Cruise speed and CG position

[Head Turner]

In the Rotorcraft Flight Manual that I am currently reading and in particular the 'Performance Section', I note that there is no mention of CG position on any graphs.
Does this mean that the performance will not vary if the laoding is between the forward and aft limits.
I get the gut feeling that there is a difference, and that with the loading close to the forward limit, cruise IAS is higher and when CG is close to aft limits the IAS is lower.

My question is:-

Does CG position influence cruise speed at a constant applied power setting?

[dangermouse]

The performance section would tend to show the WORST case, whatever that would be for a particular type.i.e the performance can always be met regardless of any other effects, obviousl;y external configuration can change this.

CG will effect the performace mainly due to drag effects on the fuselage due to changes in pitch angle with different CG, it can be significant.

In answer to your question the cruise speed will change for a given power with CG change due to the drag change mentioned above:ok:

[GLSNightPilot]

My experience is the opposite from yours - aft CG increases the airspeed, forward CG lowers it, because with most helicopters the fuselage is closer to level with an aft CG, thus less drag. It depends on the design to a large extent, though. A Bell rigged for offshore has so much drag already built-in that a small difference in fuselage attitude really isn't going to make a lot of difference. Slicker designs will show more difference, but in every type I've flown the difference is really minimal. You get more change in airspeed from differences in weight, power, temperature, etc, so it's difficult to quantify, but if I wanted to squeeze every tenth of a knot out of a helicopter, I'd try to load it right to the aft limit.

[Dynamic Component]

GLSNightPilot,

What you say does make sense, but flying anything with a starflex in aft CofG configuration and trying to go fast will almost garanteed crack your starflex sooner or later.

[John Eacott]

As with so many questions, the answer is: it depends!

A helicopter with a nose up, or near level fuselage attitude, in normal flight, will probably lose a few knots if the CG is shifted further aft. The cyclic will approach its forward limit, the disk will lose efficiency, and less thrust will be available for the forward component. The early A model A109's were usually typical of this, with a pronounced nose up attitude in flight. Some S76's are also affected by this, I've flown one that had to have the fwd float chambers filled with nearly 180kg of lead to get the CG within limits!

Alternatively, a helicopter with a nose down attitude in cruise would benefit aerodynamically by having the CG shifted aft, for all the reasons given above. Added to the physical results of the CG shift, there is also the downthrust developed by (most) horizontal stabilisers to be considered, since they become more efficient with airspeed. A nose down attitude increase the relative AoA of the horizontal stab, and therefore the pitching up moment created.

My brain hurts, I need some boffin to work it all out. Father Lappos, confession time............:rolleyes:

[NickLappos]

For most machines, aft is better, due to the fuselage angle of attack. Significant nose down is doubly damning, since the extra drag costs fuel, but the extra download on the fuselage looks like higher weight, too, so the main rotor torque is higher, reducing range and speed.

Aft CG also tends to reduce the horizontal tail down load, which reduces the main rotor thrust required, as well. At forward CG, the horizontal tail must generally produce a significant down load to balance the longitudinal axis. This download looks like weight, and forces the main rotor to work harder.

[GLSNightPilot]

Dynamic, in my (admittedly limited) experience, flying anything with a starflex in any CG configuration at all will almost guarantee a crack in the starflex sooner or later.

John, I've never flown anything that had a nose-up attitude in cruise, so I certainly can't argue with you about that. Most types I've flown have a distinct nose-down attitude.

[Head Turner]

Thank you all for your knowhow.

However, from a simple sketch of the fundamental heli design e.g.
comprising of a forward tilted rotor mast and a fixed aft horizontal stabilizer with an aeofoil section to provide increasingly more down force as speed increases. I deduce that with a neutral/central CG the work done by the horizontal stabilizer will be 'X'. With a fwd CG, ie with a tail high position the AA on the horizontal stabilizer will be greater than 'X' thereby increasing down force and hence weight, hence increase in torque. Conversely, an aft CG reduces the AA on the horizontal stabilizer and applying the same logic above reduces the force and therefore the weight, and therefore leads to a reduction in torque.
But this doesn't hold up in practice and therefore I would hazard at a guess that Fuselage paracite drag is the factor here. Each and every fuselage design will ultimately give a different answer.
The 109 is, from my experience quicker through the air with a fwd CG, unless you know otherwise!

[ShyTorque]

All PROPER helicopters have the rotor mast set tilted slightly forward so that the fuselage sits somewhere near level in the cruise. ;)

The two I fly (which have said attribute) both fly slightly slower with a more aft C of G, which results in a discernable difference in the airframe attitude. One of the two appears more affected than the other though.

[Shawn Coyle]

Related to cruise speed and CG position - which CG is going to put you closer to retreating blade stall?
I would think it would be the aft CG - the head needs to be tilted more forward with respect to the fuselage, resulting in more pitch on the aft-going blades.
Any other thoughts?

[GLSNightPilot]

Shawn, what is the relation between the tilt of the blades and the fuselage? I agree that the blades will be tilted more in relation to the fuselage with aft CG, but does that make a difference to the blades? Does the entire disk tilt more with respect to the horizon? And does blade angle of attack change with fuselage tilt?