matched engines for a twin

[Phil 9]

i am pla​ning a twin engine protect but to save money can I match engines of the same type but of different (and unknown) age and use time. Or should I start with two new engines

[onetenor]

As long as both motors are in similar condition I see no problem I mean internally similarm

[PeterF]

I have had twins with engines of the same age and still had 5% difference in rpms. At one point I had 6 APC 10x6 props and found that on one engine I got something like a 3% range between the fastest and slowest prop. I never had any issue. If you have different rpms try and avoid the desire to run the faster engine rich to slow it down, herein lies the path to single engine operations.

[Phil 9]

Posted by PeterF on 08/06/2017 22:39:30:

If you have different rpms try and avoid the desire to run the faster engine rich to slow it down, herein lies the path to single engine operations.

I thought that is what you were supposed to do? what is the correct method?

[Peter Miller]

I have never checked rpm on twin engines. Never had any problem Any slight turn can be trimmed out with the rudder.

If you take Grumpy Tigercub this is a rather exceptional model as one does not know which engine has died. It just keep flying but slower.

To answer your initial question, just check that the two engines are delivering roughly equal rev and throttle roughly equally and go from there.

[Denis Watkins]

On mine Phil, I use those Hi Tec servo arms that can be adjusted by the centre screw, in tiny increments, making

The servo arm longer and shorter.

The motors are tuned for max reliability and the faster motor brought within 100 - 200 rpm by incremental adjustment

Of the servo arm. It does work

[Manish Chandrayan]

Peter,

I would be also be very interested to hear about that not richening the faster engine. I and a friend here are contemplating a DH 89 from Jerry Bates plans and parts

[Jon H]

When customers are asking about a twin i normally pick the two closest matched engines from the batch i am building. Differences between the engines are really small, but why make life more difficult than it needs to be? I have had twins in the past with mismatched (in age) engines and they were more of a pain to get running together.

When tuning, i set each engine on its own before attempting to run them together. If they are slightly out at full power i dont really care as its mid throttle that is more important. In general i dont like running the 'fast' engine rich. It increases fuel consumption and you may find that once rich the engine will perform differently to its best mate at other throttle settings as well.

The thing i pay the most attention to is making sure the mechanical setup is as close to identical as it can be with servos all square and carb openings as close as you can get them. Once each engine has been set on the needles you can then tweak the linkage length to get the idle where it needs to be on both engines.

[Andrew Hogg]

Phil,

Put your most reliable engine on the left. and tune each engine individually, don't try and match them. they might be matched on the ground but go out of sync in the air and visa-versa, just make sure you can trust them. go fly it, then when your happy, use radio to sync them by sound, I have not used my tacho for years, always by ear. reliability is the key.

Andy

[Martin Harris - Moderator]

Two well run in and reliable engines are the most important pre-requisite so buying a new pair may not necessarily be the best approach if you already have some suitable ones to hand. I would get them both running properly and set up separate throttle curves to match them as closely as possible over the operating range - there are increasing numbers of transmitters capable of this these days.

[PeterF]

Re running faster engine rich, jon has mentioned the one issue re fuel consumption, the second is if you are not careful and make it too rich it then has a higher risk of stopping at lower rpm at part throttle. Some small richening would be fine but do not overdo it. When I got a more capable radio I have set up separate throttle channels as per Martin's post above and used throttle curves to match rpm over the mid range and sub trim to match the idle.

[Martin Harris - Moderator]

I can hear a sharp intake of breath from Jon, but this might be a justification for on-board glows if you choose to go down the richening route...

...and on consideration, surely you can richen the top end while maintaining the correct idle/mid range mixture on the idle screw with appropriate iterative tuning?

Edited By Martin Harris on 09/06/2017 11:38:28

[Jon H]

Posted by Martin Harris on 09/06/2017 11:34:36:

I can hear a sharp intake of breath from Jon, but this might be a justification for on-board glows if you choose to go down the richening route...

...and on consideration, surely you can richen the top end while maintaining the correct idle/mid range mixture on the idle screw with appropriate iterative tuning?

Edited By Martin Harris on 09/06/2017 11:38:28

on board glow....the root of all evil

But yes, careful tuning of the various needles can be very helpful but its not guaranteed to iron out any mismatch. But to be clear, 100rpm of mismatch is not going to be a problem at all and i wouldnt even bother touching it unless they are well out of sync.

Throttle curves can be helpful though although i have never used one myself. I tend to go for the simple option of a y lead and as closely matched mechanical setup as i can get.

[Phil 9]

all very interesting and informative posts. Many thanks and does go to show any model with two engines is going to provide a few more challenges than a single engine model.

My thinking so far was could I buy second hand engines for a twin with different an unknown run times with the advantage being it would cost a lot less than two new engines or would this pose more problems than its worth.

I had the idea of building a simple twin ugly stick type model as a test bed for a twin engine setup and to have a bit of fun with whilst learning about a two engine setup. With the intention of then eventually transferring the setup into a scale model.

[Trevor Johnson]

Just remember whatever your set up on a twin when one engine fails the remaining one takes the aircraft to the crash site.

[Chris Freeman 3]

Many years ago I built a Dornier Skyservant that I designed as I thought that it would be an ideal twin engine test bed with the sub wing with the engines on. I used 2 new ST 45K motors and both had the same set up. In the beginning a tach was used to try set the motors as close to each other as possible. I had more problems doing it this way than setting each engine up to a reliable setting individually. I eventually found that one motor ran better with muffler pressure and the other was best without? This aircraft flew for 15 years like that until I had a brass tube break in the tank due to old age.

[Jon H]

Posted by Trevor Johnson on 09/06/2017 13:04:08:

Just remember whatever your set up on a twin when one engine fails the remaining one takes the aircraft to the crash site.

I am pretty sure this is meant in jest as its quite untrue. Most twins will fly on a single engine just fine if the correct actions are taken upon loosing an engine. Throttling back to idle and gliding home is not recommended as most twins are heavy and glide like bricks. Maintain airspeed at all costs and fly it home.

That said, a twin trainer is an excellent idea and gives a margin of error when learning the tricks of multi engine setup. Also if it does all go pear shaped then its not the end of the world 

Edited By Jon Harper - Laser Engines on 09/06/2017 13:46:32

[Martin Harris - Moderator]

Wise words Trevor. I've witnessed several avoidable write-offs at shows due to single engine failures being mishandled by (assumedly) experienced pilots. My advice, unless you are familiar with the single engined handling and capabilities of the model would be to immediately close the throttle and call a deadstick landing. An off field landing or over run is usually far less damaging (and potentially dangerous) than the typical spin caused by asymmetric thrust - often initiated by turning towards the dead engine with the live one at high power.

Words I should have heeded when I lost a motor on a rather well over powered electric Tony Nijhuis Mosquito - all was well until I decided to give it a squirt of power when it looked like slightly undershooting - the instant spin at 30 feet destroyed the model as it impacted the field moments later...

P.S. Just saw your post Jon...the saying quoted by Trevor is actually from full size aviation.  Looks like we disagree here but I have to admit that most of my twin experience is on a Slim Twin that I was quite happy to use for an entire flight on a single engine due to its very forgiving nature.

Edited By Martin Harris on 09/06/2017 13:55:33

[Former Member]

[This posting has been removed]

[Jon H]

Posted by Martin Harris on 09/06/2017 13:49:06:

Words I should have heeded when I lost a motor on a rather well over powered electric Tony Nijhuis Mosquito - all was well until I decided to give it a squirt of power when it looked like slightly undershooting - the instant spin at 30 feet destroyed the model as it impacted the field moments later...

P.S. Just saw your post Jon...the saying quoted by Trevor is actually from full size aviation. Looks like we disagree here but I have to admit that most of my twin experience is on a Slim Twin that I was quite happy to use for an entire flight on a single engine due to its very forgiving nature.

Edited By Martin Harris on 09/06/2017 13:55:33

Yea I know, hence my comment about it being a joke.

And you mossie experience is typical as you dropped below the minimum speed required to maintain control. From the sound of it you were quite unlucky as the hard part was already out of the way and a power off glide approach would have seen it home. Mossies are not the most forgiving though so that probably didn't help

[Martin Harris - Moderator]

Bad luck didn't come into it - I underestimated how much power would come in at what was meant to be a gentle addition of power - it was the maiden and I'd only completed one circuit before the motor failure. That's the problem with electric models...not nice and reliable like a Laser!

[Jon H]

Seems pretty unlucky to me

[Phil 9]

the sound track on this video make the twin idea tempting