly) would be the effect of both engines rotating in the same direction? The forces ought to be like this: -----UP-(e)-DOWN--(body)----UP-(e)-DOWN--- I would imagine it more or less being the same or even a little less than a single engine's torque effect? The center torque should cancel each other out. Would the torque be twice that of a single engine with the combined force of both? A fraction of one engines power? I can't quite find a straight answer to this via Google. My main question is really "do I need to know this", but since I am incurably curious I guess I HAVE to know it nonetheless :) Btw, the two guys talking about it didn't come to a consensus about the proper way to work out that three-engine plane's torque :) Thanks Jenni > This weekend I overheard a rather interesting conversation at an > airfield here, but did not participate myself. [quoted text clipped - 21 lines] > Nice and symmetric. Would there be any difference in flight if the > torque was exactly reversed? I should think not? In normal operation no. With one engine out you want the engines arranged so the imbalance cause by one engine out is countered to some degree by the imbalance due to torque. The P-38 did this -- after getting it bass ackwards on the first try. > But what (roughly) would be the effect of both engines rotating in the > same direction? The forces ought to be like this: [quoted text clipped - 6 lines] > the combined force of both? A fraction of one engines power? I can't > quite find a straight answer to this via Google. The two torques will add. If you don't like that, the inner torque (down-body-up) will be much smaller than the outer torque (up-e-e-down) because it will have less leverage. Trust me that the second exercise will get you the same answer as the first. What _will_ happen is that the torque effects will be different because of the aerodynamics of two smaller props instead of one big one, and the placement of the engines and wings, and all that other second-order stuff that modelers are usually happy to ignore. > My main question is really "do I need to know this", but since I am > incurably curious I guess I HAVE to know it nonetheless :) Only if you're going to build a twin. Do a web search on the Rutan Boomerang. It's an asymmetric twin engine design that automagically minimizes the torque problems in a twin. Very pretty, and weird looking -- and very typically Rutan. > Btw, the two guys talking about it didn't come to a consensus about the > proper way to work out that three-engine plane's torque :) That's easy -- fly it and find out.  SignatureTim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html >> This weekend I overheard a rather interesting conversation at an >> airfield here, but did not participate myself. [quoted text clipped - 54 lines] >> > Only if you're going to build a twin. I've built two twin-engine pusher jets. Wingspan ~2m, delta-wing, electric, both props (6x5.5 props at the moment I think?) rotating the same direction. Which is why I am so curious about the subject - I never noticed much torque effect when flying them. > Do a web search on the Rutan Boomerang. It's an asymmetric twin engine > design that automagically minimizes the torque problems in a twin. Very > pretty, and weird looking -- and very typically Rutan. Ohhhyes, I completely forgot that one... I always liked the slightly odd look. >> Btw, the two guys talking about it didn't come to a consensus about >> the proper way to work out that three-engine plane's torque :) >> > That's easy -- fly it and find out. They eventually did take it up in the air, and it flew well. It seemed to be a bit heavy though, landing looked more like a rock dropping on the runway than a gentle glide - even at relatively high speed. In the air though... it looked quite gorgeous. Jenni > This weekend I overheard a rather interesting conversation at an > airfield here, but did not participate myself. [quoted text clipped - 21 lines] > Nice and symmetric. Would there be any difference in flight if the > torque was exactly reversed? I should think not? I think there would be a difference... Look at it with the rotations reversed (swapping which wing the engines are on): (B) ---DOWN-(e)-UP----(body)----UP-(e)-DOWN--- Now imagine both with one engine out: (A) -----UP-(e)-DOWN--(body)-------(e)-------- (B) ---DOWN-(e)-UP----(body)-------(e)-------- Which would you rather try to keep flying? Remember, "engine out" means decreased lift on that side (no more propwash over the wing) _and_ more drag (stopped or windmilling prop). You're _also_ likely to be running the remaining engine at significantly higher power to make up for the loss... meaning MORE torque. The preferred rotation is left as an exercise for the class... :) > But what (roughly) would be the effect of both engines rotating in the > same direction? The forces ought to be like this: [quoted text clipped - 5 lines] > the combined force of both? A fraction of one engines power? I can't > quite find a straight answer to this via Google. However, the problems don't occur so much with both engines running - again, it's the "engine out" scenario which is (literally) the killer. Imagine the above with one engine out: ---DOWN-(e)-UP----(body)-------(e)-------- Now the other engine out... --------(e)-------(body)--DOWN-(e)-UP----- The problem is, the aircraft response is significantly different, depending on which engine is out! An "engine out" emergency is no time to have the pilot having to worry about different reactions to turns in different directions, especially if it happens on takeoff! > My main question is really "do I need to know this", but since I am > incurably curious I guess I HAVE to know it nonetheless :) > > Btw, the two guys talking about it didn't come to a consensus about the > proper way to work out that three-engine plane's torque :) Treat it as a two-engine plane, make them counter-rotate, and don't worry about the third center engine. SignatureEvaluating all GUIs by the example of Windows is like evaluating all cars by the example of Yugos. > Treat it as a two-engine plane, make them counter-rotate, and don't > worry about the third center engine. > > -- That sounds like practical advice. Taking it a step further, the slower and draggier the plane, the less it matters at all. I built a Senior Telemaster with three OS 20 FPs on it, and torque didn't make a bit of difference. I lost an engine on a touch and go a few times, and all it took was some rudder to do a fly-around and land it. I would expect a big Junkers to be fairly easy to handle. | I know that the common way to counteract torque in twin prop (i.e. the | banana heli config) HELICOPTERS is to just counterrotate the blades. Well, short of a tail rotor, that's the only way to counteract the torque :) | On my own planes I've never bothered, since I never consciously | thought about it. Torque issues are *huge* in a helicopter (and I guess in a plane like the Osprey.) In a plane, they're much smaller and can be safely ignored in many cases. | Google searches showed that counterrotating seems to be the common way | to fight it... but why even bother? Why bother? Because it makes for a slightly better (in most cases) flying plane. For most planes the torque, prop wash, P-factor and gyroscopic precession aren't usually a big deal except possibly at takeoff, but if you can minimize them simply by having one engine run the other way, that's a good thing. Why not bother? Logistics. If your engines go opposite directions, you probably can't just swap engines back and forth, and something will have to be different about each engine to make it turn in a different way. And you'll also need a `pusher' (reversed) prop for that side. Talking about models, for a glow engine I think you generally swap out the crankshaft to get an engine that rotates the other way. Of course, this special crankshaft may not even be available ... For brushed electric motors, all you do is swap the wires, and that's fine if the timing is neutral, but if it's not, you also need to adjust the timing. (And it shouldn't be neutral unless you just don't care about performance or motor life, which is often the case with cheap can motors.) For brushless motors, you can generally reverse a motor just by swapping two of three three leads -- no sweat. (If it's a sensored brushless motor, then you also have to worry about timing again, but sensored brushless motors are rare nowadays.) The ESC takes care of the timing. SignatureDoug McLaren, dougmc@frenzied.us It's always darkest right before it goes pitch black. | This weekend I overheard a rather interesting conversation at an | airfield here, but did not participate myself. [quoted text clipped - 41 lines] | Thanks | Jenni It becomes real important on full scale twins. P factor causes more thrust on the downward side of the propeller arc due to a nose high angle of attack to the relative air flow. (This also occurs with a C-150 trainer on climb out. Right rudder must be held to compensate for the left turn tendency.) With counter-clockwise rotating engines (viewed from the front of the airplane) the left engine is the critical engine I.E. the thrust is more closer to the fuselage while the right engine's thrust is outboard of the nacelle. Here is a pretty good example of the effects of P factor: http://yarchive.net/air/airliners/p_factor.html This is another article which goes into more detail: http://www.pilotscafe.com/articles/multi-OEI.pdf Some full scale twins also mount the engines with a 5° outward angle to compensate for the engine out yaw that is experienced. This has also been done with some twin engine models. In models most of us do not fly very cordinated anyway. Most of us make slipping turns, I.E. aileron only turns that would really show up as sloppy piloting in a full scale airplane. SignatureJarhead Jennifer Smith Wrote: > Nice and symmetric. Would there be any difference in flight if the > torque was exactly reversed? I should think not? Lockheed found out about that on the P-38, the hard way. Their notion was to eliminate the 'critical engine' issue on twins which is that under certain flight regimes (low and slow, for example one engine in a twin will be more critical if lost than the other. IOW if the critical engine barfs out shiny bits at the wrong time, al the pilot can do is fly as far into the crash as possible. Lockheed got it backwards, and both engines in a P-38 are critical. The procedure for loss of engine during climb out read something lik "try to land straight ahead 'cuz trying to turn gets really ugly reall fast" -- the-plumbe I believe they got that straightened out and it took "Lucky Lindy" Col at that time I think, to theach the flyers in the Pacific how to make a P-38 fly correctly. After that the fly boys really loved the plane and if I remember correctly (seen it some place) the three top aces of WWII were all in P-38's! Roy > Jennifer Smith Wrote: >> [quoted text clipped - 14 lines] > "try to land straight ahead 'cuz trying to turn gets really ugly really > fast". SignatureUsing Opera's revolutionary e-mail client: http://www.opera.com/mail/ >I believe they got that straightened out and it took "Lucky Lindy" Col at >that time I think, to theach the flyers in the Pacific how to make a P-38 >fly correctly. He taught them fuel economy measures that extended their range. Other handling issues had already been worked out before he arrived on the scene. It took years to get the bugs out (engines, cooling, controls, tactics). I read a great book about the P-38 last year, then gave it to a friend who gave it to a friend ... The first flight of the plane was in 1939. It didn't become an effective fighter until about the J version, which got the chin intake for cooling, larger fuel tanks, and dive brakes. >After that the fly boys really loved the plane and if I >remember correctly (seen it some place) the three top aces of WWII were >all in P-38's! Bong and McGuire were #1 and #2 in P-38s. That's not just a US WWII record, but all-time. In the prototype, the props both turned inward; the dominant production version had outward-turning props, except for a run of fighters for the British that had both engines running in the same direction. Marty I have to stand corrected it was fuel economy that he taught. However I don't think that Bong and McGuire are world record holders considering the Luftwaffe's Helmut Lipfert with 203 kills and Johannes Wiese with 133 kills. Also consider "The French called him le petit rouge, and he is known in the English speaking world as the Red Baron. In a time of wooden and fabric aircraft, when twenty air victories insured a pilot legendary status and the coveted Pour Le Mérite (the famous "Blue Max"), Richthofen had eighty victories, and is regarded to this day as the ace of aces." Roy >> I believe they got that straightened out and it took "Lucky Lindy" Col >> at [quoted text clipped - 27 lines] > > Marty SignatureUsing Opera's revolutionary e-mail client: http://www.opera.com/mail/ >I have to stand corrected it was fuel economy that he taught. However I >don't think that Bong and McGuire are world record holders considering the >Luftwaffe's Helmut Lipfert with 203 kills and Johannes Wiese with 133 >kills. I did not phrase what I was trying to say very well. >> Bong and McGuire were #1 and #2 in P-38s. That's >> not just a US WWII record, but all-time. I meant that they are the top US aces of all time (WWI, WWII, Korea, Vietnam, etc.), not that they were at the head of the world list. Marty > the Luftwaffe's Helmut Lipfert with 203 kills and Johannes Wiese with 133 Tallies that will now probably never be beaten. Steve | > the Luftwaffe's Helmut Lipfert with 203 kills and Johannes Wiese with 133 | | Tallies that will now probably never be beaten. | | Steve Don't forget that the Germans flew until they were killed, or to the end of the war, whereas our ace pilots were taken out of combat and brought home to sell war bonds. SignatureJarhead >> the Luftwaffe's Helmut Lipfert with 203 kills and Johannes Wiese with 133 > > Tallies that will now probably never be beaten. > > Steve And may or may not have been 'confirmed' in the way that they should have been. Totting up official German tallies of RAF aircraft downed reveals that somehow the RAF used more than three times the number of planes that it actually did.. The RAF eventually refused to confirm kills without ground evidence of a downed aircraft, and or camera gun data or an independent sighting. Often up to three pilots would simultaneously 'shoot down' the same aircraft..or flak got it as they were waving a gun in its general direction...or in fact it didn't get shot down at all, but just disappeared from sight. Some made it back. One way to rack up a good score was to strafe airfields..if the flak didn't get you you could claim a dozen aircraft destroyed in one pass. Sometimes they were only made of wood and canvas though.. In short official records of kills are only valid in the context of the methods used to assess them. > >> the Luftwaffe's Helmut Lipfert with 203 kills and Johannes Wiese with 133 > > [quoted text clipped - 4 lines] > And may or may not have been 'confirmed' in the way that they should > have been. That's partly what I meant. Steve Checking the website "Aces of the Luftwaffe" I found that most of the victories by both Lipert and Wiese were against Russian aircraft. I think, although I could be wrong, that the German fighters were far superior to the Russian's during WWII. Lipert does show one B17, one B24 also three P39's victories so it wasn't always a turkey shoot. Strangely I didn't notice any obvious British planes in amongst the list, although I only did a quick scan could have missed them. http://www.luftwaffe.cz/wiese.html http://www.luftwaffe.cz/lipert.html Roy >> >> the Luftwaffe's Helmut Lipfert with 203 kills and Johannes Wiese with > 133 [quoted text clipped - 9 lines] > > Steve SignatureUsing Opera's revolutionary e-mail client: http://www.opera.com/mail/ |
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