I have a little money from christmas that I thought I might spend on a set of 25 asp engines to build a twin with. I am in the proccess of building a 40 size sea master at this time and first thought was to build the 2 25s into the wing of the seamaster. I then remembered that I have an old Trainer Forty wing that has been sitting in the garage for years and it may be a good candidate for the twin engines if I built a fuse for it. I was thinking of a low wing. It is a symetrical airfoil, D tube design. So if I go with either, how much clearance do I want from fuse to tip propeller, from ground to prop and from water to prop. Also I assume if I am going to build a fuse for a low wing land plane, a longer fuse may be helpfull if an engine quits. So any advise on setting up/building a twin.
Thanks

I am not an aeronautical engineer nor am I an airframe mechanic, but when I designed my first twin, I built the nacelles as close to the fuselage as possible. As far as clearances go, I used the largest prop the engine called for and drew the plans with that much clearance to the fuse and the ground from the tip of the prop. Hope this helps some...and Good Luck

A standard airframe will work fine for a twin. On my twins, I build for square. Same size fuse vs. wing length. If you are building the airframe, add additional vertical/ rudder surface area for engine out. Having said that, my first 2 twins were 25 size. With the engines close to the fuse, didn't make much of a difference with one engine out. I used the one finger gap between fuse and prop tip. Remember you have to get the glow driver between the head and the fuse. Otherwise,what ever wets your whistle for design.

Here you go.


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 can not 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. 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 that 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:
a. 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.
b. 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.
c. 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.
d. 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.

4. Setting up the throttles. Here’s my procedure.
a. 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.
b. 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.
c. 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.
d. Now, hook up the throttle pushrods and go to full throttle. Adjust the clevis or pushrod connector to get the carb fully open.
e. 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.
f. 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.
g. 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.
h. 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 Moorman
25 twins and counting

Wow, Ed. This was very very helpful. I have been working on a Royal P-38 for a few years and ws about to go electric out of fear... Now I'm rethinking it especially thinking I'll build a simpler "trainer" twin first and fly that for a while.

-Skip

Super nice write-up Ed! Thanks for posting the info... it should be a sticky IMHO.

[8D]

<span style="font-size: larger"><span style="font-family: Arial">Nice write up Ed. Also well thought out and well said.</span></span>

Thanks for all that info. That will get me where I'm going alot quicker.

Ed,
Are you or anyone else aware of any other transmitters now available that have a twin engine menu like the JR9303 you mentioned?
Thanks

I don't know for certain about how the twin engine program on the jr9303 works but I just purchased a FrSky Taranis that uses OpenTX for the program and you can program anything you want in it. I didn't think about it until you just asked this question, but you could put both throttles servos on different channels and set each one up for a different curve and/or rate. In essence if the geometery wasn't perfect between the two throttle servo connections, you could correct for it in the programing of the radio. It would take a bit of trial and error but could be done with my Taranis. I just got the Taranis and am still learning all the things that are possible to program into it. I just got my flaps to go down at slow speed. That was cool.

There are some great tips here and I have one other one: Never climb in a turn after losing an engine. Always make a slight descent in turns and it will not bleed off speed and will actually help with stability. If you are very low then you may just be screwed. Unless you are over a *****ly pear forest just chop the throttles and put it down. Better to make minor repairs than to trash a plane.
One more idea is to use three or four engines. I built a quad in the 1990s and am building another now. It is tapered and swept 25 degrees with span of 82 inches and four OS .46s. Losing one engine is not a problem although losing two could be an issue. I agree completely with Ed about tuning engines. I have seen more than one twin lost after the owner tuned them together which effectively leaned out one engine. Just tune to taste each, refuel and take off.
Another idea for a good twin is to take an existing foam wing kit and build a center section that can be joined to a left and right wing panel. By doing this you can put two fuselages on one wing which will double the vertical stabilizer area. Each can have its own radio if you can link two receivers to one transmitter (Futaba). I used two Sig Kougars and, later, when they were flown separately and one crashed I just bought a used Kougar and kept flying. This should work for many sport models.
There are many various multi ideas out there including tri motors which I have had. Since this is supposed to be fun just get what makes you happy and go fly!!

Sbroome we think alike

I agree, a great write up, the only thing I would change is
" 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. Your natural reaction to a uncommanded roll will be to put in aileron to correct the roll. That aileron direction input is the rudder direction you need, put full rudder in the same direction as the aileron and HOLD, now let the aileron go back to neutral. All this will happen in a couple of seconds, before you can think out which engine you have lost and before you lose any airspeed.
Maintain full throttle and level flight. Now balance power on-rudder in, power off-rudder off."

Thanks a bunch for all the additional information. Since I started this post, I have completed my twin seamaster and it has had a couple summers of flying. Your all right, I is a lot of fun. I have had an engine go out a couple times and was able to land the plane due to the infomation I received here. The first time I had plenty of alltitude and was still climbing, the plane stapped and I throttled back regained speed in a dive, leveled off and landed on the water and taxied back to shore on one engine. The second time I lost an engine on takeoff from a large wave, I was heading out to sea so to speak and the water was getting really rough so I did not want to set it down that far out. I continued a shallow climb using rudder and was able to come around and land near the shore. It is amazing at how overpowered our planes really are. I was amazed at how one 25 engine was able to pull my 7.5 - 8 lb plane up into the air and circle around to land.

Thanks dkm, for that info on the rudder going the same direction as the ailerons. I guess I knew that but never thought about it. A great tool to use when things go wrong really fast. Natural reaction is aileron input for correction which is a great indicator to use the correct rudder input without having to think about it.

I am still planing on putting a couple 25's on the semetrical trainer forty wing and building a fuse for it. I would really like it to have retracts, but have to do more research on a twin tail dragger configuration. Seems like it could get a little squirly on the ground.