Gyrocopter Blues

[Hiker]

I'm seeking a good technical answer for a question that came up over a pint awhile back {There's bottle of scotch at stake on this one}.
What makes the boody blades keep spinning on a Gyrocopter?
Now, I know about the principles of autorotation to some degree {I'm a fixed winger, take pity on me!}.
But I've yet to find a detailed explanation about autogyros. Obviously the forward momentum of the machine is provided by the pusher engine, and the blades are pre-spun to start the blades up, but I would like to know more about the rotor disc;
1. Assuming the machine is in normal flight, at which direction is the airflow striking the blade? [eg. underside, topside]
2. What is the relative blade angle in normal flight and what if any is the range of travel? [0' being flat pitch]
Now my probem stems a massive mental block based on windmilling props, if the rotor disc is tilted aft, [as it appears] and the blades are advancing, [as the normal chord design would indicate] and there's any kind of positive pitch, the bloody things stop! even back up if it where possible.
Help me please, I'm losing valuable couch potato time thinking about this.
Cheers, Hiker..........

[Skycop]

This is difficult to explain without a diagram (and quite difficult to explain with one too!) but here goes. On a rotary wing there are three components to the airflow acting on a blade. 1. Rotational flow, 2. Forward airflow, (1 & 2 should be self evident) 3. Induced flow (the column of air displaced by the rotor disc - similar to fixed wing downwash). The actual flow experienced by a blade is a combination of these three, known as Relative Airflow. The angle of attack experienced depends on the velocity (direction and magnitude)of this relative airflow as well as the pitch angle of the blade. As you will know, lift always occurs upwards at 90 degrees to the relative airflow and drag acts 90 degrees backwards from the top of the lift vector (even on plank wings!). There is however, a difference between fixed and rotary wings at this point. For rotary wing the resultant between the lift and drag vectors (known as the Total Reaction) is resolved vertically into Rotor thrust (up the rotor mast) and horizontally along the "Plane of Rotation"of the rotor system. Now here is the crux of your question...If the horizontal component lies behind the rotor's axis of rotation it acts as rotor drag and needs power to keep the whole show turning i.e. we are in helicopter mode. However, in the case of an autogyro / gyrocopter or a helicopter in autorotation the Induced Airflow is coming from below the rotor disc (yes, the disc appears to be "tilted back" as you say, or for a helicopter it requires a rate of descent to get the same effect). This changes the Relative Airflow so that the Total Reaction moves forward and there is a "positive rotor drag" or autorotative force. For all this to happen the blades are set at a low pitch angle (permanently on simple autogyros, or the collective lever lowered to the bottom of its travel for a helicopter). The principle is similar to a fixed-wing propellor windmilling after engine failure but in this case, due to the engine axis being roughly in line with the direction of flight the effect is seen as a large dragging force rather than a vertical rotor thrust. (There, now you wish you never asked, I'll bet. Cleans off blackboard) :)

P.S. Mine's a MacAllans

[This message has been edited by Skycop (edited 06 June 1999).]

[Hiker]

I greatly appreciate the very comprehensive answer, I going to need a day or two to digest everything! Muust.....ggeetttt.....brainn.......tooo.....functiooonnnnn. ..
Thanks again,
Hiker...

[Skycop]

If that's too much to take in, try this. Get a kid's plastic windmill and hold it so that the axle (axis of rotation) is facing down at 45 degrees or so at the front. You now have a simple model of a "tilted back" rotor system). Hold it at the end of the stick and stick it out of a car window to get a good old airflow (I don't recommend you drive the car...) The blades will go round and you should feel the drag (backwards)and rotor thrust (upwards). Same thing really. Don't do it at motorway speeds and don't let go, as I might be behind you on my motorbike ;)P.S. Apparently smiling and wearing a grass skirt?

[This message has been edited by Skycop (edited 07 June 1999).]

[neverinbalance]

Hiker, you could try this:-
1. Print out Skycop's first reply.
2. Give to your mate.
3. Grab the scotch and neck it quick.

[Skycop]

I deliberately didn't mention the fact that the outer part of a blade system in autorotation is tending to slow down whilst the inner end is tending to speed up due to the differences in relative airflow. Also the "positive rotor drag" is the resultant of these two effects...Hope this helps the brain!

[Hiker]

The windmill idea worked, I hit him with it, and ran out the pub door with the scotch, but whilst running through the streets dodging countless cars being driven by nitwits holding propellers out their windows at breakneck speeds, I couldn't help noticing that the propellers though tilted at a 45' angle had a positive pitch and were rotating backwards with the exception of one fool who had his at a negitive pitch, thereby his was rotating correctly!
Alas the life of a fixed wiger is a diffcult one, and often misunderstood.
Cheers.......

[Skycop]

Hiker, you are correct to say that most windmills have positive pitch. However, it is the angle of attack that matters i.e. not the pitch angle. (Remember that the pitch angle is controlled by the pilot but the angle of attack depends on the Relative Airflow). As far as I am aware, no rotary wing aircraft has a negative pitch angle in flight as this would result in a major rotor overspeed (and a nil-wing aircraft). I say "in flight" as I believe that the Lynx has a negative pitch capability to hold it onto the ship's deck (prepared to stand corrected on this). Told you it wasn't easy to explain :)

[neverinbalance]

Hiker, you say the life of a plank driver is not an easy one!!! Wanna job swap?