Friday, August 12, 2011

Helicopters: Forward Flight Transition

Heli Forward Flight Transition.

By Lood Birk

Ermelo Radio Flyers

I have been flying helis for some time and when I was asked how to move from a hover into forward flight I had to sit down and think about it quite deeply. You see, I have been doing that quite a bit and it has become second nature. None the less, it is a very good question. I am going to try and explain to you the type of stick movements that could expect when you transition from a hover into forward flight and vice versa.

You might think that there is nothing to it, but you are mistaking. To explain why I am going to give a very simple aerodynamic lesson. When a heli hovers, or flies, it generates a downdraft of air. For the sake of simple understanding, call it an air bubble. It is this air bubble that makes the difference when you transit between flight modes (hover and forward flight). During hover the air bubble generates lift for the chopper (keeping it very simple). Because the chopper is stationary, the lift is more or less equal on all sides of the rotor blade. To put it another way, the heli is balancing on the air bubble. When you fly, the heli is always trying to get away from the bubble and the lift forces becomes different and the heli starts reacting a bit differently. Making the move from the one to the other is called flight transition. Geesh, I make it sound so complicated, but it’s not really that bad. If you have mastered the controls on your heli, the corrective inputs normally comes quite naturally and you don’t even think about it. Coming to think about it, on a properly set up heli the inputs are quite small.

Ok, onward ho. You are hovering your heli and the mood strikes you to try some forward flight. No problem. Just push the stick forward. Right. Partly yes, but don’t forget that you are now moving away from your air bubble, so two things are going to happen. Firstly the heli’s nose will keep tilting forward as your air bubble suddenly gives more lift at the rear of your rotor than the back and it will lose height because the lift balance changed. So, as your heli starts moving forward you will give some back stick and some pitch. Now comes the fun part. As the heli speeds up the rotor becomes a like a Frisbee and start to generate lift like an aircraft wing. Because the rotor is nw generating more lift, your heli will start to climb, so you can bring back the pitch a bit to keep it at one altitude. Once you are in forward flight, your heli will more or less become balanced and you have transitioned into forward flight.

So now you are flying around and as your fuel becomes less and less you make the decision that maybe it’s time to bring your heli back into a hover and ultimately land. By this time you are shaking because you know you have to do that transition thing again. Once again, it is no real biggie. If you had the skill to move into forward flight, going back to hover should be no problem. Lets look at the procedure. Firstly you will have to slow your heli down. You do this by pulling back on the stick. Now because you are pushing your rotor disk into the wind (remember to thing of your rotor as a Frisbee) it will start to generate a lot of lift, so you will need to reduce the pitch a bit. As your heli slow down, the lift will become less and you will have to give a bit more pitch. Remember now that you are almost stationary and you will be riding on top of your air bubble now, so the chopper will tend to fall backwards on it’s tail. Here you must give it some forward cyclic to bring the heli nose down again and there you go. Nicely established in a hover again.

I just want to emphasize that it all sounds so complicated, but it’s really not all that bad. It is something you should be aware of, but if you are proficient at hovering your heli, the control inputs to counter the transitions will come naturally. So, don’t be scared.

Hope this helps a bit. (for those who asked)

Happy Landings

No comments: