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NR droop or not in OEI CTO

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NR droop or not in OEI CTO

Old 10th Oct 2020, 06:04
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Question NR droop or not in OEI CTO

Does anybody know which way has more lift in OEI CTO of FADEC (EEC) helicopter to droop NR at some extent or no droop at all?

The instructor I met in the past said to me that to droop NR to the minimum of OEI NR range if needed below Vtoss gives us more lift results in smaller height loss but after passing Vtoss to recover NR without droop gives us more lift.

I found the article on AOPA HP that the combination of best range speed and minimum continuous NR in autorotation gives us best range due to lift to drag ratio.

Is this true to OEI CTO until Vtoss mentioned above?
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Old 10th Oct 2020, 06:12
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You don't specify which helicopter you are talking about but generally, drooping Nr to achieve Vtoss in a OEI CTO is a standard procedure as per your RFM as is recovering Nr to normal after Vtoss to improve rate of climb..

It is also quite common to droop Nr in auto to get extended range - this works better on lighter singles than heavy twins.
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Old 10th Oct 2020, 09:25
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Probably one of the only scenarios in life where droop is a good thing.........
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Old 10th Oct 2020, 10:05
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Maybe this is a language thing.... But I feel a confusion between "droop" and "adjusting Nr".



To me, droop is something that happens when the engine cannot deliver the amount of power required, hence the RPM drops.
(this is something we regularly test on maintenance flights... In oei conditions, pull pitch until the rpm drops....).

Adjusting or lowering rpm to maintain a certain power setting is not "droop" as far as I understand it...

Correct?
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Old 10th Oct 2020, 10:58
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I don't think we are talking about beeping down Nr in either configuration, it is definitely droop.

In the OEI CTO the remaining engine is at max so raising the lever will decay Nr - iaw the RFM procedure.

In the autorotation scenario you are also drooping Nr by raising the lever such that the autorotative force cannot drive the rotors as fast.

In both cases the aim is to produce a change in the L/D ratio to gain more lift for a specific purpose.

Last edited by [email protected]; 11th Oct 2020 at 07:44.
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Old 10th Oct 2020, 12:00
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Crab has it: "the aim is to produce a change in the L/D ratio to gain more lift".
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Old 11th Oct 2020, 06:27
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The lack of understanding is largely due to poor explanation in the RFM. In the types on which I teach, you are supposed to droop Nr to continu the T/O "to min 90%"

The point is - drooping the Nr is the only way that you are extracting the full power from the remaining engine. If its not drooping, there is something left which you are not using.
The figure of 90% of course is a LIMIT, but lots of people, including many instructors see it as a TARGET
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Old 11th Oct 2020, 12:32
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Well, you are partially right in that if one droops the NR, the pilot has put the engine on whatever the limiter has installed, power-wise, but that isn’t the point Crab made, which is to set the collective at an NR providing the maximum lift to drag ratio for the rotor-meaning at that Nr, the pilot has achieved, balancing engine power and rotor lift efficiency, the maximum rotor lift.
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Old 12th Oct 2020, 04:00
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The type is Leonardo AW139, minimum cautionary of NR is 90% in OEI and 102% without any droop.
There is no specific NR described in RFM to get the maximum lift to drag ratio at certain speed.

I wonder which NR gives us max L/D?
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Old 12th Oct 2020, 05:53
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The 102% is set for Cat A/PC1 profiles to give you more energy to convert in the event of engine failure during the transition - drooping the NR to 90% extracts energy from the rotor to enhance lift so you can either cushion the touchdown if you abort or reach VToss if you continue.

The actual best L/D ratio will be somewhere between 102% and 90% but that is only part of the equation, the other is extracting kinetic energy from the rotor.

Just follow the RFM procedures, you don't have to worry about trying to calculate L/D ratios.
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Old 12th Oct 2020, 09:02
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My understanding is that setting the NR slightly high also takes into consideration that acceleration of the engine is not instantaneous. It’s advantageous to give the rotor energy “a little in hand” if one of the engines doesn’t perform as advertised, especially on a low inertia system.
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Old 12th Oct 2020, 09:35
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The next part of the procedure requires lowering the collective (once VTOSS and a rate of climb are achieved) to recover 102%. This follows fairly shortly after the initial droop and acceleration and results in an increased ROC. But if more lift/rotor thrust has already been achieved by drooping the RPM somewhere below 102%, why does lowering the collective again result in an increased ROC?
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Old 12th Oct 2020, 10:02
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Maybe because the best L/D ratio is at 100% Nr. At least that’s my understanding. It wouldn’t make sense to cruise at any other setting than the most efficient L/D setting.

The 102% setting gives the rotor system more inertia advantage than 100%. Having said that, it really wouldn’t be your day if the other donk stopped.
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Old 12th Oct 2020, 10:28
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Originally Posted by Torquetalk View Post
The next part of the procedure requires lowering the collective (once VTOSS and a rate of climb are achieved) to recover 102%. This follows fairly shortly after the initial droop and acceleration and results in an increased ROC. But if more lift/rotor thrust has already been achieved by drooping the RPM somewhere below 102%, why does lowering the collective again result in an increased ROC?
As mentioned already, the droop before Vtoss may not solely be for the best L/D ratio, however the Nr for the best L/D ratio may also change as airspeed increases, so that is also something to consider (as well as the fact you're sliding down the power required curve).

I agree with ShyTorque though that a margin for some droop in the acceleration from TDP to Vtoss allows for the short lag in power delivery when an engine suddenly is asked to spool up to 100% output and gives pilots some accepted margin to work with. That's something that was pointed out to me when first flying PC1 takeoffs, as well as the need to convert some rotational energy into added lift so that obstacles can be cleared (or the touchdown can be cushioned) while on the back end of the power required curve. I liken the second point to making mountain takeoffs in a piston; limited by full throttle, there were some sites where a little Nr droop (only between 2-3%) gives some extra lift between ground effect and translational lift so that treetops/obstacles could be cleared on the edge of the landing site.
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Old 12th Oct 2020, 10:35
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Makes sense Shy. Which fits with Crab‘s point about extracting kinetic energy to achieve the initial acceleration (an effect which presumably would only be a short-lived). Thus lowering their collective is putting the energy back and attaining best L/D ratio again. Ergo a counter-intuitive increase in ROC.

Crossed posts. Thanks ApolloHeli. Perhaps amend the above to “best L/D for a given speed“.

Last edited by Torquetalk; 12th Oct 2020 at 10:47.
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Old 12th Oct 2020, 11:47
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Originally Posted by Torquetalk View Post
..But if more lift/rotor thrust has already been achieved by drooping the RPM somewhere below 102%, why does lowering the collective again result in an increased ROC?
Because you're getting closer to Vbroc. i.e. the gap between power required and power available is closing, meaning you'll climb faster with your engine producing the same maximum power.
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Old 12th Oct 2020, 15:32
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Originally Posted by gulliBell View Post
Because you're getting closer to Vbroc. i.e. the gap between power required and power available is closing, meaning you'll climb faster with your engine producing the same maximum power.
Check gulliBell, iac with ApolloHeli‘s point about sliding down the power curve as the aircraft is accelerating from VTOSS to Vy.

Last edited by Torquetalk; 12th Oct 2020 at 15:43.
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Old 12th Oct 2020, 18:25
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FADEC's/DECU's are great. You only need to focus on 2 numbers, rotor speed and airspeed, all the while avoiding whacking anything. 30 second power with drooped rotor gets you to Vtoss, 2.5 minute power gets you to Vbroc, then with MCP selected just set the collective where the rotor just starts to droop from normal flight RRPM and accept whatever airspeed you get.
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Old 13th Oct 2020, 03:16
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Crab....Just follow the RFM procedures, you don't have to worry about trying to calculate L/D ratios.
gulliBell....FADEC's/DECU's are great. You only need to focus on 2 numbers, rotor speed and airspeed, all the while avoiding whacking anything. 30 second power with drooped rotor gets you to Vtoss, 2.5 minute power gets you to Vbroc, then with MCP selected just set the collective where the rotor just starts to droop from normal flight RRPM and accept whatever airspeed you get.
Crab and gulliBell nailed it. The OEMs have already done the performance optimization work as part of the Certification Process. They measure power required to fly at all airspeeds between Vtoss and Vy over a range of rotor speeds between about 90% and normal operating Nr, and build that data into their RFM computer performance models. FADECS with limiters are performance enhancers and workload reducers.
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Old 13th Oct 2020, 06:27
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I particularly liked the AS365 N3 which had the audio for low NR set at exactly where you needed to droop it to on an OEI event - that meant you could pull to the audio and then back off very slightly and you were at the optimum Nr for the landing or continued take off - no need to look inside for the Nr gauge.
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