JollyPopper

My Feedback: (6)

Is it absolutely vital in a multi engine RC setup to have the engines all running at the same speed when they are at full throttle? We spent the day today getting a huge C130 ready for her maiden flight. It has four brand new Saito 180s on it and they were running anywhere from 6800 to 7600 RPM at full throttle. Mind you, these engines are not broke it yet, so what we have today is not what they will eventully end up being, but we were wondering at the field just how closely the[img][/img][img][/img] engines should be at full throttle or at any throttle setting for that matter. We didn't fly her today. This was just a shake down to find out what all would need attention. The engines started right up and ran fine but with a lot of variation in WOT. If we were to synchronize them, should we do it with the needle valve settings or just how. And if we do synchronize them, we will have to tune them all to the slowest turning of the four. That means we would have to detune three of them to match the slowest running one or try to adjust them all mechanically with the linkage some how. Anyone have any suggestions?

Altered1

Hey Jollypopper,

First up - I know nothing about multiple engine setups, this is just my thoughts... see if others agree

I believe you are never going to get exactly the same RPM on all 4 engines. The best you could hope for would be approximately the same thrust on each side of the aircraft ? Any minor thrust difference between left and right sides can be taken up with trim on the control services ?

Are all 4 engines set to one channel ? For each engine, setup just on the rich side of peak, take the average RPM for each side, and apply a throttle mix on the transmitter?

You may also have better luck posting this in the glow engines thread ?

[edit] Great looking bird BTW ! I'd love to see a spectre version

Cheers

TomCrump

A friend just designed and built an early Boeing airliner. This is not his first multi engined aircraft.

He told me that it is more important to get the engines running reliably, than to have them syncronized.

His view is that any differences in thrust can be dealt with by using rudder trim.

JL1

My Feedback: (93)

I have built and flown several twins but no experience with four engined planes. With twins, I have found that +/- 500 rpm is fine and they tend to sync when in flight. What is far more important is not getting the carb settings too lean trying to sync the "weaker" engine to a stronger one.

TomCrump

You guys down in Mason seem to know a bit about multi engined aircraft.

init4fun

The OP is mentioning an 800 RPM difference between engines , that's quite a bit in my opinion .

One possible idea would be to use a digital servo on each engine's throttle , so that the travel and end points can be set for each one depending on the RPM required . Yes I realize you may not want to slow them down to perfectly match the slow one , but to slow em down a bit , maybe to get the RPM difference down to 500 or less , may be a worthwhile use of the digital servo's ability to store it's own operating parameters ....

Gizmo-RCU

My Feedback: (27)

JL1 expresses exactly what I would have advised. I have seen more that one plane distroyed by someone trying to make the slow engine keep up with the faster one (lean run). Just set them individually to run
as you would any individual engine and go fly after establishing they are loose enough to be reliable.
Very recently I helped a friend with his new Saito 40. It had almost no compression out of the box and was very tight during the first several runs surging and changing RPM's, the compression did develope after running. Both valves were very loose
and required resetting..........this is not what I expected from Saito. We were running Byrons 15% 4 stroke fuel.
After three tanks the engine was good to fly and really break-in.
I suspect you need more running to have reliable engines for a multi engine, I know I would run them until there is no question they would not quit.

mscic-RCU

My Feedback: (6)

I concur with Gizmo. I have flown several twins and I read an article a number of years ago in RCM. The builder and flyer in that article said to tune each engine individually, start them and go fly. They will "sync" in the air and they always do. I have owned a lot of Saito's over the years and if you break them in as described in the directions, they will run flawlessly. Another tip in the article was if you have an engine out, shut down the other engine and deadstick the landing. Of course he was describing twins and not multiengines. Good luck and that is an awesome looking plane.

tailskid

My Feedback: (34)

I talked to Don Parsons a while back and he "only" flew twins and he said he would rarely have an engine quit...because he worked on ONE engine at a time and NEVER tried to 'sync' them...and he flew a LOT!

JollyPopper

My Feedback: (6)

Thanks, all. I really appreciate your input. I agree with the opinions that each engine should be tuned to its peak and backed of a few hunderd RPM and let her rip. Unfortunately, I do not own the plane, I am simply the guy who makes the engines run correctly and reliably, and it really went against my judgement to detune three of them to match the weakest one, but that was what the owner of the plane wanted. Fortunately we only ran the engines for a few moments. One of the most glaring problems was that the nose wheel did not have enough weight on it to steer it properly and the wind was blowing about 30 MPH so it pretty much went where it wanted on the ground. That will be fixed by Wednesday. And armed with your guys' expertise, I hope to persuade the owner to tune each one and fly it.

The guy who owns the plane built it from nothing. He scratch built it from photos only. He is pretty much a genius in that regard, but he is not good at tuning engines. One of the other guys in the group is a machinist with his own shop and he builds landing gear and such for him. There are several of us who have a hand in this project and it looks like it will turn out well. We intend to maiden this bird on Wednesday. If I recall correctly, the span is 14'8" and it is 91 inches long. The horizontal stat is four feet and the vertical stab stands 26 inches from ground to tip. It weighs right at 60 lbs. Wish us luck.

combatpigg

My Feedback: (3)

If your plane doesn't have enough rudder authority to contend with a severe variance in thrust [such as an engine outage] between wing panels...then that plane is just an accident waiting to happen.
Always turn towards [against] the side that is making thrust and create your own condition that is best for a deadstick landing

DavidAgar

My Feedback: (108)

I agree with alot of the comments being made. I fly twins and I always put a servo on each engine. I then can adjust the end points on the servos as it is very difficult to get the carbs all set the same with bell cranks or cables. I can get my engines within 500 RPM and have not had any issue's with them. Some of the horror stories on flying twins I have seen to be untrue. One of my flying buddies was flying his twin one mourning and upon lift off on take off, he was at 10 ft elevation and lost an engine. Everyone would have you beleieve that this would be the kiss of death for a twin, however he used rudder to cordinate his turns and landed the plane with no problem. I always set my engines up so they will go dead if I want them to, so if I lose one in flight I can kill the other if I need to. I have used this feature on a couple of occassion's as I would rather land dead stick than with only one engine running. Multi engine are a little more work, but so worth it. Good Luck, Dave

Bax

My Feedback: (11)

The best advice I've ever seen was to make sure the engines are each, individually broken-in and tuned for maximum reliability. If you can get within 500 RPM or so, you're good. If you have too much variance, then your props may not be quite what they're supposed to be or the odd engine is not right. It's better to discard the engine and replace it with one that will turn where it belongs than to risk such a wonderful model. The cost of a new engine will be far, far less than the cost of the model.

Reliability is what your main goal should be.

Edwin

My Feedback: (2)

I've run mine as much as 1000 rpm off and flew fine. (ATI B-17 and H9 B-25.) My DX18 has a neat function called servo balance that will match assigned channels at 7 way points along the master travel. It still wont match them, differences in linkage will keep that from happening. But if I am within 500 rpm, I'm happy. Just my opinion, thats as good as it gets.
Edwin

topspin

OK, so if you can find a way to install a magnet on each prop shaft you can use a hall effect device similar to the ones used in ignition systems to measure the RPM of each motor. The RPM for each engine can then be quantized every 2 or 3 hundred milliseconds and the result fed to a comparator. Using a microprocessor the rpm of each engine can then be adjusted via a feedback circuit to each engine servo. Violla, an electronic engine synchronizer. Sounds like a cottage industry project for someone who is more into the technology than making any real money.

Edwin

My Feedback: (2)

Been done.
http://www.troybuiltmodels.com/items/WRCTWINSYNC.html
Edwin

Rotten40851

My Feedback: (2)

I'd say if your within 400-800 rpm differance and they are all running good, you're good to go

topspin

Huh, swiped my idea. Oh well, there is always room for a four engine system.

TexasAirBoss

My Feedback: (22)

4 X Saito 180s ? Whew ! Thats alot of power ! What does the Herk weigh ?

JollyPopper

My Feedback: (6)

Yep, four 180s. She comes in right at 60 lbs. We were holding it by hand last Sunday with all four of them wound tight and it was hard to hold. It also produced some awesome sound.

Well, the maiden didn't happen today. The wind came up to about 20 steady and gusting to 35 or so, thus the delay. It is now scheduled for Sunday, weather permitting.

I didn't really research those governors, but first glance looked like they were for twins. I'm not sure how they cold be adapted for a four engine application.[]

beepee

Jolly,

Should be able to use the twin version. Get two systems. Connect the two outer engines to one and the inner engines to the other. I don't know why that would not work. This way, if an outer engine shuts down, the opposite outer engine will drop to idle, but the inner engines will remain at original throttle setting. Should result in close-to symmetrical thrust.

Good luck with it. I love the Herc.

Bedford

P47Tbolt

My Feedback: (20)

Have you tried moving the props from one engine to another. swap the prop from the engne with the lowest rpms to the engine with
the highest rpms. I have done this on a twin and it got the rpms closer. I tune each engine then check rpms,then try to get the rpms closer by swapping props around to get them as close as I can,then I take the prop from the engine with lowest rpms and sand the end of the prop down a little at a time.this lightens the prop up and reduces drag at the tips letting it spin faster.to increase 200-300 rpms does not require much removal of the prop tip,then recheck balance.

JollyPopper

My Feedback: (6)

beepee, that very thought crossed my mind. That could get us close to where we wanted to be. I'll talk to the builder of the plane.

P47, it didn't occur to us to change props to make RPM changes. It sounds like a possibility. We'll try it. These are just off-the-shelf three bladed props fitted to the engines in no particular order, so maybe switching props on the fastest running engine and the slowest running engine might possibly make a change. An easy thing to try.