Where to go for twin engine design help?
 

I'm looking for a tutorial on twin-engine design; specifically, how does one figure out the optimum thrust angles for each engine? I don't want to get so deep into twin theory that it requires a math degree to understand. Something similar to the great piece that the late Chuck Cunningham wrote for the floatplane guys.

I've had an idea for several years now to turn a CG Senior Falcon into a taildragging twin using the nacelles, cowls, and (optional) Robart retractable landing gear from a TF DC-3 kit. I thought of simply duplicating the thrust angles of the TF kit but I don't know if they would be appropriate since the airframe would be completely different.

Your thoughts are appreciated.

Harvey

Harv,

I'd be glad to help.
-My buddy and I built 25 twins and I flew them all. Many were scratch.
-I wrote the "Fun Aerobatics column for R/C Report magazine for 10 years.
-In my 2-part column, I taught an aerobatic maneuver and wrote a feature. A couple of them were on twin-engine flying.

Drop me an e-mail at [email protected] and I'll send you a copy of the twin article and I'll also answer your questions.

Ed Moorman

Thanks, Ed! E-mail sent.

Harvey

Yea, sure. Ed's big thing is "Twin, don't do them". But he does and well also........

Hi!
Don't be so doubtful! Just go ahead and build! Engine thrust really doesn't matter ! I don't use it ,certainly on on a full scale plane that doesn't have it anyway.
What matters is pilot ability and knowing to land as soon as one engine cuts out (if one does).
Also ,it's very important to know you Engines so be prepared to run both engines in a single engined plane before you put them to use in a win.
Last, It's very important to choose reliable engines and use a good tank set up and good tanks.

JAKA, since you mention it, and cause I'm curious ,what about tank placement? I've been modeling and flying for about 6 years now and have an itch to try a twin design. most likely something scratch built. I have two new broken in, os 55ax engines and already plan on on board glow driver setup. the thing I'm curious about is tank placement. is it possible to feed two of these off one tank? I have three of these engines and lovem. not the easiest things to get used to, can be somewhat finnicky . but they are very torquey with the right prop and type of fuel. the engine requires good tank pressure to run right. so,with a single tank setup , with pressure from both engines mufflers ,would you suppose an auxilliary pump of some type would be necessary? thanks in advance

To add to Rcjunky67's question, would it be beneficial for the single fuselage-mounted tank to feed individual (smaller) nacelle-mounted "header" tanks instead of going directly to the engines?

Even though it would add some complexity, the header tanks would then serve as "local" fuel supplies for each engine while the main tank and pump would keep the header tanks topped off. I would think that such header tanks would supply a more stable fuel delivery to each engine since long fuel lines between the fuselage-mounted main tank and carburetors would be eliminated. The added benefit would be that the engine nacelles wouldn't have to be extra large in order to fit decent-sized tanks.

Harvey

..

I recommend you do not complicate things.

TWIN NOTES BY ED MOORMAN
10 years as author of Fun Aerobatics column for R/C Report magazine
([email protected])

Engines for twins notes:

1. Rule numero uno is “Reliability is king.” You won’t have any fun if your engines don’t keep running. Use the most reliable engines you can find. Call me picky, but I want engines that I rarely have to adjust and ones that never quit in flight.
2. Break in your engines first. Fly your engines in a single engine plane first and get them tuned before installing them in a twin. I cannot emphasize this rule enough. Do not put a brand new engine in a twin unless you just love trouble.
3. Use a fuel filter. I have a filter in my fuel jug, but especially on twins, I have a fuel filter in the fuel line to the engine. This goes along with reliability. Get a little trash in the carb on a single and you throttle back and land. Get a little trash in the carb of one engine of a twin and you may not hear it until that engine leans out and quits. The good engine will mask the sound of the bad engine.
4. Stick to lower nitro fuel. Fuel with 30% nitro may make your engine scream, but it also may not keep going. Fuel with 10% nitro has a much better chance of running reliably.
5. Do not peak the engines out. They’ll quit. Set them to leave a nice smoke trail. If you normally peak, then back off 2 or 3 clicks, that’s probably too lean for a twin. Peak, then back off 4-6 clicks. If you are a person who compulsively tweaks the needle every flight, go fly electrics.
6. Do not adjust one engine with the other engine running. You will invariably set it too lean. Set each engine by itself. Hold the nose up to check for leaning out. Shut down, crank and set the other engine. Crank the first and fly or shut both down, top off the tanks, re-start both and fly.
7. I do not bother to synchronize my engines. If the engines are the same brand and size, they’ll be close. I have flown planes with a 1,000 rpm difference and I have also flown planes with two different engines (Thunder Tiger .46Pro/11-6 prop & Thunder Tiger .42GP/10-6 prop) and you probably won’t notice the difference. Bottom line is: Don’t get wrapped around the axle trying to get your engines within 50 rpm. The idea is to have fun, not mess around with your engines. Here is Ed’s axiom: “10,000 rpm difference is a lot worse than 1,000 rpm difference.” For those of you who don’t understand, 10,000 rpm difference is with one engine out. One of your engines may just run better than the other. If you try to match them, one will be too lean or the other too rich. The best I have found is to get each engine running right separately and forget the tach.

Airplane notes:

1. Do not start off with a heavy, scale twin. You will just be looking for trouble. Start with a good flying, sport twin and learn how to handle a twin.
2. Consider using out thrust. It will help you keep control during an engine out. 8 degrees is the magic number, but it does look like a lot. If you do the math, you are losing less than 1% of your forward thrust (Cosine 8=.9927 or 99%+ of forward thrust). On my test flights with one tank short fuelled, I couldn’t tell any difference except for engine noise when I was flying along and a slight decrease in speed. Any out thrust is better than no out thrust.
3. Consider side mounting your engines. Side mounted engines seem to run slightly better and more reliably than upright mounted engines. If you really want to shoot yourself in the foot, mount your engines inverted.
4. A twin fuselage design generally flies better than a twin nacelle design. This is especially true with one engine out. If you are going to kit bash for your twin, consider making a twin fuselage design.
5. Twins are heavy. They are generally heavier than a comparable single engine plane and have a higher wing loading. If you are going to kit bash a .46-size plane into a twin, figure that you’ll add 3 ½-4 pounds to the overall weight. That’s what we have found.
6. Twins land faster. Since they are heavier, a twin tends to land a little bit faster, not excessively faster, but slightly faster, than a comparable single engine plane.

Twin construction notes:

1. Keep the servo arm and throttle arms accessible. When you build, make sure you don’t bury the servo arm and the throttle arm. You will need to get to them to make adjustments.
2. You’ll need a stronger landing gear. If you are scratching a twin from a single engine kit, the landing gear probably won’t be strong enough because of the extra weight. You’ll probably need 3/16” wire or a sturdier aluminum gear.
3. Use a strong servo for rudder. I prefer a digital since it has greater holding power compared to even a strong analog servo. If one engine quits you want to be able to hold rudder against air loading.
4. A pull-pull set-up for rudder is recommended. Beware of long, unsupported rudder pushrods. After you have made the rudder servo installation and set up the radio, hold full control and try to straighten out the rudder. Do this in both directions. If you can more the rudder by hand, I would consider changing your servo or pushrod.

Notes on Flying Twins

1. As long as both engines are running, twins fly just like a regular airplane. A sport twin will fly like a slightly heavy sport single. A heavy twin warbird will fly like a heavy single engine warbird.
2. You are going to lose an engine. Learn how to handle it because I can guarantee you are going to lose an engine. Handling an engine out means using the rudder. You must learn to use your left hand on the rudder.
3. If your plane rolls over on its own, you have lost an engine. Starting an uncommanded roll is a very good indication that you have lost an engine and are getting roll due to yaw.
4. Engine out: throttle back, regain control, and then power up. When you lose an engine, throttle the other engine back so you don’t lose control. If your plane started a roll, it’s a lot easier to roll level if you aren’t fighting a big yaw. With the power back, the plane will be basically straight. When you get things sorted out then you can put in rudder and add power.
5. Know your airplane. Some planes will fly on one engine and some won’t. If the plane is a big heavy scale plane, hold rudder and steer for a nice flat area to set it down. Do not try to stretch the glide, you’ll only snap, crash and really tear the plane up. If the plane is a sport plane, hold some rudder, fly back to the runway and land.

Radio set-up Notes:

1. You really need a computer radio. Flying a twin with a 4-channel is like practice bleeding, in my opinion. It can be done, but it takes a bunch of work to do it.
2. The worst throttle set-up for a twin is a single servo and belcranks or dual cables. It can be done, but it is really hard to get both engines adjusted evenly.
3. Set-ups for two throttle servos:
   1. Two servos and a Y-connector. This is just slightly better than belcranks. Both of these will work, but it takes a lot of effort to set them up and get both the idle and full throttle settings equalized.
   2. Two servos and a JR Matchbox or Smart-Fly Equalizer. Either of these will work great. They allow you to use one channel and individually reverse servos, adjust the center and end points as well as use a separate battery pack, if desired.
   3. Two servos and mixing. You plug into separate channels and mix. This works very well, especially if you are comfortable with using the mix on your radio.
   4. Two servos and a twin engine set-up on some radios. The JR 9303 transmitter has a twin engine menu that makes doing a twin very easy. It also has dual throttle curves that allow you to match your engines all the way up and down the range.

1. Setting up the throttles. Here’s my procedure.
   1. Before you hook up the throttle pushrods, take both engines and rotate the carbs fully open. Look at the angles of the throttle arms. If they aren’t the same angle, loosen the set screw on the throttle arm and adjust the angle. You do this at full throttle since most all carbs will stop at full open and they might not stop at the same place at the low end.
   2. Turn your radio on and set the throttle stick in the middle. You still have not hooked up the throttle pushrods. Set both servo arms as close to 90 degrees as you can, then go into your Sub Trim function and blip the arm to exactly 90 degrees. If you are using 2 channels and mixing, you can set both midpoint Sub Trims.
   3. What all this does is give you a preliminary throttle set-up. It also tends to equal out any adjustments for top end and idle and make them smaller.
   4. Now, hook up the throttle pushrods and go to full throttle. Adjust the clevis or pushrod connector to get the carb fully open.
   5. Go to idle, set your trim to wherever you like it for starting your engine. Use End Point Adjust to get both engines together at idle. You may need to adjust after running the engines.
   6. You should now have the engines together at 3 points, idle, mid-range and full power. You’ll need to run the engines to check the settings, but if they are together at these three points, I’ll bet they will be together all through the throttle range.
   7. Obviously, if you don’t have enough travel to get the carb fully open or closed, you’ll need to move the clevis on either the servo arm or the throttle arm.
   8. If you are using only one channel, get the idle positions together first, then worry about the top end. Most throttles are non-linear and you can pull back to three quarters throttle and lose very little power. You want the idle settings together so you can taxi and so you can slow down for landing without killing one engine.

Twins are double the fun. You need to try one.

Ed,

Do you own stock in the printer cartridge companies? You KNOW we're all going to print out this terrific list for our files!!!

Thanks!

Harvey

Glad to have you guys copy anything you want.

I remember one of the guys at my field spending nearly 30 minutes with a tach synching up 2 OS .46AX engines. I could fly 2 planes in the time he spent tuning.

I was flying my Twin Air 45 powered by 2 Magnum .52 XLS 2-stroke engines not long after the "synching" incident, when on one flight I noticed some yaw on takeoff. I also noticed more smoke coming from one engine in flight. The plane didn't fly any differently, so I forgot about it until I got home. I ran up both engines, checking them with a tach, and found one was too rich and turning nearly 1,000 rpm less than the other engine. A thousand rpm-that's a bunch, and I normally fly at full power most of the time. Flaps and I thought that this could be a project that would be fun, seeing as how "everyone" said you had to sync your engines.

One thing my late flying buddy, Carl "Flaps" Laffert, and I did was build planes to test things. We talked about this and Flaps said he'd change the engines out for the test. He wasn't sure about synching either and wanted me to test it. I could also write it up in my R/C Report column.

I grabbed another twin and took it and the TT engines over to Flaps' shop where he removed the OS .46AX engines. He replaced the left engine with a Thunder Tiger .46Pro, only with a Tower .46 muffler. When you could get those mufflers, they would add on the order of 800 rpm to your engine. I tested this and since they only cost $15 so I bought several. On the .46Pro we ran an 11-6 prop. The right nacelle got an old Thunder Tiger .42GP, plain bearing trainer engine. Stock muffler and a 10-6 prop. Now I don't think anyone would believe that both engines ran the same speed or pulled the same. We both felt they were a complete mismatch.

At the field I cranked each engine separately, tuned it, then did the other. I topped off both tanks, restarted both engines and took off. Shoot, the Twin-Air flew just as well. I couldn't tell the difference.

Later on I flew the Twin-Air at the McDonough, GA (just south of Atlanta) annual twin meet. I had flown the same plane with the matched engines the year before. No one noticed or commented on the mis-matched engines until I told them.

Now, I'll admit that if you're flying a full scale recip twin and the engines aren't matched in rpm, the noise will drive you nuts. With RC planes, I don't notice it. As far as I'm concerned, if the engines are both are the same type, same age and both broken equally, you shouldn't have any problems.

Ed

Ed,

When it comes to full-scale multi flying, out of sync engines are more than an auditory discomfort. Out of sync engines, combined with heavy prop blades, can generate strong pulses that are detrimental to bearings, airframes, and avionics. However, unsynced engines are not a big deal in our models because our props are lighter and our balsa airframes are somewhat flexible. (They absorb and dissipate the destructive harmonics.) In addition, our model engines typically turn much higher RPMs than the engines in full scale planes. What might be a thumping vibration in full scale is simply a high pitched buzz in our models.

Therefore, the urge to keep our little engines in sync may very likely be a result of full-scale multiengine operating habits trickling down to our models.

Harvey

Harvey,

I did not know that out of sync was that big of a problem in full scale planes. Not having much recip time in the Air Force, I don't have any experience in that area. My twin time was what the USAF called "center line thrust." Jets were easy, and don't let anyone tell you any different.

Ed

Now you're talking my language! I used to own a '67 Cessna 337B (the civilian version of the O-2 that you flew in the USAF).

Out-of-sync engines in full-scale planes can produce destructive harmonics that can reduce crankshaft bearing life, cause fatigue cracks in engine mounts, weaken solder joints in avionics, shorten light bulb life, etc. Of course, these problems are generally worst-case scenarios and are not common; but then again, maybe that's because full-scale multi pilots are taught to always keep our engines in sync.

Harvey

Harvey,
Here are some nice examples of what you want to do.

http://www.rcuniverse.com/forum/atta...mentid=2018044http://www.rcuniverse.com/forum/atta...mentid=2018045

Another thought go Electric on multi-engine models. The TF DC-3 has an option to use electric motors. Save the glow engines for single engine models and increase your twins life span.

Good Luck

Dont know why pics did not show.
http://www.rcuniverse.com/forum/atta...mentid=2018165
http://www.rcuniverse.com/forum/atta...mentid=2018166

Nice! Tell me more about them.

Thanks Stan.

Harvey

I was doing a search senior falcon a while ago and found these pictures. nothing else about them.

As far as twin design info, I would check out multi-engine arfs and plans built twins. Most of the arfs assembly manuals you can download
in PDF format. These manuals usually have all the setup info for the model. There is a wealth of design and model setup info in these manuals.
.
if you like to design or mod a standard kit like the Falcon you can get a lot of ideas looking at professionally designed models and comparing their specs.