I know nothing about multi engine planes. I have never seen the inside working of one either. One day I thought that I may want to tackle such a plane. But then I got to wondering how on earth the throttles are hooked up to a servo? After all, I would have to use trim on one engine at a time. How could that possibly done if they are all hooked up to the same servo? Is this a very technical thing or what. I am trying my best to use my imagination, but can't quite see it. Any explanations out there in this forum? Is this the type of thing that is explained in kits?

SF:

Using rods and two bellcranks two engines can be throttled with one servo. The linkage is set to have the throttles fully shut and fully open at the same time, then working with the angles good engine sync can be reached. It's a lot of bother to get it right, and after a period of time it all has to be done again as the engines wear, never equally.

For these (and other) reasons most builders use one throttle servo for each engine. With a single throttle channel on the radio the mechanical linkage has to be set precisely for sync, just as when using a single servo for both.

Next step up, and some consider it the best of all, is using a separate radio channel for each engine. This way, if your radio will do it, you can use the throttle curves in the transmitter to adjust the sync of the engines.

There are two electronic devices on the market to keep the engines in sync.

EMS/Jomar has their "Engine Sync Device" that measures the rpm of one engine, always the same engine, and matches the other to it. It is what is called a "Master/Slave" device. Last time I looked this cost $80, and you had to supply some minor parts to go with it. It requires separate throttle servos.

The ultimate electronic sync device is the "TwinSync," now available from RCS for $130. It also requires separate throttle servos, but it's complete, no additions to buy. For the added $50 you get a unit that is not master/slave, it reads both engine's rpm and changes the throttle of both to reach sync. It also has many other built-in facilities including a programmable glow driver, the ability to run the engines up individually for mixture checking, even the capability of tieing the throttles to the rudder for more crisp ground steering or extremely sharp yaw turns in flight. The device is fully programmable down to the point of varying the response time.

As a final note, synchronization of the engines on a twin is not really important, a few hundred rpm difference wont affect the flight of 99% of the twins we fly. But the sound when they are running in sync can make tghe enjoyment much greater. And when a cheap twin is going to represent $1200-1500 what's another $130?

Bill.

Bill, thank you for all the explanation. It's not completely clear to me, so I guess this is going to be one of those areas that Iwill have to find someone who has a muti engine plane and have them show me how to set it up.

My solutions have been to simply use two servos and slave them together with the radio. The solution is simple and inexpensive.

Bill

Bill,

Just out of curiosity, what if one of those "V-tail mixer devices" was added in the throttle linkage setup? Of the two servos, one would serve as the 'volume' and the second would adjust the 'balance', allowing in-flight adjustable (albeit manual) sync. I know it would add complexity to an airplane but it would probably still be simpler (and a lot cheaper) than one of the electronic sync units mentioned above.

Here's a suggestion to anyone wishing to dabble in twin engine planes, at minimal cost and risk.
I created a SPAD to try out my ideas. Here is a link where you can get an idea of what I ended up with:

http://www.krcm.net/krcm_fourm/showthread.php?t=189

One of the pictures shows the single throttle servo arrangement. Engine synchronization is not a problem, with
my design, as I have large tail volume. I know that they engines are not exactly in synch all the time, by their
sound, but it is not really noticeable in controllability. Extreme slow speed, hovering, harrier landings, high speed
flight, it all works well.

Now, if I were building a "conventional" model, in particular something that is at all scale, I would be a lot more
concerned about engine synch and reliability issues. That is where e-powered looks more appropriate, to me. My SPAD
experiment started out as a means to learn a bit about twins before embarking upon a Twin Otter model. I have
now lost interest in the latter, the risk/benefit isn't in favor, for me, as I am not into electric aircraft.

BillS,

If $99 is too expensive for a true sync unit with the added fail safe of idling an engine if one quits (it saved my P-38 last weekend) -what do you think the price should be? I would really like your feedback because I would like to get the Twinsync to "high volume" production. It is kind of a chicken and an egg problem though - if I built 10,000 of them they would be really cheap but no one (including myself) would spend that kind of money to get the price to $10-30 range. However, I think if it were "cheap enough" a lot more people would fly twins. If I have a few built at a time they cost more to make than what they sell for. So what is the right MSRP for a sync unit. <$100 was my goal. Should it be lower?

I always flew my twins with "Y- Cables" until now?

Bill

I have never used a “V-tail mixer� and simply don’t know. But I do believe the engine out alert is more important than synchronization.

Bill

Bill,

What should the price point be? You have asked the absolute hardest marketing question imaginable. After 30 years in business I still get the answer wrong more often than not. By comparison the engineering decisions are simple.

Marketing:
You must make a profit. A company’s first products however are unlikely to be home runs. There are likely to be 140,000 or so modelers in the US. A very small percentage fly twins and an even smaller percentage will appreciate the need for engine out protection. A thousand unit sales might be considered highly successful. Inevitable product changes due to design or cost or production changes will disallow building in volume.

Business and the IRS:
You will probably find that for business purposes you only need to prove that you intend to make a profit. Along the way you will meet a lot of interesting people, discover many new ideas and hopefully have a lot of fun and pay for your modeling habit. Although the comments may at first blush sound a bit negative I highly encourage you to move forward.

The current crop of P 38’s have a wing loading above 40oz per sq ft in addition to widely spaced engines. The new Cessna 310 ARF has a wing loading of 50 and a dangerously narrow flight envelope. However the modeler’s ability to rationalize away the laws of physics, convince himself that engines will be reliable, and believe that he can learn engine out flying under emergency conditions is amazing. My good friends are included. On any Sunday at the flight field one can observe several single engine dead sticks by knowledgeable flyers. There is no legitimate reason to believe twin engines will inherently have significantly less engine shutdowns.

You do need to make a profit. My suggestion would be to price reasonably while feeling out the market, educating the user about engine out alert, and building a customer base. The customers will attempt to educate other customers but the battle will likely be uphill. Initially the margins are likely to be small but production efficiencies over time improve the margins on most all products.

More after a few days thought.

Bill

Thanks for the feedback....

I just put a TwinSync on ebay for $50 to "test" the market and maybe sell one cheap enough that even you might be interested!

check out item # 220057481734 on ebay.

Bill

Bill,

My response to sugarfoot was never intended to imply anything about price. The response was simply intended to inform about a solution I have used starting in 1992.

The TwinSync has been recommended to two other twin flyers in the local area along with the suggestion that we travel to Raleigh to discuss the device with you.

Not enough was known about sugarfoot’s intentions or requirements to make a recommendation except that a single servo is inappropriate. Everyone flying a P 38 or Cessna 310 or other ill-mannered heavy gal needs an engine out alert.

Bill

No offense taken... I know that in order to get the volumes up the price needs to be as low as possible. The electronics business is completely volume dependant so I am in the process of looking at ways to get the price down in the future. More volume is the key to getting the price/cost down. Secondly, I think the time is right for twins to start gaining momentum. I have seen more twin arfs come out the last 6 months than I can ever remember. Send me a PM or emeail and I will call you or send you my phone #.

I agree that one servo per engine and a "Y" cable is the minimum. I had a twin engine combat plane I did with one throttle servo but the time required to get it right was not worth a one servo savings.

I still would like to hear feedback on the "sweet spot" for TwinSync pricing w/o glow drivers.

Bill,

Yes the time is right.

There are at least three P 38 ARF’s. P 38’s are notorious at turning over and entering an inverted flat spin on engine out. The fundamental reason (design characteristic) appears to be wide spaced engines and high wing loading in the 40’s. The two high-end P 38 ARF’s will require well over $1,000 of gear in the air.

For some time Twinman has advocated rudder gyro’s to dampen the effect of P 38 engine out and to give the pilot an extra half second or so to recognize the out and react. The solution is probably wrong headed but may have been the best solution available at the time. Although I don’t mean to speak for Twinman (he articulates very well for himself) he might be open to a rudder bias on engine out. Twinman where are you?

It is guessed that those without twin experience will be inclined to throttle up to full after an engine goes down. The human mind seems to be naturally programmed to believe that more is always better. Since no one practices engine out the ‘more is better’ mentality is working in reverse as they attempt to learn in emergency time. Another destroyed airplane.

The new Cessna 310 is expected to be an ill-tempered monster. It is pretty, overweight and underpowered. Even the manufacturer suggests that a one engine go around is questionable. An engine out 400’ away means landing at almost full throttle on the good engine at a stall speed of 35 mph probably in the high grass or trees assuming the engine out is recognized. Scary. Yes with over $1,000 in the air every Cessna purchaser needs engine out alert.

A Dual Ace with .45 engines measured weight was about 12 1/2 pounds. It should fly and land properly on one engine. Yet failure to recognize engine out before the cascading mistakes put my friends Dual Ace in a far away tree. The tree climber cost $150. A new wing and a few engine parts put the crash cost above $300. Yes an engine out alert would have saved money.

The Phoenix twin should be somewhat less critical with .60 engines and a target weight of 12 to 13 pounds. It remains to be seen if the additional power will adversely affect single engine performance.

An email has been sent.

Bill