NR 'rules of thumb'

[hovering]

I am looking to create a list of little NR rules...in relation to OAT, Temp, Alt, IAS, range, lift, fuel burn, etc.
Things like "reducing NR on a hot day, if Temp. limited, allows for more torque"

Thanks!

Hovering

[IHL]

OFF the TOP

Generally speaking a 1% reduction in Nr results in a 2% reduction in TR RPM - depending on type- which could affect the aircrafts ability to handle out of wind situations/ critical wind asimuth , i.e. LTE.

One would want to reduce NR for conditions of stuck power pedal.

Reducing NR and increaseing speed usually extends the RANGE of an Auto.

You want lots of NR at the bottom of an Auto to cushion the crash.

As a general rule:
"TOO much or TOO little NR will kill you."

[Gaseous]

In a manual throttle aircraft ( e.g. Enstrom) flying with Nr at the top of the green will use about 1 US gallon per hour more than flying at the bottom of the green for the same airspeed.

Watch out for recovery from low Nr by opening the throttle(if its not already open!). The extra torque may overpower the limited TR thrust. i.e pilot induced LTE. Safer if possible to lower the lever a bit first. This type of LTE is easy to (inadvertantly) demonstrate in an Enstrom and easy to recover if you are on the ball.

[VeeAny]

IHL

Forgive my ignorance but how can a 1% reduction in main rotor RPM result in a 2% reduction in TR RPM the ratio is fixed by the gearing but % changes affect each equally.

a 1% change in NR will result in a 1% change in TR RPM.

No argument that less TR thrust and thus reduced control margin but as percentages they will be the same.



V.

[MightyGem]

a 1% change at in NR will result in 1% in TR RPM
Otherwise the TR would be stationary with 50% Nr. :eek:

[Recuperator]

pilot induced LTE

Sorry Gaseous,

I have never heard of of that term "pilot induced LTE". I understand the aerodynamics of LTE or loss of tail rotor effectiveness and all other factors surrounding it.

I agree the tail rotor is less effective due to low Main Rotor Rpm, but surely this is not the same as LTE?

Would you be so kind as to explain your terminology?

Keen to learn something new, so fire away! (and sorry for hijacking the thread):\

[212man]

If you had 50% Nr the last thing to worry about would be the tail rotor control!;)

TR effectiveness varies as to the square of the speed, so at 90% Nr you have 81% of the thrust at 100% and at 80% you have 64% etc.

I guess pilot induced means induced by not controlling the Nr.

[Gaseous]

Recoup.

Pilot induced LTE is my terminology for this phenomenon which happens in Enstroms and possibly other aircraft with manual throttles.

1)pilot negligently lets Nr droop to say 250 RPM (75%, yes it will hover at 75%!!) by not controlling throttle while concentrating on hovering or whatever. (yes it does happen). The aircraft is still flying just fine and the TR thrust adequately balances the main rotor torque. The pilot may not have noticed his left boot is well forward to counter the low RPM.

2) Pilot scans dials and thinks Oh Sh**, opens the throttle wide and gives it full left pedal. The extra torque generated overpowers the reduced TR thrust despite the full left pedal and the aircraft begins to revolve. The tail rotor is no longer effective and the pilot has caused it by whacking open the throttle, hence my terminology.

3) Pilot either reduces power or stuffs it into into the ground.
The pilot says "LTE.... honest" to AAIB and his boss and probably believes it himself.
This happened a lot in the early days, often at low level so quite a lot of aircraft crashed and Enstrom got a reputation for a weak tail rotor.

Have I done it? Yes, when very low on Enstrom time after converting from Robbie. I landed going sideways but got away with it!

Enstrom put wider chord TR blades on to help reduce this happening.

At normal Nr the Enstrom tail rotor is fine - even the narrow chord ones, so keep it in the green and this won't happen.

edit to make it clearer.

[Jack Carson]

For turbine powered aircraft, Engine Shaft Horse Power (ESHP) = Torque (Q) X Rotor Speed (Nr) X a torque constant (Kq). As you reduce the rotor speed the torque will increase for the same ESHP. As an example, the H-53 Kq = 0.32. At 100% Q and 100% Nr, the H-53 is generating 3200 ESHP per engine. At 95% Nr and 100% Q the engines are generating 3040 ESHP each. Since we typically fly to a torque limit your are reducing the engine power available at the reduced rotor speeds.

In addition, as you reduce rotor speed the torque required generally increases while at the same time your anti torque capabilities are decreased (reduced tail rotor speed). Rule of thumb "Rotor speed is your friend":ok:

[hovering]

At what point, or in what atmospheric conditions does a decrease in RPM help? (if ever) Say you are climbing out of a hole, I mentioned a TOT limited helicopter decreasing RPM to lower TOT, and increase Torque, and climb out...does this actually help, or are you just increasing induced drag through the rotor, and not increasing lift at all? What about in cruise? I am under the impression that increasing RPM on a hot day at altitude can increase range...is this true?
I am flying a S76 at the moment, do these rules apply to all types, or do different rotor designs have different rules?

Hovering

[paco]

As for types, it can depend on the blades - for example, you can beep down a little on a 204/5, but the 212 has to be flown at 100% as the blades are asymmetrical

phil

[Dave Anderson]

Bell 204/205's have a green arc on the Nr/N2 gage that "allows' you to operate as low as 97%Nr for cruise. The UH-1H manual allows cruise at 97%, in fact all of the cruise charts are set up for 97%Nr (6400rpm, N2)cruise. In the UH-1H there is a chart that allows for a correction to 100% Nr. These birds were certified under the old Civil (CAA) rules which have been changed numerous times under the FAR system. The original reason for the 97% Nr cruise was for extending range, which amounted to about 4% advantage.
I had the experience of certifying a UH-1H under the newer FAR's and found a major problem with the aircraft response to sudden engine failure. I was at 97% Nr, 4000 ft Hd, 108KIAS(Vne), and 9400lbs.(9500 is maxGW). I chopped the throttle and was supposed to wait 1 second before reducing collective -- needless to say I didn't wait. The Nr dropped through the floor and I had to use the engine to keep it from getting below recovery Nr. During the recovery I bumped the mast, though slightly, and damaged the rotor mast dust boot.
Later, I was able to uncover two US Army test reports and one US Air Force test report that identified the same problem. Needless to say we certified our aircraft at 100%(6600rpm N2) and no lower. In fact with our installation of the PT6 there is no beeper switch for Nr.
I would never recommend flying a single engine helicopter in cruise at a lower permissible rotor rpm than its stated 100% because of the above. If you are in a high Hd, high GW, high speed condition and the engine quits, the recovery will be a monster and may not be possible. If you have to fly in that condition, "spring load" yourself for an engine failure.

[IHL]

Veeany:
Please accept my most humble and contrite apology. You are correct, what I meant to say was with a 1% reduction in NR you will experience a 2%-3% reduction in TR THRUST dependent on type.

Hi PACO , what does blade symmetry have to do with NR. The 76 has assemetrical blades and it can operate between 96-107.

[hovering]

Referring to NR in percentages (100%, 97-107%), are just lines in the sand, so to speak. It says nothing of tip speed, although I know this is published for each type. Symmetrical vs. Asymmetrical? Long blades with low RPM (214ST) vs. short with fast RPM(MD500)? Tip speeds are likely similar. Do they react similarly to variations in NR. There should be some graph of NR vs. Lift in a hover @ a specific density Alt. or in cruise flight. Lots of variables...engine limits complicate matters.

[Helibelly]

If you can see the blades individually as they go round, the Nr is probably too low.