R44 Max Glide Configuration

[mausermolt]

I had a student ask me yesterday why Robinson's Max glide configuration says to set the RPM at 90%. Never been told why the 90% and 90 knot combo produce the max glide distance.

I get the 90 knots but why not 102% RPM? Any inside intel on this subject?

[Mikemv]

Mauser, the helicopter will descend at a slower rate at 90% Rotor RPM and thus have more time to travel at 90kts. and cover more distance.

 

The faster the MR RPM = the higher the rate of descent

the slower the MR RPM = the lower the rate of descent

 

(the above within limits of course)

 

Mike

[mausermolt]

Mauser, the helicopter will descend at a slower rate at 90% Rotor RPM and thus have more time to travel at 90kts. and cover more distance.

 

The faster the MR RPM = the higher the rate of descent

the slower the MR RPM = the lower the rate of descent

 

(the above within limits of course)

 

Mike

 

but WHY does the lower RPM create a slower decent rate? if the MR is spinning slower wouldn't it be producing less lift therefore descend faster?

[Lindsey]

Perhaps because of the increased pitch in the blades to keep rpm down to 90%, therefore decreasing rate of descent.

[Pohi]

but WHY does the lower RPM create a slower decent rate? if the MR is spinning slower wouldn't it be producing less lift therefore descend faster?

 

Like Lindsey was saying, when the pilot is raising collective the reason the RPM is slowing down is because the main rotor blades are trying to produce lift. The whole Newton's Law thing, where the equal and opposite reaction thing happens.

 

With the collective down, the air is traveling through the rotor and nothing is trying to slow it down very much, so the rotor blades spin really quickly and there is a fast decent rate of the helicopter. When there is pitch applied to the rotor system, the upward energy is being used by the rotor blades to produce lift, which slows down the rotor system and also slows down the rate of descent of the helicopter.

 

I'm sure the engineering weenies at Robinson did quite a few calculations to figure out the best numbers that took into consideration all of the different forces of lift and drag, along with a nice fudge factor for the pilot when they came up with the numbers.

[Mikemv]

Mausermolt, are you CFI-RH?

 

PM sent for clarifying info!

Edited September 28, 2011 by Mikemv

[kodoz]

Good topic...something I didn't realize for a long time. My instructors were too concerned about maintaining rRPMs that we never discussed what is going on in the max glide configuration. I'm sure there are several others here who learned something from this thread.

 

The way I'd put the pieces together is that as collective is raised, lift is increased. With the increase in lift, there is also an increase in drag, hence the lower rRPM.

[brettjeepski]

"I'm sure the engineering weenies at Robinson did quite a few calculations to figure out the best numbers"

ha ha I found this hilarous!

[Mikemv]

To All,

 

I sent Mauser a PM with some clarifying info so I thought that I would also share it here for you.

 

Without getting into the 3 elements that control the MR RPM stabilization point covered in my C&E Sems, let me just give you an easy way to perceive this.

 

We know that the air rushing up through the rotor system is what is driving the rotor in autorotation. Agreed?

 

But the air is not rushing up but rather the helicopter falling down! So falling due to gravity(descending) a smaller volume of air (lower rate of descent) will drive the rotor slower than a higher volume (higher rate of descent).

 

Think about doing 180 autos and how when you turn the rotor RPM increases and the rate of descent INCREASES.

 

The aerodynamic discussion can be so much more than what is stated above but this gives your students an easy way to perceive/understand/remember it.

 

To take things further. Now from a stabilized autorotational state (constant RPM and ROD) increasing collective does apply pitch, increase lift and drag and slow the rotor RPM and ROD.

 

Most lower inertia rotor systems are so FLYABLE to stretch the glide by reducing the rotor RPM and descending slower over a given time frame at the same airspeed. The B407 I flew was a great system to work with in autos!

 

Note to follow training curriculums and guidelines and do not now go out and reduce the rotor RPM on all practice autos. Follow your schools maneuvers guide for training!

 

Fly Safe,

 

Mike

[RagMan]

Thanks for sharing that info with us, Mike!

[iChris]

I had a student ask me yesterday why Robinson's Max glide configuration says to set the RPM at 90%. Never been told why the 90% and 90 knot combo produce the max glide distance.

I get the 90 knots but why not 102% RPM? Any inside intel on this subject?

 

Each rotor system has an optimum rotor rpm for each flight condition, hovering, turns, pull-ups, level flight, high-speed flight, low-speed flight, autorotation, etc. At optimum rpm, most blade elements are at the best angle of attack for maximum local L/D ratio. That point of best efficiency is related the optimum blade-load coefficient for maximum rotor thrust for any given flight condition.

 

You have to wonder how they could get maximum performance under all these flight conditions with such a narrow range of power on rpm. They must compromise.

 

As a reference, most rotors have their peak efficiency in forward flight when the blade-load coefficient is .08. This coefficient is based on rotor blade area, radius, normal rpm, gross weight, and air density. However, many helicopters operate at a higher set rpm under normal conditions, which relate to lower angles of attack as a compromise for better overall performance across the board.

 

Raising the collective thereby lowering the rpm until the rotor is operating around five degrees of average blade element angle of attack can often yield increase rotor efficiency during an autorotation at low gross weight and altitude. This is all known from calculation and test data.

 

Rotor rpm values are set in the design and the pilot has little control other than what's given in the RFM. Take note from the figure that lowering rpm to stretch glide may have different results (taking into account conditions and aircraft design). Similar curves could be drawn for the R44.

 

 

Edited September 30, 2011 by iChris

[IHX]

Drag is proportional to velocity squared. Lower rotor rpm means less aerodynamic drag on the blades thus less frictional energy "bleeds" from the rotor system at 90%. As iChris points out if rpm gets too low then the blades will be too far off their optimal L/D ratio. More pitch is needed to compensate for lower rpm = higher induced drag on the blades. Plus if rpm is allowed to drop too far (below 80%) the ship stops flying. They told us this frequently at the RHC safety course

Obviously RHC have found from testing that 90/90 is the most efficient glide.

[amphibpilot]

I had a student ask me yesterday why Robinson's Max glide configuration says to set the RPM at 90%.... Any inside intel on this subject?

 

Ray Prouty does his usual fine job explaining "Stretching the Glide" in Chapter 13 of his 1985 edition of Helicopter Aerodynamics [hopefully it's also in the worthwhile 3-volume 2007 reprint of the same name on Amazon].