Call for more information on our Club 504-669-5836
 
 

So you want to fly twins?

I found this on line a few years ago and thought I would share. I personally like twins, I have not had any major issues with any or my twins.
JeffC

A list of suggestions for success with Twin Engine Aircraft.

Rules:
1. Don’t do twins – four times the trouble and worry of a single engine airplane.
2. See #1.
3. As you can’t follow instructions (you skipped #2) please read on for my personal suggestions for success to enter the “exclusive” (see “eccentric” in the dictionary) and challenging world of twin engine radio control airplanes.

Twin Engine Tips for Success:

Twins are for experienced pilots. Not a second airplane! Pick a plane where the engines are close together, as this minimizes problems during an engine out situation . DO NOT start with a warbird for a first twin.

Long tail moments and large rudder control surfaces are also a strong advantage to control differential (uneven) thrust from two separate engines.

Learn to fly with the rudder! It is very important during take off and for control when an engine out emergency occurs. This is far more important than in single engine airplanes, set for near maximum control deflection.

Pick a tricycle landing gear arrangement. During the take off run, the two engines never come from idle to full speed together. This causes the plane to wonder on take-off far more than you are accustomed to. Tail draggers are much harder to control in twin engine configuration.

Engine control:
Always bring the engines up slowly on take off to avoid engine flame out in transition. Do Not immediately fire wall the engines. If an engine sags on take off run – do not take off! (Burn in brain).

Never, ever, “peak” the engines for maximum RPM by leaning the mixture. This almost guarantees a dead engine . run them slightly (Read “more than normal” for a single) rich for increased reliability.

Never lean an engine to match the stronger engine. Yes, one will always be stronger. Richen the strong engine down to the weaker (that is running slightly rich …remember ).

Check that idle speeds match and idle mixtures are consistent for reliability and smooth transition to full power.

Most important!!! Before take off, always (use a helper if possible for safety) go to full power and hold the plane straight up for 5-10 seconds. If you lose an engine at rotate or climb out you will crash! This is the most vulnerable point in the whole flight. Lose an engine here, and the plane will immediately rollover and spin in. (Spectator note: Applause is really not appreciated in a twin take off Crash!)

If an engine sags or screams to a lean out RPM – Don’t take off until you correct the problem! Personal suggestion – do it once at the pits and again before take off on the runway. Yes, things change from a cold engine to a warm engine. It may work OK in the pit, but lean out on the runway. Do it every time you fly!

Engine performance can change as the daytime temperature increases. This can cause lean runs.

Idle speeds need to be reliable and reasonably close to the same speed to avoid differential thrust during landing.

Next issue………Flying Characteristics

Flying Characteristics:
After the normal 3 mistakes high altitude is attained, (yes 3 mistakes high not 2 as is normal for single engine) the plane will fly very similar to single engine airplanes.

You will note the maneuvers, rolls, turns, etc., require greater input as the wing loading is heavier than a single. The plane feels slightly heavy. Note the wings are heavier due to the greater mass of engines located on them and away form the center of the fuselage as is normal for single engine airplanes. It takes more force to start and stop this mass.

Landing (Normal Twin Engine Operation):

Wing loading is higher than standard airplanes and so landing speeds are faster be prepared for this. Do not try to slow down and risk a stall like a single. The plane is more likely to fall out of the sky than is a single at stall speeds.

Make your approach with reduced power; not full idle as you would with a single. This will prevent stalls due to low airspeed. It also increases the reliability of the engines because of the slightly higher engine speed.

If (and it will happen) a go around is necessary. Do not firewall the engines!!! You are near stall speeds and differential thrust at this speed and altitude is very difficult to handle. Lose an engine here and you will crash. Gently bring the engines back up to flying power. Fly straight to gain speed during power up. Climb after engines are at full power.

During power up listen carefully to the engines. Any sagging of one engine means go to idle and land now. Do not risk an engine flame out at landing speeds. Damage from a grass landing and weeds will be less than a high RPM inverted flat spin. (More tree branches are better than tree trunks). Taking off with two engines does not guarantee landing with two engines. Thought for today.

Single Engine Out Emergency:
If possible fly alone as you want to hear the “harmonic” of the two engines. Loss of this sound means trouble, perspiration, elevated pulse, blood pressure trembling hands, etc. Prepare to land NOW!

If flying speed is high, you are relatively safe as control surfaces can control differential thrust. I suggest you pull back power to an absolute high of 75%. With 50% being safer.Use the rudder!!! If you lose the right engine the plane will yaw and try to turn to the right. Apply left rudder to control yaw. You can apply left aileron to hold the wings level, but you must control yaw or the right wing will stall. Use the rudder! Prepare to land- now! Keep your speed up.

Left hand turns are very difficult with the left engine running at high power. If you must turn, I suggest powering back to less than 20% throttle, to reduce differential thrust, and keep the nose down to maintain speed. Make the turn and slowly bring up power if necessary, slowly! No more than 50%. The plane will react as the power comes up, be prepared for it, and prepare to reduce throttle if yaw cannot be controlled. Now use only 50% throttle max as you have lost airspeed and control-ability during the turn.

Right turns can be made easier than left turns with the left engine running, but the risk of a spin is higher if power is not reduced during turn. Keep the nose down to maintain air speed.Make a normal approach – keep up airspeed. When lined up, cut engine to idle, (this eliminates differential thrust) and land.On final approach do not bring the power back up. You will crash as airspeed is too low for the rudder to control yaw, use of the ailerons will actually cause the under powered wing to stall. This will cause the plane to roll over and dive into the ground.

Landing short or long it is always better than an inverted crash. Do not try a “go around”! The slower you fly the less engine power you can use as rudder effectiveness is reduced as air speed reduces.

Twins are definitely fun to fly, draw attention, sound neat, look cool, and will demand your best flying skills. Big point never let them see you sweat or shake.

If at any time during single engine flight, yaw increases, a wing rises, you enter a spin, bring the throttles to idle. This will eliminate differential thrust so you can gain control. Lower the nose to gain speed and Land!!! Trying to power up again is very dangerous. You are too slow for single engine control.

Your first take off with a twin will be less stressful if, after mandatory vertical power check, a helper holds the plane on the runway at full power and releases it at the pilots signal. This eliminates differential thrust during run up at take off. You are already nervous, and this reduces some of the potential problems. If you are starting with a tail dragger this is strongly recommended.

Futaba 8 channel, Hitec, and I assume JR all have mixing menus that will allow the two engines to be slaved to the rudder.

Example:
Right rudder will slow down left engine and increase right engine if below max throttle. This could produce fantastic hammer head turns, reversible spins, easy spin recovery, knife edges etc. I have not tried this…. Yet! But I do suggest you be able to cancel this function very quickly.

Additional Twin Points:
Do not mount twin engines inverted. They are less reliable due to carb flooding and tank positions. You will also experience more problems due to fuel puddles at glow plug. A 90 degree mount is ok with easier access to the carb needles for mixture adjustments and better fuel tank positioning.

Using 10% fuel produces less power than 15%, but the engine runs cooler and helps prevent lean out due to heat. Increasing the oil content of the fuel will also help with this. As stated in previous articles the reliability of the engines is the most important part of multi engine flying.

Flaps are a good idea, as it significantly reduces landing speeds that accompany high wing loads, as well as on board glow drivers, which are good for increased reliability at idle and power up. If you use the on board glow driver keep the power on up to the 25% throttle setting.

Safety:
Always start the engines from an angle to avoid propeller arc and “in front” dangers. Always start engine farthest from you to keep from having a running engine near your body. I suggest using a helper whenever possible. With two engines, two props, there is twice the possible dangers. Only adjust needle valves from behind airplane.

What kind of plane:
You can convert standard single engine to twins. I suggest using one with thinner wings for less drag during single operation and higher speeds. Just add a nose cone over the existing mount and build nacelles on wings. You will have to set the Thrust Alignment, however it is much too complicated for this article. Another option is to mount quarter inch plywood beams across the existing single engine mount. Install two engines on this in a “hammerhead” design. Plan on using a lot of tail weight. You will also need two separate fuel pick-ups if you want to use one tank.

If have two like fuselages, can marry them together for a “twin Mustang” effect. The advantage to this is there are also two rudders in the prop blast for better low speed control. There are also less fuel problems with this design than a hammerhead due to separate tanks.

All of this is fine if you are already flying twins or have a lot of scratch building skill. For first timers however I suggest you get one of the twin ARF’s or kits.

Suggested Models:
Twin Star – 25 size, AARF and good flight characteristics.PBY- as engines are very close together and big wing.

If you can find one of the inline push pull twin designs used by the US army and German Army. This eliminates the differential thrust worries. Example: Cessna Skymaster.

ARF Twin Mustang– two rudders and the engines are close together, but it is a tail dragger.

Twin Sukoi ARF- More aerobatic, engines close together but it is a tail dragger.

Texas twins has a Bobcat series of twins 20, 40, and 60 size. My personal experience with the 60 size is very good. Yes, I have 90’s on it, but that is another story. (Note: Over powering a twin can get you into trouble faster than normally powered planes.) These designs are now handled by a Florida company. They are a tricycle gear arrangement.

Personal suggestions of planes to not to start with:P-38, Mitchell B-25, Cessna 310, or any war bird with wide engine spacing and high wing loading. Build up to these. Yes, I have two P-38’s …… Do as I say not as I do! The DC-3– a tail dragger and wide engine spacing. Fly’s well, but not for your first twin. The Cessna 310 wide engine spacing and short fuselage again not a good first twin. In short if you are not sure talk to people that fly twins for their recommendations.The main point of flying twins is to impress people around you. So never let them know how close to disaster that last “really neat maneuver” really was!!!

Remember losing an engine does not cut your power by one half. It can cut climb power by 90%. If you have two 1.6 HP engines for a total of 3.2 HP Assume the plane needs 1.4 HP for level flight. Lose one engine and you have .2 HP reserve and this does not take into account the drag of the dead engine or heavy rudder deflection necessary for straight flight. Now you want to climb too – get real!

On board gyroscopes for the rudder will significantly reduce unexpected turns due to differential (unequal) thrust. They will help in take off runs and avoid (with out control limits) or minimize ground loops. They are not able to fly the plane completely as they are not heading hold systems and cannot compensate for engine loss after airspeed drops below the ability of control surfaces to counter engine torque. They will give you more time to react to engine failure problems. If used, practice first on single engine airplane and be able to turn it off in flight. Rolls can be quite “interesting” with a gyro on.The Futaba 8 channel (I don’t know about the others so don’t yell at me) has the ability to have separate throttle control. With this you can practice single engine performance and flying characteristics. (Minimum 4 mistakes High). Learn to use the rudder! While this is important for all airplanes it is mandatory for a twin.

Do not ‘slave” the rudder to the ailerons. It will not work to counteract engine torque during single engine operation. This is due to changing rudder requirements due to airspeed and engine thrust settings. Not only is it a crutch but inverted flight (where rudder functions reverses) becomes impossible. Forget knife edge etc, etc,. If you can’t fly (Or won’t) with coordinated rudder control, don’t fly twin engine airplanes; you are a crash waiting to happen.

Use reliable engines that you are familiar with. Do not “mix old and new engines, mufflers or engine types. They need to be a matched set capable of similar power (thrust) and reliability. If you need to buy a new engine to match your current old engine, rebuild the old one. This is easy and inexpensive if it is a ring type engine.

Try to get the engines to idle, midrange and hit peak RPM as close together as possible. This is important to minimize differential thrust. I realize this is tedious, time consuming, and a pain in the a__ , but very important for twin operation. Note earlier comments on setting engine RPM.

Don’t use bell cranks for throttle linkage. This just adds to the play and set up problems. If you use nyrods (which is OK), make absolutely sure they are the same length along the nylon push rod length. These rods expand and contract with heat. If they are the same length these changes in length will be uniform and so not affect engine synchronization.

Any Body for Four Engines?
2 engines are 4 times the problems of a single.

I do not want to think about four . How about the Spruce Goose – 8!!!

 

Tags:

 

More Posts in Uncategorized

 
 

Share this Post



 
 
 
 

Leave a Reply

Your email address will not be published. Required fields are marked *