Main Rotor RPM

[Peter-RB]

I have looked in many place and not found the answer to this question:-

Could a helicopter(any helicopter) benefit by having a higher RPM on its main rotor(plus ratio'd up for TR), or is the RPM controlled by Blade/head/design strength, I have a feeling that one or two answers may well point to things that we PPLs dont really know about, but if a blade cord was increased slightly, shortened the blade in length with the relative alterations to final drive gearing and bearings would this not allow a greater safety margin in lighter helicopters, with better flight/manouver responce .

This question has partly been egged on by the comments on the series called Dangerous flights(The ferrying of A/C around the world) the people there were given a twin F/W that had more powerful engines and different props(from original design), so if it works on a FW would it on Heli's.

PeterR-B

[RINKER]

Peter I am by no means an expert on this but apart from centrifugal load changes, perhaps reduced by shortening the blades I think this would have an adverse on head inertia and hence autorotaion characteristics.

R

[ShyTorque]

A decrease in aspect ratio = a less efficient blade.

Think of a glider: Long, slender wings. Disturbs a lot of air a little bit, doesn't need much "power" to stay aloft.

Compare the wings to the lower aspect ones of a heavy military jet fighter bomber. The latter needs a long take off and very large amounts of power to fly. Weight for weight, a shorter wing which disturbs little air, but disturbs it a lot. Sinks like a brick when the engine(s) stops.

[Gemini Twin]

Short answer, yes. The AS350B3e has such a feature. Long answer, blade RPM is established during initial design and based a peformance capabilities of the helicopter. Many factors must be considered including VNE, number of blades, IGE and OGE peformance goals.

[timprice]

Some helicopters I have flown have a higher RRPM for take off, I think the main reason is to give a greater margin of safety (ie Inertia) during this critical point in flight.
Also other helicopters I have flown if you reduce the RRPM in the cruise the blades/engine's become more efficient.

[Sloppy Link]

Main limitation as I understand it is the rotor tip will approach Mach 1 and the attendant issues with compressibility, effect on following blade etc

[topendtorque]

the series called Dangerous flights

If that is anything like "Worlds Most Dangerous Roads" or "Top Gear" then you should first up view it through only a quite jaundiced eye.

[Peter-RB]

Thank you for the answers, it now gives me somthing to delv into, to try and expand my further understanding,

Re "Dangerous Flights".. I think the title is to attract an audience to that show, but its mainly based on a couple of US pilots who seem very well versed on all aspects of commercial flying, transporting by flying any type of Aircraft to anywhere in the world, as far as I have seen its really only the marginal fuel left after such flights that seems to be the Risk they are prepared/happy to take, plus the fact that it is highly over dramitised for the TV film.

Peter R-B

[Thomas coupling]

Hi Pete - how's life in Yorkshire, sorry Lancs:oh:
I can sense the armchair experts out there desparate to chip in with their years of knowledge and experience - keen to overload you with science, mirrors and smoke.

K.I.S.S.: If rotor blades were reduced in length to stop them bumping into confined areas, they would have to spin much faster to maintain their lift capability.
This then imposes serious strain on the bearings/hinges/materials due to centripetal / centrifugal forces. In addition, the tip of the advancing blade starts working its way towards compressibility etc etc.
Increasing the chord length increases the lift vector which increases the loading on the disc.
Smaller blades also make more noise than larger ones and Mr ICAO doesn't like noise for downtown commuter helicopters so he puts a noise limit on them. [Ever noticed a 109 coming? You can hear it in the next county!]

You still fly the Robbo: Measure the blade length, look at its nominal Nr and work out how fast your tip is going (2 x Pi x r). See how close it is to the speed of sound when it's going at full chatter @ say 100kts still air.

Fun and games.

From here on it gets murky:
Chord twist.
modern materials.
wing fences
winglets
berp
anhydral

yawn yawn..................:}

[Peter-RB]

Hi TC,
As they would say in Yorkist County, " I arnt yawning" in fact asking this question has made me realise my simple question has opened a huge area of my knowledge that is lacking in deeper understanding of the way the Rotor helps the heli to fly, I had not given thought to the increase in centifugal forces that possibly would do for the likes of the R22, by the way, it is many years since I flew one of those following my near demise, and I should have picked up the answer to my question from that example of overloaded Rotor bearings.

But I am still grateful to all for the answers to my question!

One other thing TC, please dont get me mixed up with any of the Yorkists, I will buy you a drink without the need to be prompted.! ;)
Peter R-B