View Full Version : Question from a fixed wing Pilot
7th Aug 2012, 02:14
I think I have a very basic understanding of how a helicopter flies.
My question is: how do you make a helicopter fly as fast as possible ?
My theory is that you would have the collective all the way up for maximum lift while the cyclic would be well forward to tilt the rotor disc forward 'vectoring' the most lift possible in the forward direction.
Anywhere close ?!
7th Aug 2012, 02:53
That's pretty much it in a nutshell, just with a little more finesse.
There are a few limiting factors when you try to go as fast as possible:
Engine power output
Limit of forward cyclic travel
Onset of retreating blade stall
Hope this helps your understanding.
Start high and nose dive......Collective will be lower, thereby less angle of attack, therefore reducing the chance of retreating blade stall.....
Arm out the window
7th Aug 2012, 05:12
Re the "collective all the way up" bit, most often you will be limited as to how much you can pull by either a transmission torque limit, or by some engine limitation (be it temperature, rpm or sheer ability to produce power - in this last case, if you pull more collective pitch and create more drag than the engine is capable of overcoming, the rotor rpm will start to 'droop'.
I know you were talking about forward flight, but to illustrate this last point, imagine taking off on an angled departure out of a confined area - you pull pitch, put in some forward cyclic to translate forward, continuing to pull collective to fly the machine up the desired slope.
If the engine can't crank out enough ergs to keep the rotor turning steadily at the required pitch angle, the rotor rpm starts to bleed off, reducing lift and making you drop below your desired flight path. You pull more pitch to compensate but, as Scotty says, she canna take it, Captain, and rotor revs drop even more. With blade pitch well past the most efficient angle of attack now and revs dropping sharply (with associated warning horns going off), you settle unceremoniously into the trees.
Some helicopters are blessed with bucketloads of power and don't often run into this situation, but like old taildraggers, it's good to learn on one that teaches you to work within your limitations!
8th Aug 2012, 04:27
Absolutely, genuinely fascinating, thanks for the replies.
9th Aug 2012, 06:29
Ok, another question if you would indulge me.
I have read a few books about helicopter flying, particularly in Vietnam.
'Chickenhawk' was just brilliant.
If I remember correctly there was a twist grip throttle control on the collective to control engine speed on the UH1
Is this still the convention on modern helicopters. Do you 'throttle up and down' with a twist grip or does one just set the engine speed in a 'detent' and leave it alone ?
On a piston engined machine you would use the throttle all the time, but even some of those are now governed, so you set the throttle to full and just leave the fuel governer to get on with it (also with turbines). If there is an emergency (tail rotor etc), you can take it out of the governed range and use the throttle manually.
Modern turbine machines tend to have their throttles in the roof, or on the floor, but hardly ever on the collective. Personally, I think the collective is the most logical place for it.
On some of the first machines the throttle was on the cyclic.
10th Aug 2012, 11:42
That's interesting, throttle on the stick. Any reverences or illustrations?
10th Aug 2012, 12:20
Its a twist grip like on a motorbike.
10th Aug 2012, 12:37
The Bristol Sycamore had a twist grip that was on and at right-angles to the collective. Where on a motor cycle to increase power the throttle was turned so that the top of the grip went rearwards, on the Sycamore it was towards the front. This was to compliment the natural turn of the wrist when lifting the single collective lever, which was quite heavy, because there were no power controls. The throttle pointed towards the RHS so that flying it from the LHS, as it was a basic helicopter instructional aircraft, required the left hand on the cyclic and the right hand on the collective. Because of the natural movement of the wrist lifting the collective then the hand was placed underneath the throttle; so you had a cyclic each and shared. quite often, the collective and throttle.
I did my initial instruction on the Sycamore, both Right and Left seats and believe me, after that, ANY helicopter was a piece of doddle.
Old and Horrified
11th Aug 2012, 13:15
If I recall correctly from my days at Westland, it is also compressibility effects on the leading blade tip that limits ultimate speed, i.e. it can become close to supersonic. Westland took part in the BERP (British Experimental Rotor Blade) programme in the late 1970s to improve blade tips with respect to both retreating blade stall and leading blade compressibility. The result can be seen on all EH101 aircraft and most Lynx. The sweep back improves leading blade performance and also created vortices to improve retreating blade stall - similar to the way the Concorde wing works at low approach speed. The conventional helicopter speed record is still held by G-LYNX, a specially modified Lynx with BERP blades at just over 216 knots. Westland held (still holds?) several patents on the BERP technology (there was a lot more than just tip shape) and tried hard to sell it to other helicopter manufacturers, but without success.
11th Aug 2012, 15:22
Mhale71, he said cyclic, not collective.
11th Aug 2012, 20:09
One thing not mentioned yet is that the rotor system is run at a fixed RPM, chosen as a compromise between the most efficient angle of attack and enough RPM to give you a chance to get into autorotation if the engine stops. Newer machines might take off at one RPM setting, and cruise at a lower setting for efficiency.
To keep the RPM constant with varying pitch (and load) settings will require the engine to put out differing amounts of power.
With a turbine, the throttle is usually set to full open, allowing the governor to use any amount of engine RPM (N1)up to its design maximum. In older piston engines, it is up to the pilot to work the throttle to keep the rotor RPM in limits. Usually the engine RPM (ERPM) and the rotor RPM (RRPM) needles are on the same tacho, so we always want to see the needles matched.