Beeping/Switching Up MR Rotor RPM for Takeoff

[BlenderPilot]

I was having a conversation yesterday with a friend who flies an EC135P2 offshore, and this pilot was telling me that on some instances where they are very heavy and it get's diffcult to get out of the platform, what they will do is use the Cat. A switch that the EC135 has to bump RPM to 103%, and that this will give them an extra power margin which will allow them to clear the platform with more margin, they tell me performance improvement is VERY noticable. (they know it's not supposed to be used that way, since it's not a Cat. A takeoff, but it works)

Why does it give the them more "power" for takeoff? My friend tells me that although the FLI (first limit indictator) is being used, under those circumstances the limiting parameter is Torque. But doesn't having to turn the rotor even faster require even more torque?

Or is the rotor just more efficient at 103%? and if so why can you only use it for Cat. A operations?

Also the A109E has a similar switch that bumps RPM from 100 to 102%, you are supposed to use this for TO and LDG, but what this will do is when you go to 102% it will give you a momentary rise in TOT of about 30 degrees, then it will stabilze to a permanent 10-15 degree rise in TOT while using 102%.

I don't appreciate having to use 102% on hot days where it's very common for us to be able to tightly clear obstacles due to TOT limits. And you don't really see any performance improvement from it, and it "steals" margin from your TOT scale! (although using 102% gives you a little more TR authority, especially needed whey you fly an A109E with the new composite blades I have to admit)

Then I don't understand why on the A109E the TOT rise is permanent? I understand why you would have a transient rise of TOT as the engines work to go from 100 to 102%, but then if you maintained the same flight condition you would need less pitch for the same hover altitude say, the TOT would decrease again. Unless the added drag from increased RPM caused the permanent TOT increase?

As I see no definte impovement in performance for the A109 at 102% I am guessing they just have that there in case one engine fails it gives more time for the working engine to come to play before the RPM drops too much??

I am trying to outweigh and understand the benefits and drawbacks of changing the RPM for takeoff as they seem to be contradictory, of course I mean theoretically since the POH specifies the way they will be used, but still.

Any thoughts, opininons, experiences??

[Phoinix]

I'm not sure why the guys shouldn't use the "High NR" switch on EC135 as they should use it everytime the TOW is more than 2850kg for Cat.A. I'm talking about EC135P2+. You didn't specify the version but if you are talking about CatA switch it must be a P/T2.

The higher rpm gives a little bit more lift at same collective pitch, so you don't get as much torque as you would at max torque normal rotor speed (more collective pitch). I've written a simple equation in my TR book, but I don't have it with me at the moment.

[delta3]

When a rotor is at high angles of attack its efficiency decreases (lift/drag ratio is the same way as in a plane wing has an optimum angle at a certain speed)

Increasing RPM improves the lift/drag ratio because thanks to the higher foil speed we can lower angles of attack to produce the same lift and make the wing more efficient.

In helicopter:
If we lower collective at higher RPM we will have the same lift with less power (and because of higher RPM with even less torque).
If we maintain the collective both power and lift will go up.

Same is true when disk is lightly loaded (low TOW), decreasing RPM will increase efficiency.

There are however several catches, on of which is that RPM influence vibration levels.

d3

[212man]

The fundamental reason they are doing it is that they are torque limited - this being an MGB constraint, not an engine limit (per se)

Power = K x Nr x Tq

(where K is a constant )

so, for any given power output from the engines - a function of Ng and TOT - a higher Nr will result in a lower Tq. Therefore, if you pull to the torque limit you will actually be obtaing more engine power.

If you are TOT or Ng limited this works against you.

Any aerodynamic variations between the Nr values are secondary to the power/torque relationship.

[Shawn Coyle]

The temporary bump in TOT is due to the engines having to run the RPM up to the new values.
Once the new, higher RPM is established, the TOT will return to close to the original value.
As 212man stated, the value of this is that power is torque times RPM. 100% torque at 100% RPM will equal a certain horsepower, where 102% RPM AT 100% torque will be 2% more power.
Power is needed to overcome the drag of the blades to provide the lift you need. Higher RPM means slightly less angle of attack is needed to produce the same lift than a lower RPM, so the drag will be slightly less than trying to produce the same lift at the lower RPM, and you will magically rise into the air...

[212man]

Power is needed to overcome the drag of the blades to provide the lift you need. Higher RPM means slightly less angle of attack is needed to produce the same lift than a lower RPM, so the drag will be slightly less than trying to produce the same lift at the lower RPM, and you will magically rise into the air..

Of course, that is correct but, with respect, I would argue that the fundamental gain in the practical scenario described -of trying to get off a deck at high AUM - is down to the increased torque margin available, and the greater power going into the rotor once the MGB torque limits are reached.

[Non-PC Plod]

The reference to A109 seems to be missing the point of the Nr switch. The increase in Nr is not to improve performance with 2 engines running, its to give you a fighting chance if one engine fails. Thats why it is part of the Cat A procedures, which are designed to give you a safety margin in this eventuality.
If you are operating in an environment where the 2% Nr change is encroaching on your ability to stay inside the TOT limits, then you are almost certainly way outside the WAT limits for Cat A anyway. In this case, you are taking an informed risk assessment about operating Cat B and the possibility of an engine failure at a critical moment. The chances are, something will get broken if the failure happens at the worst moment, so its up to you to decide if the 102% Nr or the extra Tq/TOT is going to leave you best placed for your circumstances. Its not anywhere in the flight manual that you can only use 102% Nr if you are operating Cat A.
As for the EC135, I am not familiar with the newer variants, but I guess the switch is there for an identical purpose. Just because its designed for Cat A, doesnt mean you cant use it if you are Cat B!

[RVDT]

Power = K x Nr x Tq

(where K is a constant )

so, for any given power output from the engines - a function of Ng and TOT - a higher Nr will result in a lower Tq. Therefore, if you pull to the torque limit you will actually be obtaining more engine power.Where K would be the VEMD needle (given that you are torque limited). You would achieve more power for the same indicated power. Another issue on the 135 is that the RRPM varies with altitude and is only constant above ~ 8000' or with the Cat A or more recently High Nr switched on. + or i models have "High Nr". The difference over the Cat A switch is it is connected to the ADC and will reduce/increase Nr dependent on IAS. Similar to VARTOMS on the 145 which is dealing with analogue engines.

Also keep in mind that the higher Nr in itself and if unintentionally used in the cruise phase, plays havoc with all sorts of other issues. It goes some way to reducing design finite component lives.

Unless the added drag from increased RPM caused the permanent TOT increase?Drag in the inlet? V squared? The 109E nearly always has a TOT split as one engine breathes harder than the other. You can regain some of the margin using the TOT matching switch.

Power is needed to overcome the drag of the blades to provide the lift you need. Higher RPM means slightly less angle of attack is needed to produce the same lift than a lower RPM, so the drag will be slightly less than trying to produce the same lift at the lower RPM, and you will magically rise into the air..My information would say the opposite. A better L/D is achieved with lower RPM. If power is limited (i.e. N1 or Ng at max) you will achieve more lift at a lower RPM. Easily proven by the difference in range with a lower RRPM in autorotation or talking to a UH1/205 pilot. The higher RRPM normally used is for RBS margin and TR thrust margin. Be very wary of the TR thrust margin. DO NOT try this at home if the procedure is not in the RFM.

[Shawn Coyle]

Be careful about inferring performance truths from the UH-1 series. They have a very high tip mach number that causes problems (particularly in cold weather), and may have had some RPM issues that were not strictly blade-related. For example, inertia may have played a part in choosing the hover RPM. (We see this in the increased RPM for Category A often).
And as for better performance at lower RPM in autorotation - that doesn't translate to hover performance. There are a whole different set of issues for performance in autorotation than in the hover.

[NickLappos]

The question of how much benifit you get at higher rpm in the hover/takeoff regime is complex, and I implore you to not think that what works in one model helo will benifit in another.

These are turbine helo generalizations (hats off to RVDT, who gets it right):
Generally, hover performance (weight for power) gets worse at higher rpm because the drag of the blades increases with rpm and tip mach number. If you are up against the transmission limit, the extra rpm gives you more power for the same torque, but the rotor probably eats some or most of it with that drag increase, leaving no great improvement in lift. Remember, the rotor blade area is sized for high speed cruise, and the blades have far too much lift for perfectly efficient hover, so the blades should be slowed down for optimum angle of attack in hover.

But the higher rotor rpm gives you more transient power during the droop in an engine failure (energy stored in the rotor helps you), and the extra rpm gives you much more tail rotor authority, so many helos beep up for take off because they anticipate the engine failure, and tune the flight situation for the reject.

Follow the flight manual for your type, at all times, and ask the factory pilots (a phone call is always accepted in the TP offices I have worked in!)

[victor papa]

That is why on the EC130 and B3+ the rotor rpm is determined by the DECU which selects a nominal N2 according to the tail rotor/fenestron pedal position. I can not remember the exact range, but you do not have a set rotor rpm but a rotor rpm range which is a range of +/-15 rpm (or close to it). Of course this also assist with noise on especially the 130, fuel consumption, performance, etc

[BlenderPilot]

I really appreciate all of you very interesting responses, I am going over them carefully and sorting out what really happens, will play a little with the 100-102% in hover today and let you know what happens.

[DeltaFree]

The extra Nr will give a handy increase in rotor energy in the event of an engine failure, a benefit to give a little extra umph as the Nr decreases post engine failure, this is the main reason for its use in Cat A ops. If you are pushing limits on two engines you sure as hell aint going to get that performance if one fails.
The main lift advantage when heavy is due to more power being available at higher Nr, if tq is the limit. That is true. All this about rotors being more or less efficient is mute. In the hover compressibilty at the tips should not be too big a problem as this would then be hugely magnified on the advancing blade in the cruise. As with fixed wing there will be a best L/D angle of attack, but lets not forget this is not pitch. For a constant hover we need the same amount of airflow down through the blades, when Nr is slowed the Relative AirFlow has a smaller horizontal component, so the RAF becomes steeper and pushes the lift vector back so increasing drag for a given lift. In reverse higher Nr reduces lift induced drag.
Now in forward flight the vertical component of RAF is reduced because you are flying into clean air and the effect is less noticeable, but the form drag (the fixed portion of your drag coefft) is reduced by the square of the airspeed (Nr) and has a greater effect on overall power, and since most machines, when OEI, will be engine limited, not tq, a lower Nr is better during the flyaway phase.

[skadi]

The extra Nr will give a handy increase in rotor energy in the event of an engine failure, a benefit to give a little extra umph as the Nr decreases post engine failure, this is the main reason for its use in Cat A ops.


Thats exactly the point. It gives an extra amount of time to react before the RRPM drops too low. I heard, thats a part of certification process and deals with the average time of reaction of about 1.7 seconds. Thats what I´ve heard from a ECD Pilot.

skadi

[Jack Carson]

DeltaFree, you are correct. For the EC135T2+ or P2+. and the increase in rotor speed has little to do with rotor efficiency. The increase in rotor speed provides for 3% more ESHP. Using factory specifications the following computation is provided:

From the EC-135 Factory specification the MGB Duel engine max. cont. torque limit and corresponding ESHP is:

• Q = 69% ESHP = 380

Solve the equation for Kq = 0.055075. This can now be used to compute the ESHP for all torque / rotor speed combinations. Since we fly to a torque limit one can see the advantage to having the rotor speed increased at higher weights.

Takeoff at 100% rotor speed = 78 X 0.05507 X 100 = 429.5 ESHP per engine. The total takeoff power available for use is 859 ESHP. The EC135 power available is flat rated up to more than 5000 Hp.

Takeoff at 103% rotor speed = 78 X 0.05507 X 103 = 442.5 ESHP per engine. Increased rotor speed means more lift and power available for use at the same indicated torque.

OEI Max power at 100% rotor speed = 128 X 0.05507 X 103 = 704 ESHP. At 103% rotor speed the engine would be producing 726 ESHP.

Once the OAT increases above 30° C the EC135 T2+ becomes TOT or N1 limited. At this point there would be little if any performance advantage to operating at the higher rotor speed. There is still a very signifcant advantage realizied during takeoffs and landings realized should one suffer a single engine failure. Increased rotor speed equated to and increased rotor inertia. Rotor Inertia = I X Ω squared. 3% increase in rotor speed is 6% increase in inertia.:8

[Phoinix]

Pulling up the discussion, I was looking at JAR-27 and I couldn't find any relevant information about "HIGH NR". Or should I say, where can I read about the intention of raising RPM. It is logical to me, that rpm increase gives you a fighting chance in the event of critical powerplant failure, but its hard to have a forum debate as a reference.

Any literature, standards?


Another thing; is there an "AMC" to JAR-27 like in JAR-FCL, that would require standards like 135's 110 kts to autorotation safe speed of 90 kts in 1.7 seconds? I'm thinking this standard should contain increased rotor speed...

[Shawn Coyle]

Phoinix:
The advisory circulars for Part 27 and 29 might give some advice on this, but then, they might not talk about this. (been a while since I trolled through them).
High RPM for Cat A procedures is commonly used to provide larger margin of safety when an engine fails - it takes longer to decay the rotor RPM, with all attendant benefits.

[Geoffersincornwall]

I agree with Shawn's assertion. It was explained to me that on the 139 the optimum performance is obtained from 100% Nr but during Cat A certification the test pilot must allow a two-second reaction time before intervening with the appropriate procedure (reject of Cont TO). To compensate for the additional droop caused by this 2 second wait the Nr is raised to 102%. Arguments about performance gains in this context may be interesting but not strictly relevant.

No mention of using 102% Nr in any other context than Cat A, Sling Load or Hoisting is made in the 139 RFM but I would exercise caution in using the philosophy that the absence of prohibition implies acceptability.

As always, if you wish to interpret the RFM in a particular way then you would be wise to seek a 'no objection' letter from the OEM. However you look at it, if you survive the crash and have to defend your actions in a court of law then not doing what the RFM says makes your position just a little weaker. As there is no mention of 102% Nr in any other context than those mentioned I would read it that 102% is not intended for other applications. If you disagree then by all means write to the factory and seek clarification.

G.

[Fareastdriver]

On the Bristol Sycamore we used the 'Jump Take Off' technique. On this aircraft the rotor control was manual and it was powered by a supercharged radial piston engine.

IIRC the powered on rotor limits were 200-240 Rrpm with a boost limit of 46'MAP

One would sit on the ground and carefully accelerated the rotor to 245 rpm. When steady one would pull the collective rapidly towards its upper limit at the same time opening the throttle to maximum boost. The aircraft would 'jump' into the air and as soon as one was airborne one would go into forward flight milking the collective to maintain at least 200 rpm until transitional lift.

Then you would fly away normally.

Easy.

[army_av8r]

I think it has already been said, but i use this technique when im taking off from an "off airport" confined area or when forced to land/hover with high tailwinds. of course, only when im limited by a transmission limit. the higher Nr results in a lower collective pitch setting to hover, less coning due to centrifugal force which provides slightly more lift, higher tail rotor authority, more RPM in case of power failure, more collective in case of power failure. all for a small rise in compressor RPM and slightly higher Temps. if you look at the charts for blades, they are typically much more efficient at low angles. they generally resemble a parabolic curve. where efficiency drops drastically at high angles. cant comment on the legalities for every situation, but it works for me, and falls within allowable rotor limits for my aircraft!

[A.Agincourt]

A long time ago we used to 'beep up' simply to increase tail rotor efficiency therefore using less pedal and reducing power bleed from the main rotor via transmission system. Used for both take off and landing - but there again that was a long time ago.

[Geoffersincornwall]

Owner of Dauphin C equipped with tricycle undercarriage orders skid kit to achieve a lighter airframe. The manual explains everything the guy needs to do but he cannot find the page in the manual that tells how to remove the tailwheel mounted at the base of the fenestron fairing.

Assuming this is a mistake, he uses his 'common sense' and takes the thing off. Some flying hours later the aircraft crashes with fenestron failure. The accident investigator finds that the tailwheel is used to tune the fin structure to avoid resonance.

QED

If you want to do something that is not in the manual then it is wise to check first.

'He applied his common sense' is not what you want to have on your headstone.

G.

[spinwing]

Mmmmm ...

Geoffers .... with respect

.... there would have been nothing in that aircrafts 'Flight Manual' to do with the removing of the tailwheel ... THAT would have been a maintenance/Engineering issue and I suspect that the maintenance manual said nothing about NOT REMOVING the assy. either.

I can see what point you are trying to make ... BUT as you know aviation safety has by and large been improved by the 'generating of f**kups' that people have then discussed and found the answers to ....

And as far as the law (in aviation) is concerned whilst you can be just as easily damned if you don't ... you can also be damned if you do ....

OR just possibly praised for NOT doing exactly by the book ... as Capt. Chesley Sullenberger discovered ...

[Aser]

Geoff
Regarding the 102% in the 139:

IN FLIGHT PROCEDURES
Hoist Operating Procedure
1. RPM switch — Set 102%.

Regards
Aser

[Geoffersincornwall]

Spin - You are absolutely right in the same way that the difference between a medal and a court-martial is sometimes nothing at all, it is the outcome that is important. Notice that in my example I was talking about a 'headstone'. This rather implies that it didn't work out. My message was meant to convey the caution required when treating the absence of prohibition as tacit permission. If in doubt - check.

Aser - Yes I believe that in an earlier post I mentioned that 102 should be used for Cat A, sling ops and hoisting.

G.

[ATN]

To feed the discussion:

EC 155
Use of RPM HIGH is limited to CAT A helipad or increased slope operations for IAS < Vy. (FLM section 2.4, 1.3).

AS 365 N and N2
CAT A clear area, OEI at take off after TDP, beep the rotor to 325 Nr, in the
2nd segment for the N, the 1st segment for the N2.

EC 225
OEI at take off after TDP, adjust collective pitch as necessary to maintain NR at approximately 96 % (FLM section 3.1, 6.1 page 8).

Cheers

ATN

[Aser]

Sorry Geoff I misread your previous post.
You made a very good point about treating the absence of prohibition as tacit permission.
We have been talking about the use of 102% in cat B, when to arm floats, the use of tq limiter etc. For years...
The question is, Is a normal procedure in the flight manual a limitation?