Green band in Autos

[TheFlyingSquirrel]

During Autos, I understand range is best achieved with the RRPM at the bottom of the green and endurance at the top. I've got all of the books out but i'm still not getting it. Can the experts enlighten me as to why this is the case? Thanks.

[mfriskel]

Are you sure the min rate of descent and also the max glide distance aren't both achieved with the lower RPMs?

[overpitched]

The more collective (pitch) you use the slower your rate of descent, so if you want best range (distance) or best endurance (time in the air) your rotor rpm should be at the bottom of the green. What would vary would be your airspeed. eg.

Bell 206

Minimum rate of descent 52 kias.

Maximum glide distance 69 kias.

[ConwayB]

Now I'm not an instructor, and my explanation may not be text book or 100% correct, but I'll throw in my two cent's worth. (I hope it's close to being right, though). But Overpitched is correct.

In autorotation, the key is keeping the RRPM in the 'green range' because it is at this speed that lift is produced most efficiently and that's what you want to do; keep the rotor producing as much lift as possible to keep you in the air (endurance) or to get you travelling across the ground fast (range). In either case, RRPM is key to ROD and/or range.

By pulling in pitch, you increase AoA and the driving region of a rotor blade moves further outboard (and that's OK) but it also causes the stalled region of a blade closer to the blade root (where Angle of Incidence is highest due to wash out) to increase and the point of equilibrium to move outboard because the resultant angle of attack also increases to the point where more of the blade stalls. (Clear as mud, so far?)
RESULT: Pull in pitch > more of the blade stalls > less driving force and more drag force > RRPM slows down to outside green arc > less lift.

By letting RRPM increase too much (say in a turn during a 180 auto), then the opposite occurs whereby the driving region moves inboard and the stalled region decreases which sounds good, but the DRIVEN region also increases and this region doesn't produce useful lift.
RESULT: Keep pitch the same > increase RRPM because of turn > driving region moves inboard > stalled region decreases > driven region increases > RRPM increases to outside of green arc > less effective lift.

Most effective speed is usually in the bottom of the drag curve (VminD) which for a Chinook is about 80 knots. This is the speed where the rotors have the least work to do to overcome the various forces of drag... and so lift is the most effective. This is great for keeping ROD to a minimum.

Most effective speed to cover distance is 20 knots faster at 100 knots. ROD is a little higher because of the attitude of the aircraft and the resultant change of airflow through the rotor disk but the aircraft will be able to travel further for the least increase in ROD.

In the Schweizer (now Sikorsky) 300, the VminD is 55 knots so that's the speed you're looking for in an auto.

So short answer: Keep the RRPM in the green arc and sweat on your attitude and thus your IAS for minimum ROD or maximum range.

Anyone who thinks I've got it wrong, please feel free to correct me and help me see the error of my ways.

Hope this helps.

[ThomasTheTankEngine]

Basicly the slower the rotor turns in auto the slower your ROD will be, Watch out there is a minimum & maximum limit (R22 90 - 110 RRPM, This doesn't cover the green ark on the RRPM, So RRPM at the bottom of the green won't give you your maximum range in this situation)

The faster you fly the more distance you will cover in auto, Once again there is a optimal airspeed that will give you a distance covered V ROD.

I teach 3 types of autorotation in the R22
Normal Auto 65 Knots IAS / RRPM in the green

Range Auto 75 Knots IAS / RRPM in the green (This will make you glide a little further but your on the ground a little quicker)

Max Range Auto 75 Knots IAS / RRPM 90% (This will allow you to glide further than the range auto as it reduce's the ROD)

One thing I would say is if you have a choice, A landing area closer to the helicopter which you can manouvre into is better than a landing area further away which require's one of the range auto's. (Just incase you can't make it)

[TiPwEiGhT]

The R22's rotor dics is most efficient at 90% RRPM, hence that it is used for range autorotations. Lower airspeeds in autorotation (min RoD - 53kts & constant attitude 40kts) with low RRPM will make life harder for you in converting airspeed into RRPM if you need to.

TiP



*edited for being a pleb and not being able to spell*