DamonTX

Can someone give me some suggestions for procedures you are using for tuning engines for your twins? So far I've been doing it this way:

1) Break-in engines as recommending by manufacturer.
2) Install on plane and tune in to the most reliable setting. (one engine at a time)
3) Start both engines and see how close the are in RPM and temp.
4) Adjust throttle linkage until RPM are within 200-300 of each other.

This has always seemed like the most logical way to do it in the past but for some reason I'm getting engine outs (on each engine) every 2nd or 3rd flight. I'm using 2 brand new .25 FXs each on it's own throttle servo. The flights are very smooth with no aerobatics because I'm afraid I'll lose an engine. Engine-outs occur mainly on takeoff at 3/4 throttle and ocassionally in the air at 1/2 throttle. The tanks are mounted in the wing and are surrounded by foam with no contact at all to the frame. There are no air bubbles coming through the fuel line and each engine runs great on the ground with idle RPM at 2200 and wide open at 13000.

Please give me some ideas/suggestions if you've got them.

Thanks,
Damon

Ed_Moorman

My Feedback: (1)

Damon,

I have been running experiments with one of my twins. If you are flying a sport twin, I will say not to worry about it very much. It is much, much more important to have both of them running than to have them together. Tune each engine individually for its best setting. I like mine so they leave a little smoke trail whether I am flying a single or a twin. If you have the same types of engines, they will be close enough. Big scale planes, I can't speak for.

I have flown with the engines 1,000 rpm different no ill effect. I now have my twin, a kit bashed Goldberg Tiger 2, with a Thunder Tiger Pro .46/11-6 on one side and a Thunder Tiger .42GP/10-6 on the other. Still flies great.

jaka

Hi!
I do as you do most of the time but most imortant is to check the running of the engines carefully each time you intend to fly the airplane.
Hold ithe plane straight up (do not shake it, just hold it ) and check the throttle setting at full power. It should be slightly rich even if you hold the plane straight up. Also check throttle action from idle to full throttle in this position to see if the engines can throttle good.
Then let the airplane sitt idleling for a couple of minutes.....then give full throttle and see how the engines respond. Both engines should not hessitate at all, but go to full power instantly.
To have this reliability I mostly run OS engines (LA , fsr and different size fourstrokes) and use 5% nitro and 20% castor oil (nowadays I use 15% all synthetic oil , Motul "Micro" ) OS 8 or Enya 3 glowplug all twostokes and OS F plug in the fourstrokes and well padded fueltanks with Uniflow set up (old control line secret)or Better yet...Tettra "Bubbleless fueltanks without any clunk.
Regards!
Jan K
Sweden

clscale-RCU

My Feedback: (7)

I have a Grumman Tigercat with a pair of OS-15's and I have used this procedure on other multi-engine models I have flown in the past:

1) tune each engine by itself, tune the high speed needle valve for a good running and a good transistion from idle to full power

2) Tune the other engine by itself

3) Start both engines and slowly pull back the throttle to idle and then slide the trim back until the engines quit, pay close attention to which engine quits first. Do not adjust the needle valve

4) adjust the clevis on the engine that quits first by turning the clevis by 2 or 3 turns. turning the clevis makes the throttle pushrod slightly longer, making the engine speed up. Do not adjust the needle valve...

5) re-start the engines and repeat step #3 and #4. I repeat this step until both engines quit at the same time and in general the high speed RPM is close enough

Good luck
Fred Cronenwett

jrf

My Feedback: (551)

I agree 100% with clscale-RCU. Notice that he said nothing about trying to sync them at top end. If you have identical engines and props, and they are set for optimum individual running, they will run within 100 or so rpm of each other and that is close enough.

I would only add that with larger engines (double needle carbs) you will need to adjust the low speed needle of each engine separately for best transition.

If you absolutely must have the top end in sync, shave the prop on the slower engine. Never try to sync them by adjusting the needle valves.

Jim

BillS

Your comments indicate that transition is not quite correct or fuel is sometimes foaming.

Might not be your problem but my experience indicates that harmonic vibration in a twin makes tank foam padding critical to avoid fuel foaming. Foaming is difficult to identify and tends to come and go in a random manner. Simply removing a tank and putting it back will sometimes introduce foaming due to slightly different packing. I have started using double-sided Velcro to hold the foam and tank because the firmness against the foam can be controlled. If you discover a sure fire way to check for foaming please let us know.

I also don’t believe top end synchronization is very important. Five to eight hundred RPM difference is no big deal. If you use the rudder the top end difference can be huge. Although the top end RPM was close I recently encountered a 1500-RPM spool up difference in the 7000-RPM range. The left engine was pulling harder than the right and take off runs were squirrelly to the right until 3/4 throttle was reached. Had three close encounters with the big shade tree on take off which would have been a disaster if one quit.

Bill

DamonTX

Thanks everyone, you each had great points. It looks like I should relax a little on point #3 and not worry so much about having the RPM so closely matched. I should, however, focus a little more on point #4. I built the plane so the tanks are easily removeable so I'll pull them out and go over everything checking to make sure they aren't packed too tight...which is a good possibility. I've never really had any ongoing problems with engines so this has really had me frustrated over the past few weeks.

JohnBuckner

My Feedback: (1)

There is one method to absolutely eliminate foaming and unporting without the need to resort to a sealed vent pump system such as used on YS engines. That is a bubbleless tank system by either Tetra or Jett without a clunk. Foam is impossible to form and foam padding is not used. One additional advantage is its not neccessary to have the tank close behind the firewall, they will reliably deliver fuel at distances up to 7 or 8 eight inchs from the engine. It does require a syringe tanker for fueling like this: http://www.darrolcady.com/Jett_Produ...t_products.htm

Many early flights of my Quad Cadet with standard tanks would tend to lose outboard engines during some manuvers such as loops, rolls and variations. That is now a very rare occurance with the substitution of bubbless tanks.

John

jaka

Hi!
Argee to John!
Use Tettra "bubbleless " tanks and you will have no more foaming problems.

Jan K
Sweden

RCer

How come noone mentions to adjust servo calibration curves to tune the engines to each other? Is it because it is not a viable solution?


Regards
RCer?

DamonTX

It's flying on a Futaba 8UAF. I did a quick search in the manual as well as another book I have on it before starting the project but didn't see a way to tune both servos individually. I'm about due to get a new radio and would love to find one that will allow me to do that.

Damon

BillS

Damon,

Sure you can. I have a similar radio and last Friday changed a triple from JR to the Futaba. What would you like to know?

Bill

Super Hot

With Futaba 8UAF, you can use Throtle Needle mix.

DamonTX

Awesome! I've had this radio since 1988 and am still finding out new things about it. I had it probably 2 years before figuring out expo and now I can't live without it.

What I'd like to be able to do is (in theory) have both engines running and be able to adjust the throttle setting individually on each throttle servo. Sure I can stop and start the engines, adjust the clevis and then start them back up but it sure would be nice to be able to do it while they're warm & running. This would make it much easier to readjust things on a day-to-day basis if needed.

Update...... The airplane I was referring to when starting this thread is still undergoing minor repairs. On a deadstick landing, I tore off the gear which is easy to fix but the underside of the fuse got raked by the gear which made things look ugly cosmetically. Gotta fix that before moving on.

Damon

BillS

Damon what I wanted to accomplish was an easier way to trim adjust the engines while running relative to each other primarily at low speed or idle. In addition the main throttle trim needed to move all engines up or down in unison. For full throttle the barrel simply needed to be wide open.

First problem was one of my servos centered different that the other requiring a different horn position and substantial sub trim. Second problem was desensitizing pots 6 and 7 so they act as trim rather than full range of movement. In addition the mechanical aspects of the carb linkage (throttle arm position) needed close attention.

I also used switches to un-slave each outboard engine but simulating engine out is an entirely different subject. Changing the engine differential in the air can be very informative.

The 8UAP or F is a very capable radio and likely there are several programming ways to accomplish the same results. Changing the throttle curve would not be my choice. Sub trimming the channels might be appropriate. The pots can also be used for sequenced shut down, which can be impressive. Of course for sequence shutdown you must first land with everything running which is the hardest part.

Bill

RCer

I did not mean to change the throttle curve. On my radio (a Multiplex Evo 9) I have 1 throttle curve that is the same for both engines. The output of the throttle curve is then passed to a servo calibration curve (function) before being transmitted. This allows amongst others to calibrate both servos to turn an equal amount with the same throttle input. This is especially important if the servos used are not syncronizer very precisly. For example this calibration could also be used to syncronize to elevator servos acting on the same surface. What I was trying to suggest is that the syncronization of the engines can be fine tuned using these calibration curves.


Regards
RCer.

BillS

RCer,

Interesting and unknown to me. Thanks for the information.

Bill