Nothing quite draws attention at the club field like a multi-engine aircraft.  The sound of a twin is unmistakable.  All eyes watch as it taxis out for take-off. Throttles are advanced, and the power of two engines has the airplane airborne in a moment.  (Climb-out is going smoothly until the airplane makes an abrupt turn. The sound Changes a moment later. You realise you have lost an engine! Events happen rapidly now. The plane is starting to roil, and you add aileron to keep the wings level. The plane starts to lose altitude. You instinctively add up elevator to compensate for the loss of power. The throttle is at maximum - more bank, more aileron. Which way to turn to get hack to the field? The airplane slows even more. Your mind is racing, when suddenly the airplane rolls on its back and dives straight into the ground. As you approach the crash site. You consider building a glider for your next project.

The purpose of this article is to give radio control multi-engine pilots practical information for coping with engine failure during flight. It is important to understand some of the basic aerodynamics of multi-engine flight, and how to apply this to an airborne engine-out emergency. The end results will be a set of procedures for handling engine failure thought out before your airplane leaves the ground. The desired result is recovering your pride and joy in one piece when an engine quits.

Note that these discussions cover multi-engine aircraft with engines located away from the aircraft's centreline. Multi’s with both engines on the aircraft's centreline (such as the (Cessna Skymaster) or engines located very close to each other (PBY Catalina flying boat) have less asymmetric thrust penalties than "normal” twins with engines on the wings, such as the Cessna 310. Our following discussions will assume a twin-engined aircraft with the engines located off the aircraft's centreline (see figure to the left).

There are two important concepts regarding multi-engine flight that R/C pilots must understand. The first regards single engine climb capability, and the other aircraft controllability with only one engine working. When you lose one engine in a twin, you lose 50% of your power. Many pilots erroneously assume that the airplane also loses 50% of its performance. By performance, we specifically mean climb rate. In fact, when an engine quits, you lose up to 90% of your climb capability! The reason is the decrease of what makes any airplane climb, and that is excess thrust.  For example: a full-scale twin has two 160 horsepower engines, for a total 320 horsepower. In this case, about 140 horsepower is needed to maintain level flight. Thus, we have 180 horsepower available for climb.

Now take out an engine. You have a total of 160 horsepower available or flight. Remember, you still need 140 horsepower to maintain level flight, and this does not take into account the additional drag of the propeller on the inoperative engine. You now have 20 horsepower available for climbing; quite a decrease from the 180 horsepower available with both engines cranking away.  In fact, you have about I 0% of normal climb power available; ie there is a 90% reduction in climb capability when one engine fails.

The other important understanding with multi-engine flight concerns aircraft controllability when one engine is out. A twin engine aircraft behaves more or less like any other airplane when both engines are working.   However, when one engine fails, an asymmetric force condition is set in place.  The pilot must overcome this asymmetrical condition by sorting up a countering moment with the rudder.

The yawing force (or moment) of the single operating engine is a constant at full throttle.  As airspeed decreases you will need to add progressively more rudder for the two moments to balance.  Eventually, your airspeed (or what concerns us here, airflow across the rudder) will be low enough that the rudder moment with full rudder will not he able to counter the opposite moment of the single engine. You are about to lose control: full rudder, full power, and the plane starts rolling opposite your rudder input. This is called minimum controllable airspeed.  The airplane will eventually roll on its back if you continue to lose airspeed.  The nose falls and you head straight down for the proverbial one point landing.

Controllability is determined as the single engine sets up a yaw force, balanced by the rudder, around tie aircraft’s Centre of Gravity. Ailerons have no real effect on this situation. By the time airspeed is low enough that the single engine moment overcomes the rudder input, the ailerons do not have enough control authority to keep the wings level.

What is unique about minimum controllable airspeed in a twin is that you can be flying, albeit with only one engine, have the rudder at its full limit, and yet still have the aircraft turn opposite your rudder input above stall speed. Minimum controllable airspeed is so important in full-scale twins that it is marked on the airspeed indicator with a red line. Fly below this speed immediately after an engine failure and you risk losing control of your aircraft. What can we do to avoid this unpleasant situation?

Rudder is the key control to counteract the yaw on the single operating engine. Power is a constant, assuming maximum on the operating engine so elevators will control airspeed. Nose up, you slow down, lower the nose, airspeed increases. Your troubles are being generated by the asymmetric thrust condition established by your one operating engine. But keep in mind there is a way to fix this situation. Simply bring the operating engine to idle. Now the thrust is essentially equal on both sides, with one engine at idle and the other inoperative. The entire problem of maintaining control with one engine dead is temporarily resolved. Your performance may not be to your liking, as you will be in a descent. But you have maintained control.

When an engine quits on a twin, your overall goal is to get the airplane down in one piece. Landing on the runway is nice but not a requirement. You have to convince yourself that two engines on departure do not guarantee a return trip to home should one fail. In many cases you can make it back, but there are segments of the flight where you may have to recover off the field. It is always better to accept an off airport landing under control than a crash out of control.

First, anytime you lose an engine (or see or hear anything unusual with the airplane) during the take-off roll, there is no question but to abort the take-off. Throttle back to idle and stay on tie ground. This is carved in stone.  Even if you are near the end of the runway, discontinue the take-off and go into the bushes. Damage will likely be a lot less than trying to get airborne on 10% of normal climb power, without even discussing the difficulties involved with circling to land under these circumstances.
 

MORE ON FLYING TWINS
(From WRCS Newsletter November 1999)

Once you get airborne and are starting to climb out, engine failure response actions enter a grey area that depend on your aircraft’s flight characteristics. If engine failure occurs just after take-off and there is any, clearway in front of you, consider throttles to idle, lower the nose, and landing straight ahead.

If you have started the crosswind turn, you are usually high and fast enough to he committed to flight when an engine quits. You will already be at full power. Add rudder to counteract the yaw from the working engine. Use elevator as required to maintain altitude. With power a constant, elevator will also control airspeed. Make all turns gentle. The instant you feel you are losing control, as evidenced by the start of an uncommanded roll, reduce power. By decreasing the yaw' moment of the operating engine, you, in effect, increase the rudder moment, hence aircraft control. If necessary, go to idle on the operating engine and land straight ahead. Again your goal is to maintain control.

With an engine failure on an R/C multi, you may not know exactly what is wrong at first. You are observing the situation at a distance from the ground. Don't spend too much time analysing the situation, trying to determine the exact problem. An engine may have failed, a prop nut might have come off – you just know something' is wrong. Your first reaction should be to reduce power and head hack towards the landing zone. Try to align yourself to a downwind entry leg, so you are in a position to fly a shortened glide pattern should you lose the other engine or he forced to throttle back the operating one.

In any event, the airplane is under control. Slowly add power. With an engine failure on one side, application of power will result in yaw. There is no major problem at this point; Just use opposite rudder to counteract the engine induced yaw.  If you go to full rudder, Reduce power and lower the nose slightly (increase airspeed) to maintain control. Do not confuse performance and controllability, all we want is control.