View Full Version : Looking for Input from Shawn Coyle or Similar Authority
23rd Jan 2010, 21:58
Here's my question, for a real helicopter technical expert who can answer it authoritatively, maybe it'll get to his/her attention:
What is the effect of longitudinal CG on helicopter range?
To clarify, at a given gross weight (and stipulating, for the sake of the question, that the CG stays at a constant location during the flight), what CG location within the envelope will result in maximum range?
As I understand it (but I'm sure others will have some comments):
The effect of the C of G on power and range are mostly due to the resulting angle of the fuselage, with some contribution from horizontal stabiliser download. Any range data in the flight manual is usually conservatively placed at worst C of G.
Aft C of G: The nose is up more, so the fuselage is pointed more squarely into the relative wind, for lower cruise drag. Also, the horizontal stabiliser is hardly working, its download is small, so the range is furthest. On the other hand, the main rotor must be flapped down to get the high speed thrust, with a risk of bending the main rotor shaft
Neutral C of G: The nose is down about 3° further than it would be above. There is little change in performance, but the main rotors will not be down so much at the front
Forward C of G: The nose is down, so the fuselage has its top exposed to the free stream, requiring higher thrust, and there is a down load that looks like weight to the main rotor. This is usually the slowest and the worst for range, and the least comfortable for passengers
24th Jan 2010, 03:36
Paco has it exactly right, nose down attitude and horizontal tail download take the cake. Most flight manuals use forward CG fuel burn when range is calculated. The difference is not terribly great, perhaps 3%.
I suggest you track power needed at the various CG's at a constant cruise speed (or fuel flow if you have it) and prove this statement.
24th Jan 2010, 12:39
Paco beat me to it, and Nick (welcome back!) seconded it. Forward CG gets - which is lucky, because most of the time when you load up the helicopter, the CG is forward...
24th Jan 2010, 13:52
Shawn ...i am no expert and dont feel qualified to comment but here goes anyway !!! Have you not got it the wrong way around ? I agree forward C of G is the most likely with pax in small helis but i think that is the worst scenario ?? Better to put the fatties in the back and get more miles in more comfort ?
ps sorry if i am way off track .....
24th Jan 2010, 14:00
Sorry Shawn but for the sake of clarity (your answer appears to contradict Nick's) - all other things being equal - would a fwd CG normally be expected to decrease or increase range?
24th Jan 2010, 14:09
Mmm - seems a very confusing response from the experts!
My take is that the best range will be with the fuselage at the angle to give the lowest drag - maybe level, maybe not depending one the defintion of level and the shape of the fuselage. Since the horizontal stabiliser is an aerofoil (with good lift/drag ratio), its likely that its far more efficient to fly with the stabiliser doing some work and the fus level, than with the fus nose up just to unload the stab.
But as I alluded, the shape of the fuselage and the definition of level will vary with the particular helicopter type, so there can be no generic definitive answer except that "somewhere in the middle" is probably best.
Now awaiting the experts' flaming...
24th Jan 2010, 15:38
Personally I think this question is helicopter type specific. Having experience on the Ec120,130,350,330,332 mostly and then on the otherside of the spectrum the 206,76 and 61, I had an interesting query a couple of weeks ago from a operator who operates 119,109,R22,R44,206 who started operating a EC120. Despite a detailed balance sheet and load sheet they were convinced the c of g is to far forward due to the feel during take off and landing especially despite the load sheet saying they are within the block. Being a 120(like the 130) this is a inherit characteristic if you are not used to the other EC models which is not as prolific but you tend to not notice it moving from a 350 to a 120/130 as much. The EC guys went to reweigh the machine and once she was levelled for the weighing procedure called everybody over. All was shocked to see the 2 degree nose down attitude of the 120 on the jacks when levelled on the ground which was further pronounced by the front of the skids barely being of the ground whilst the rear of the skid is about 15-20cm of the ground-this despite the MRH slant 2 degrees forward as per most EC models(330 and 332 as much as 5 degrees)-different discussion. Seeing her level position for working out the C of G, everybody understood the fact that she will not do what they are used to ie taking off from the aft skid and landing aft skid first.
If you look at a s76 taking off you will clearly see the complete opposite. she lifts her nose wheel quite a distance of the ground and once the nose and tail angles are almost in equilibrium will she go vertical.
Can you thus expect these machines to fly the same and be effective at the same attitude? I personally do not think so as if we look at for instance as mentioned the stabilisers, there is a clearly visible angle difference in them between say the 76 and 120/130 due to the designed difference in the flying attitude and the individual machines characteristics. Where 1 might be more efficient level, another will be more efficient 2 degrees nose down if so designed. Guess it is the same as the 350 theory where you need to be halve a ball out on the turn and slip to be flying straight and it and the woolometer to be in sync:O.
Er yes, we do seem to have a difference here..... :)
24th Jan 2010, 21:50
Certainly, as I re-read my response, I was not at all clear.
Aft CG is better for range, generally, by about 3%. This is because most helos are near level for better fuselage alignment (low drag) and also because the tail download is much less.
3%? If that makes all the difference between success and failure - hardly worth the argument.
Now - where were we?
with a risk of bending the main rotor shaft
I fly with one of those inflight "mast bending indicators" - never seen it move in flight!
25th Jan 2010, 18:21
There is really no way anyone can comment constructively on this subject as it is related to the profile and parasite drag of the airframe, aerofoil sections and its rotor system as it penetrates the relative airflow.
Whilst it might seem simple I suspect that different helicopters will produce very different results.
Might be better just to put a bit more fuel in next time!!!
25th Jan 2010, 19:34
Maybe that's why we used to tip the mast a bit forward on the old '47, and play around with the settings on the sync elevator until we extracted the most that we could in airspeed. after all, the standard load for a full time mustering pilot was not a big amount. An extra 2 to 3 knots was acheivable
Polishing the blades was an extra 2 to 3 knots. Then along came these flash strobex's, wow an extra 4 to 5 knots of smooth flying straight away.
After another light year then there was the really flash GPS, another saving of up to 2 to 3 minutes every hour. doesn't sound much but every day in areas where 1.5 to 3 hours positioning was involved with bloody difficult winds, last light and a cold swag was the common enemy.