How auto-rotations work
Airplanes have the virtue of being able to glide large distances after a power outage, however the helicopter is not so forgiving. Helicopters rely on reverse air flow through the rotor to drive the system and create lift. This is accomplished by reversing the blade pitch to a negative value. If we didn’t do this the rotor rpm would spool down very quickly with a resulting loss of lift and cyclic control. The sooner one reacts to an engine out condition by reducing collective to a negative value the better off the rotor speed will be. This does not mean that a loss in rpm cannot be recovered if the prevailing conditions are in your favour. Altitude and air speed will all play a role in the emergency autorotation. Altitude can be equated to an “rpm recovery insurance policy” and forward airspeed as “money in the bank”. Forward airspeed inertia can be traded off immediately for a quick increase in rpm through aggressive flaring of the rotor disk where as altitude will give a slower recovery time.
The rate of descent in an autorotative state can vary according to the weight of the machine, rotor rpm and airspeed. A purely vertical descent will yield a rapid loss of altitude. As air speed increases this rate of descent will be reduced until a point is reached where it will start to increase.
This glide slope or rate of descent will be controlled with the cyclic stick. Since our models can be rigged for high negative collective blade pitch angles we have a further method of descent rate and rotor speed control. As for the weighted state of the helicopter or rotor disk loading, this can vary depending on fuel load and is not a large factor in models. Technically speaking for similar conditions, a heavier loaded machine will descend faster and with a higher rpm. A higher rate of descent also takes more rotor inertia to arrest than a lower rate. Even though we have a higher rpm (with increased inertia) the trade off is not equal. From the above one can conclude that there is a most efficient combination of pitch, weight and forward airspeed for the resulting lowest rate of descent. Autorotations preformed at very high or low airspeeds will be more critical than those carried out at the proper airspeed for the lowest rate of descent.
We are very fortunate to be able to change airfoils, blade length and weight. A semi-symmetrical airfoil will produce better cushioning at the end of an auto than a symmetrical. Because the end of the auto is very critical and directly related to inertia, a heavier blade will make the manoeuvre much easier. If I was only into autos, the pick of the crop would be a heavy semi symmetrical. As with all things there are draw backs and the above choice might not be the best for aerobatics. I’d certainly recommend the above blades for the individual learning autos.
The act of successfully performing an autorotation depends on a certain amount of skill which only comes from practice. Improper methods of learning autos have skinned many cats, usually by making the first incision dead center of the bill fold! For myself I chose a conservative approach with no mishaps that I shall share with you. Like most difficult things, this manoeuvre can be broken down into smaller parts to be practiced separately. Later the competent pieces can be put together one at a time to ensure the best chance of mechanical survival. The beauty of this method is you can hang around in a hover or forward flight and try which ever slice of the pie you are weakest at. The only prerequisite is that you must be into forward flight to use the complete package. This does not mean you cannot familarize yourself with the hover portions. You’ll need minus 2-3 degrees collective at bottom stick and 11-13 on the top end.
I started from the ground up by studiously performing hover chops at 6 inches altitude with moderate winds of about 10-15 kph. This makes the rotor more efficient due to translational lift, which in effect gives more cushioning lift. Make sure the clutch has disengaged with the activation of the hold switch, if not adjust it so that it does. Otherwise expect a larger than necessary yaw in the anti torque direction (same as the rotor). This is because the tail rotor is compensating for non existent rotor torque. Simply point the beast into wind with a stable hover, then engage the throttle hold. The machine will settle slowly with little application of collective needed for cushioning. As you work your way up in height the collective will have to be blended or milked in close to the ground. Sooner or later you will reach a height where you have not enough inertia left in the rotor to arrest the descent. The cure here is to reduce the collective immediately by a small amount after flicking the hold on so as not to use valuable rotor rpm by hanging around and wasting rotor inertia. As you get more experienced you will be able to chop more and more collective out at the top until you reach the best compromise between descent rate and cushioning.
This will equate to the highest hover chop and will vary from machine to machine. I was doing 4 foot hover chops before taking the big step. There is a big difference in applying the hold switch in a hover as compared to up high in forward flight, it is almost like it moved to another location on the radio. Don’t worry as there is plenty of time for that later. Practice full low collective descents with no idle-up and a good reliable idle at low stick (minus 2-3 degrees). Don’t fool around here and bottom the stick at once and at a good safe altitude. Keep the machine flying forward. If you go too fast the thing will rapidly increase its rate of descent. Try to maintain about a 45 degree glide slope.
This will vary slightly from machine to machine, but 45 degrees will certainly put you in the ball park. Bring the power back in half way down. Do this over and over gradually working closer to the ground. If you play around at quarter collective the risk is there for rotor decay, depending on your throttle curve. The other option is to check the “coming in” range at altitude, noting the rotor speed as the collective is slowly applied from low stick and then make the appropriate adjustments to the throttle curve, either electronically or mechanically. At your personal comfort level begin to zigzag around the sky on the way down. This will also help to dump altitude if you are in an over-shoot condition. Poking the nose over will cause a very rapid increase in the rate of descent and steepen up the glide slope….more so than the zigzag approach. It is now time to use the hold switch but only after the collective if fully lowered in order to prevent rotor decay. Gradually work the termination of the auto closer to the ground. The next step is the cyclic flare. Aft stick is applied which causes the machine to pitch up exposing the rotor to a larger mass of air. In effect you are trading forward inertia for lift and additional rotor rpm. One can actually hear the rotor spool up in the flare while forward speed and descent rate are effectively reduced. At this point we have not yet dipped into our now larger “inertia bank account” which will be used for final cushioning. The height at which the flare is preformed will depend on blade inertia (weight), airfoil type and disk loading. The wind will also play a large role. The flare must be removed before landing so as to avoid a tail strike with the ground. During a hard landing with the softer flapping machines and/or those having short masts, aft cyclic can cause a boom strike even thought the tail has not contacted the ground. The proper cyclic levelling of the machine and application of collective can augment one another through a proper and timely blending of the two before touch down. It’s kind of like putting the brakes on and gearing down with a automobile while approaching a sharp corner….somewhat above the legal speed limit. You can practice the flare without the collective cushioning segment up high and do a power recovery, using the hold switch if you so choose. This practice flare can be worked down closer to the ground. Well, one day the engine is either going to flame out or you might simply say, “screw this” and go for the big one. A word of caution, bailing out after the flare at a substantially reduced rotor rpm may well cause more damage than a hard landing. As a brief review, set yourself up and lower the collective fully before engaging the hold switch, find a glide angle of about 45 degrees, do a cyclic flare levelling 3-4 feet high and finally cushion the landing with collective. Abort the auto before the flare if it is going to be completed more than 20 or 30 feet away. You now have the knowledge so keep cool and practice safely. Even after you have mastered autos, you should keep your skills honed.
Sometimes engine failures occur at the most inopportune moment, like shortly after takeoff and downwind. By the time you get the collective reduced the rpm has done the same. You can’t turn into wind because you are too low. A flare is out of the question because the air speed is so marginal and what little translational lift you do have is holding the machine in the air. The “airspeed bank account” is empty and your “altitude insurance policy” has expired! By now you feel like you have just had sex, and quite possibly so. All is not lost because you have an option….the downwind run-on landing similar to a fixed wing. What you do is keep the airspeed up which will seem fast because after all you are flying in a moving mass of air. As the ship approaches the ground level it and apply enough collective to skim it over the ground, applying more and more collective so the brakes are not put on quickly with a heavy machine. With rubber skid protectors sliding on pavement you risk a ground tumble. While running-on machines with grass and pavement I have actually inadvertantly done spin-outs like a car sliding sideways. This method may be used to salvage a practice but flamed out auto, which has been screwed up at the bottom with low rpm. Timing is critical so don’t wait too long to make a decision.
Another favorite of mine is a tail rotor drive failure which happened only once due to a failed setscrew on a used machine. It was basically my own fault for not going over the machine closely enough. Anyway, this little bugger started to pirroette in forward flight. Awed, I waited about 4 cycles before it dawened on me to try to salvage the situation. So I selected the hold with the pole bottomed and sustained a bent boom support that just happened to find a lonesome boulder edge. That day I was happy, very happy to be wearing brown pants! If I had not been practicing early in my heli career, my wallet surely would be lighter today. If you cannot completely save the situation the resulting damage will likely be less, even if the landing is some distance away. A lot of damage usually occurs from hitting the ground with power on……with the hold switch on there is no torque to further mangle things.
Here are a few final tips to keep in mind with your setup. I watched a fellow hit the hold in a hover chop some time ago, and the machine went skyward because the two pitch curves did not match at half stick. You should have seen the look on his face…and he was not a beginner. Another helpful item concerns the T/R curve…it should go close to a flat T/R pitch on throttle hold, in case the tail is still spinning with partial clutch engagement or it has a slipper system etc. If you have a heading hold gyro selected to this mode the gyro will take care of matters and no alteration to the linear or normal unmodified tail rotor curve should be made. I feel the more helpful we can be at fighting “Murphy’s Laws”, the better off the everyone will be.