Wednesday, March 12, 2008

Lessons : Stall Turns

Here I got Flying Lesson From, i'm sure this lesson will help you Guys.....

Lesson objectives:
1. Continue to develop fast forward flight skills.
2. Identify relationship between collective and cyclic controls for vertical climbs.
3. Identify amount of rudder required to execute 180 degree turns.
4. Timing of collective vs cyclic for vertical climbs and dives.

Stall turns are one of the first aerobatic maneuver that a pilot will do once they begin Fast Forward Flight. They begin as a small chandelle (for want of a better explanation: a steep climb, followed by a good amount aileron till the heli has ceased forward speed, then rudder to set the machine back the way it came) through to 540, 720+ stall turns.

A stall turn, in the true meaning of the term means entering the maneuver at speed then pulling up into a vertical climb. Once, the helicopter has ceased forward momentum (stalled), then rudder is applied to turn the helicopter around (180 degrees) so that it is facing the way it came. A good stall turn will end with the helicopter pulling up out of the dive at the point at which it entered. Variations such as 540 stall turns mean that the rudder has been held in until the model has completed a 540 degree piroette, a rolling stall turn is one where the model completes a half cyclic roll in either the climbing or diving aspects of the maneuver.

I do my stall turns in Idle Up One which is about -5 at bottom stick through to +10.5 - 11 (this is a 91 powered Vigor :)) degrees pitch, and the gyro is in rate mode (many, if not most, people like to do it in heading hold).

There are some things you should know about this setup. On a very windy day doing a stall turn into wind, you often need a lot of negative pitch to stop the heli 'travelling' backwards (being pushed by the wind), therefore, in this case I do stall turns in Idle Up 2 (-9.5 to 9.5 and gyro in Heading hold).

First of all, you should be able to conduct straight level flight in either direction. Flying in a line parallel to where you are standing, do some straight and level passes back and forth and constant altitude. Continue to do this until you are confident of controlled level flight. Once you're confident, you're ready to setup your stall turn.

Make sure you're at a good altitude, 100 feet is a good start as it will give you ample room to fix up any mistakes. Give yourself plenty of run up room for your entry. Fly out 200-300 feet downrange and then begin your turn to come back on a heading parallel to you. The reason for this is to give you plenty of time to 'straighten' the heli out and chill out before entering the maneuver. Build up a good amount of forward speed, you need not enter at full speed, 3/4 stick should be plenty.

Entering the maneuver
Now that your helicopter is lined up and all straight and level, you'll want to think about entering the maneuver. I found for the first attempts it's better to let the heli fly past you for a distance of say 50 feet or so so that you are looking at the top and back of the helicopter when it executes the maneuver. DO NOT execute it right in front of you at this time! The helicopter will go straight up and likely go over your head which will result in an orientation problem (I'm speaking from experience here!).
When the helicopter has reached your designated entry point, GENTLY ease back on the cyclic and then release as the helicopter goes vertical at which point you should be lowering your collective to approximately mid stick (say 3.5 degrees or so). You should find that your helicopter will then climb in a near vertical fashion.

Executing the maneuver
Now that the helicopter is climbing, it will soon come to a stop (stall). You now have two options, one, you can bail out using one of two methods, inputting a forward cyclic to push the nose over so that the helicopter level's out, or, you can execute the 180 degree turn and increase collective and the helicopter will fly away without loosing altitude. There is nothing wrong with any of these options because at least you're getting a feel for how the helicopter climbs.

The next option is to execute the full maneuver which at this point means completing the 180 degree turn. Now, one of the biggest mistakes you can make is to give it FULL rudder, with today's modern gyro's, this will probably result in some bluringly fast piroettes and it will be hard to stop the piroette with the nose pointing straight down. It is more likely that you'll come out sideways which can be very scary if you didn't expect it!
Therefore, the better plan is to gently feed in rudder and let the helicopter come around slowly, this will make stopping the piroette in the nose down attitude much easier and will result in a much better looking stall turn.

The helicopter will appear to 'hang' for a moment and should then start falling down the same trajectory it came up on. Again, you should still be at about mid stick or lower (ie 3.5 degrees or so) and as long as the nose is pointing straight down, you should not have any cyclic input.

Exiting the maneuver
When the helicopter reaches the point where it went vertical in the climb, you want to GENTLY start pulling back on the cyclic and SLOWLY feeding in collective. As the nose of the helicopter comes up, keep feeding in the collective until the helicopter has a slight nose down attitude and is flying level. The helicopter should then be flying back down range along the path it entered on.

Congratulations! You've completed your first stall turn!
Things to watch out for

'Yanking' back on the cyclic controls.
You will lose precious forward speed that will mean your vertical climbs will be shorter. It will also mean the maneuver won't look very smooth.

Not entering straight and level.
If you enter the maneuver with a slight aileron attitude, the helicopter won't
climb straight, it will either climb up and away from you, or up and coming at you.

Don't be in a hurry to execute the 180!
As discussed above, if you whack in full rudder it's going to be hard to stop it with the nose pointing straight down which will mean that the helicopter will 'fall out' of the maneuver sideways. This happened to me with the Z230, it wasn't pleasant.

Manage the collective
If you don't reduce the collective as the helicopter is climbing or falling, the helicopter will 'travel'. This means that the heli won't climb or dive straight and your stall turn will look crazy.

Weight in the nose
If you're having trouble getting your helicopter to climb very high when in the vertical stage, putting a little bit of weight in the nose can help. I added about 40 grams of weight (using tyre weights from mag wheels - they have double sided tape and come in 7 gram blocks) to the nose of the Vigor and wow, that made a difference!

Points to perfect
The following are a list of aspects I look for when judging a stall turn.

Smooth transition from horizontal to vertical (and back again)
You want to make the the transition from horizontal to vertical flight as smooth as possible, ideally a quarter arc.

Clean vertical climb/dive
Learn to manage the collective to ensure that the helicopter climbs and dives as vertically as possible.

Timing of 180/540 turn
Practise timing the execution of your turns so that they begin when the helicopter ceases it's climb and is about to enter the dive.

Smooth pullout and exit
A smooth pullout replicating the quarter arc you scribed on the entry and then smooth forward flight out of the turn.

Get out there and burn some fuel!

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