Author Topic: Twin engine question (Read 3011 times)

gene poremba · « **on:** October 11, 2021, 04:42:10 AM »

I put together a modified twin engine Twister to resemble a HS-129. I kept all the moments of the Twister, but have a slightly larger tail and rudder to closer resemble the HS-129. Its powered by 2 LA25's. I have APC 9-4 props on it right now with Sullivan 4oz clunk tanks on tongue muffler pressure. I previously ran both these engines in the past weeks with this set up on my Sig Skyray with no issues. The plane balances 1/4" in front of the main spar. The ready to fly weight is 50.7 oz. My question is, which engine should i try and set up to quit first? Does it matter? My start procedure is, i start the in board engine first, then the out board. I have both engines tac'd within 100rpm. This is my first twin engine model.

gene poremba · « **Reply #1 on:** October 11, 2021, 04:55:59 AM »

I wanted to add that i did spend time looking thru the archives of past posts and found lots of pictures and video of twins flying. It was hard to tell on the video though which engine quit first. On some flights, it looked like the inside quit on one flight and on another flight the outside engine quit. However, it didnt seem to matter much at least from viewing the video much which engine quit first. So i figured i'd ask before flying it and regretting asking.....Gene

Robert Zambelli · « **Reply #2 on:** October 11, 2021, 05:35:34 AM »

Gene - I’ve built and flown a number of twins and I always set the outboard engine to quit first.
If the inboard quits first, there’s a chance the aircraft will yaw in at you.
That happened on one plane and I lost it.
Bob Z.

gene poremba · « **Reply #3 on:** October 11, 2021, 05:52:43 AM »

Thanks Bob. I'll put a tad less fuel in that engines tank. .....Gene

Dan McEntee · « **Reply #4 on:** October 11, 2021, 07:02:54 AM »

Search out whatever you can find on twins by John Miller. He and Gordan Delaney designed the twin Pathfinder, a 700 square inch design and it flies on two .15's! He has some great detail on the plans for engine nacelle set up, and other twin trimming information. John recommends setting the out board engine just a tad leaner than the inboard, because it has to fly farther in the circle and generates a bit more centrifugal force and this will richen up the engine. It's a very subtle difference. Your set up with the inboard mounted tanks may change things a bit. I believe John has said that on this model it doesn't matter which engine quits first if built to plans. You will have plenty of power with two LA.25's! I have some ides for twin stunters based on twister also and will use .15's or even a couple of )S FP-10's I have stashed that are set up for C/L. I love the look of your model and would like to see some photos of your tank set up and plumbing. Good luck with the project!!
Type at you later,
Dan McEntee

gene poremba · « **Reply #5 on:** October 11, 2021, 07:13:43 AM »

Thank you Dan. I'm at work right now and can post pictures of my tank set up when i get home later today.....Gene

BillP · « **Reply #6 on:** October 11, 2021, 07:44:57 AM »

Outside first.

Dan Berry · « **Reply #7 on:** October 11, 2021, 08:22:30 AM »

If I was right handed and flipping a prop with the right hand I would be starting the inboard engine first.

944_Jim · « **Reply #8 on:** October 11, 2021, 09:49:38 AM »

Mr. Dan,
That plane build log is on RCGroups. There is a good shot of his tank setup in one of pictures. Look in the RCG C/L forum.
https://www.rcgroups.com/forums/showthread.php?3987847-Twin-engine-C-L-warbird

944_Jim · « **Reply #9 on:** October 11, 2021, 10:02:24 AM »

Gene,
My BHM Mosquito could take off with the outboard engine out...as long as there was ZERO wind, and given time to reach take-off speed (two laps or more). It would fly tolerably on just the inboard engine.

HOWEVER (inboard failure),
It would fly level, and light climb to above the head with the inboard engine out in limited wind.
Any wind with the inboard engine out would (did twice) doom a take-off. Also flying after inboard flame-outs was very dicey.
Keep in mind this WAS a profile model with slab-flying surfaces and zero effort to lighten during construction. Fueling was always a problem due to tank location and construction. The last flight was in wind, with an inboard flame-out just after take-off...coming across the wind and still climbing. The wind caught under the wing, rolled the plane inward, and slacked the lines. As the plane zoomed by with its tail to the wind, I back-pedaled as much as possible, only to prolong the flight until it just missed me from 20 feet up, and slammed outboard engine first into concrete. Moral of the story: Don't fly a barely tolerable twin with a new tank set-up on a windy day! Yes, I will build and fly twins again. I am addicted to, and love the sound of the engines in sync!

Gary Dowler · « **Reply #10 on:** October 11, 2021, 10:09:20 AM »

You always want the outside engine to quit first. It will probably take some experiments with fuel load to get that sorted out, as often times multi engine setups wont burn the exact same amount of fuel in every engine. Only way to tell is to fly it. Ground tests probably won’t give you the real answer.

gene poremba · « **Reply #11 on:** October 11, 2021, 10:23:09 AM »

Dan here is a pic of my engine and tank set up. Not alot of room up front.

gene poremba · « **Reply #12 on:** October 11, 2021, 10:25:51 AM »

Ill try again.

944_Jim · « **Reply #13 on:** October 11, 2021, 11:29:17 AM »

Gene,
I fixed my earlier post to reflect WHICH engine failed. Sorry all for my engine-dyslexia.
Moral...don't type on a forum while talking about something else with someone unrelated (my youngest son and straight razors in this case).

John Carrodus · « **Reply #14 on:** October 11, 2021, 01:58:56 PM »

Gene , I love the colour scheme and slick semigloss finish to your HS 126 Twin Stunter.

gene poremba · « **Reply #15 on:** October 11, 2021, 02:21:16 PM »

Thank you John.

Howard Rush · « **Reply #16 on:** October 11, 2021, 02:49:49 PM »

Quote from: Dan McEntee on October 11, 2021, 07:02:54 AM

Search out whatever you can find on twins by John Miller. He and Gordan Delaney designed the twin Pathfinder, a 700 square inch design and it flies on two .15's! He has some great detail on the plans for engine nacelle set up, and other twin trimming information. John recommends setting the out board engine just a tad leaner than the inboard, because it has to fly farther in the circle and generates a bit more centrifugal force and this will richen up the engine. It's a very subtle difference. Your set up with the inboard mounted tanks may change things a bit. I believe John has said that on this model it doesn't matter which engine quits first if built to plans.

I have the good fortune to have that very airplane. It's hard to tell which engine quits first. You gotta look at the wheels to see which way it yaws.

The outside engine takes 1/4 oz. more fuel than the inside engine.

Yes, a right-handed person should start the left engine (looking from the back of the airplane) first. I lost a little blood learning that.

wwwarbird · « **Reply #17 on:** October 11, 2021, 06:30:35 PM »

Assuming the model is well trimmed it doesn't matter a whole lot, the main thing is just to be ready whenever one does decide to quit. On my twins I've never noticed any significant yaw issues with either engine out, a noticeable lightening of line tension, but no actual yaw. The line tension seems about the same with either engine out. Mainly, it's just best not to fly your twin when it's windy, which can increase the chance of having a problem.

Here's the simple trick for running an IC twin without a fuel shutoff...

Once at the field and prior to the first flight of the day, fire each engine separately, tune to the desired setting, and then shut off. First one engine, then the other. Don't go for the ultimate lean power setting either, leave them just slightly rich. That "slight rich" needle setting gives a safety margin that greatly lessens the chance of one engine going lean in flight and totally screwing up your chances of them both quitting together, or nearly so. Once you're happy with these prior-to-flight needle settings then top off the tanks and you're ready.
At the flight line, put a battery on each engine, fire the inboard then quickly the outboard. With this routine you should be able to have both engines fired for a flight within a couple seconds of each other. DON'T TOUCH THE NEEDLES, if the engines are pre-tuned as described they will sync up nicely once in flight. With equal fuel loads and the needle settings described you'll usually only end up flying a couple laps on one engine, no big deal.

Whatever the case, ALWAYS start the inboard engine first, you don't want to be reaching over or around a running outboard.

wwwarbird · « **Reply #18 on:** October 11, 2021, 06:51:56 PM »

Oh yeah, great looking model Gene!

gene poremba · « **Reply #19 on:** October 12, 2021, 03:23:51 AM »

Thanks once again for all the input. I'm anxious for the weather to settle down (rain & high winds) to fly this one....Gene

John Rist · « **Reply #20 on:** October 12, 2021, 08:39:20 AM »

I realize this is an inappropriate comment but with electric it is easy to have twins quit within seconds of each other.

wwwarbird · « **Reply #21 on:** October 12, 2021, 06:57:56 PM »

Quote from: John Rist on October 12, 2021, 08:39:20 AM

I realize this is an inappropriate comment but with electric it is easy to have twins quit within seconds of each other.

Yeah, but that's for wussies.

gene poremba · « **Reply #22 on:** October 16, 2021, 08:53:32 AM »

Well, i got to fly my Twin this morning. First flight i short tanked it, especially the outside tank to be sure it quit first, it did. Basically checked wings level in normal flight, did a few wing overs and a few inside loops, went well. Second flight checked inverted wings level, did a few outside loops and reverse wing overs, outside engine quit first. Third flight, i flew the complete pattern. Inside engine quit first, i felt and saw it yaw inward slightly, but then it flew the rest of the level laps fine til landing. Overall i'm really pleased with how this worked out. Its doing 5 second laps on 62' lines. Its not as fast as i was expecting it to be and stunts just like my Twisters. My pit man took 3 videos and a few stills, but the videos are not worth posting. He said Apple hates him! The videos were blurry.....Gene

wwwarbird · « **Reply #23 on:** October 16, 2021, 01:03:32 PM »

Great to hear Gene, sounds like you're off to a good start, have fun!

Gordan Delaney · « **Reply #24 on:** October 17, 2021, 12:21:26 AM »

Do not set the out board engine leaner. Set them both the same. My Twin Pathfinder was 620sq.in. weight was 46 oz. Hope this helps.
Gordy

944_Jim · « **Reply #25 on:** October 17, 2021, 07:43:18 AM »

Quote from: gene poremba on October 16, 2021, 08:53:32 AM

Well, i got to fly my Twin this morning...Overall i'm really pleased with how this worked out. Its doing 5 second laps on 62' lines.

WooHoo! I love the sound as the engines dance around being in synch. Good for you, Sir! Big Happy Dance and congrats. That plane is stunning! Do it again with a Mosquito so you can have a WWII day.

Bruce Perry · « **Reply #26 on:** October 24, 2021, 09:59:43 PM »

here are a couple videos of my twins,

this one is Gordan Delaney's Gemini that he flew at the 95 Nats....

This one is my version of Gord's Two Much ....

The Two Much has retracts just like the original.

Ive had inside engine outs on both models and like Howard said, there is very very little difference.

Bruce

gene poremba · « **Reply #27 on:** October 25, 2021, 01:07:35 AM »

Thanks for posting the videos of the twins flying. For me, a twin, was on my bucket list of planes i wanted to build and fly one day. Glad i got the chance to build and fly one. They really get everyone's attention when you fly them. The sound is un mistakeable. I have several flights on mine now and it doesn't show any bad habits while flying. Thanks to everyone that answered my questions i had before flying mine helped me feel more relaxed on my maiden and successive flights.....Gene

Fred Cronenwett · « **Reply #28 on:** October 31, 2021, 03:43:59 PM »

I fly a variety of CL Twin engine models and I always start the inboard engine first that way as I start the outboard engine I am move away from a running propeller. The idea of putting less fuel in the outboard engine would solve the problem. Just need to account for the amount of fuel the inboard engine will use before you can get the outboard engine started. I use a starter on my multi-engine models.

Now the one's I fly have throttle control but I have had the inboard engine quit in flight and when that happens I make very slow throttle changes but land immediately, making slow changes in the throttle. I have never had it yaw into the circle towards me but had I quickly throttled up the outboard engine it would have turned in towards me. I am not going to try that!

gene poremba · « **Reply #29 on:** October 31, 2021, 04:48:41 PM »

Update, I have now flown my twin enough times to have had both the inside and outside engines finish first enough times to say that at least on this model it remains steady on the lines with out any negative tendency to turn inward towards me. Both of my engines are very close to the fuselage center as i dared put them during the build, and this may help. I guess catching a bad gust of wind could possibly change things quickly though. I dont want to find out!.....Gene

wwwarbird · « **Reply #30 on:** October 31, 2021, 08:57:50 PM »

Quote from: gene poremba on October 31, 2021, 04:48:41 PM

Update, I have now flown my twin enough times to have had both the inside and outside engines finish first enough times to say that at least on this model it remains steady on the lines with out any negative tendency to turn inward towards me. Both of my engines are very close to the fuselage center as i dared put them during the build, and this may help. I guess catching a bad gust of wind could possibly change things quickly though. I dont want to find out!.....Gene

Just as I meant in my explanation earlier in this thread. As long as the model is built and trimmed decently twins are not nearly as big a deal as many seem to think it is. Glad it's working well for you Gene, it's simple, just add fuel and go have fun!

Robert Zambelli · « **Reply #31 on:** November 01, 2021, 01:31:00 PM »

I built this twin around four years ago.
Designed by Henry Bourgeois in the late forties/early fifties it is absolutely the best flying OTS plane I’ve ever flown. In fact, in the right hands I’m sure it would do well in classic.
Very simple construction as shown, it flew perfectly without any trim whatsoever.
I powered it with two early fifties Fox 19s turning Master Airscrew 9-6 plastic.
It weighed 41 ounces.
Last photo: Inboard engine quit in the horizontal 8.

Bob Z.

gene poremba · « **Reply #32 on:** November 01, 2021, 02:08:47 PM »

I see the resembelance to the tigercat. I was wondering how the fuel tank fit in that close to the wing, but then looked further down and saw the exploded view. That was a really good looking twin. ...gene

944_Jim · « **Reply #33 on:** November 01, 2021, 07:03:52 PM »

Quote from: gene poremba on November 01, 2021, 02:08:47 PM

...but then looked further down and saw the exploded view...gene

Bad choice of words, or an attempt to soften the blow?

wwwarbird · « **Reply #34 on:** November 01, 2021, 08:07:07 PM »

Quote from: gene poremba on November 01, 2021, 02:08:47 PM

but then looked further down and saw the exploded view.

Is that what they mean by "reverse engineering"?

Air Ministry . · « **Reply #35 on:** November 21, 2021, 09:04:26 PM »

I guess catching a bad gust of wind could possibly change things quickly though. I dont want to find out!.....Gene

yep , if you get stuck inverted on the outer , in A BIG WIND . land inverted .

a SPIN CAN DISMANTLE IT AIRBORNE WITH THE pROP HUB AS A WINCH ON THE LINES .

iNVERTED , engine out , do a few dis and curves / climbls . get hight and swing it through downward to recover low .
rather than climb a turn in . . .

THOUGH some fly good enough for wingovers & loops etc on the outer .

I tend to use70 ft lines in good air , also a bit of outset on the outer injun .only .

A set em nose vertical & shake it . id guess the inner tanks will give a good run . more grunt higher , 4 stroking low .

Air Ministry . · « **Reply #36 on:** November 21, 2021, 09:15:11 PM »

Someone manadged to start all three, or is that six , including a 50 yard return sprint for a spanner for the third . there was some fuel in the nitro , prop kicked loose .

74 the OS 20 262 ran No itro , but in sumer . needles are crankier & vary with weather / barometer . LAs should be ok with little nitro .Otheres you want 10 % for consistancy .