RPM droop on Cat A procedures.
Thread Starter
Join Date: Jan 2013
Location: Planet earth
Posts: 9
Likes: 0
Received 0 Likes
on
0 Posts
RPM droop on Cat A procedures.
I've just paid my annual visit to Stavanger's Sk61 simulator, to revalidate my rating, training all those Cat A take off and landings. While studing cat a procedures, one question came into my head. Most helicopters that i've flown use similar Nr droop on these procedures, of about 91-94% Nr. These similar numbers, plus sentences as "use collective to droop Nr to a minimum of 91%" or " with light helicopter it can be necessary to increase colective to droop Nr" (similar sentences can be found on AW139 manuals), make me ask some questions about this RPM droop. Is this Nr number of about 91-94% a limit not to overstep, or is a value where blades, engines or whatever offers its maximum performance??; This Nr droop is something that happens from the OEI flight condition, but as conclusion of sentences before, is this Nr droop a value that we need to find, raising collective if Nr doesn't droop??.
So, if anyone knows anything about this, some technical data, aerodynamic information, please, i'd like to know how it works.
Thanks!!!
So, if anyone knows anything about this, some technical data, aerodynamic information, please, i'd like to know how it works.
Thanks!!!
In an EC135P2, the Nr rises 3% I believe during takeoff?
In Westland SeaKings, the Nr rises during lift to the hover.
Does the above, fly in the face of what you are discussing? Or, are you talking about SEF's??
In Westland SeaKings, the Nr rises during lift to the hover.
Does the above, fly in the face of what you are discussing? Or, are you talking about SEF's??
Avoid imitations
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 14,573
Received 422 Likes
on
222 Posts
I'm not an S-61 qualified pilot so I write the following generalisation from experience on a number of other twins; hopefully with some relevance! 
In the event of OEI, the Nr is drooped to that figure with collective to ensure you have pulled to maximum OEI power on the remaining engine (be it 30 second, 2 minute, or whatever power your type of engine is designed to give in that situation).
That sort of droop figure will bring the blades to their most efficient angle of attack at takeoff/ landing flight speeds.
If you droop lower the torque will begin to increase with no beneficial effect because you are going "behind the drag curve" on the main rotor.
Also, of course at the same time the tail rotor is slowing down too, which bring you closer to a LTE situation.
In the event of OEI, the Nr is drooped to that figure with collective to ensure you have pulled to maximum OEI power on the remaining engine (be it 30 second, 2 minute, or whatever power your type of engine is designed to give in that situation).
That sort of droop figure will bring the blades to their most efficient angle of attack at takeoff/ landing flight speeds.
If you droop lower the torque will begin to increase with no beneficial effect because you are going "behind the drag curve" on the main rotor.
Also, of course at the same time the tail rotor is slowing down too, which bring you closer to a LTE situation.
Thread Starter
Join Date: Jan 2013
Location: Planet earth
Posts: 9
Likes: 0
Received 0 Likes
on
0 Posts
Sorry Thomas, i was talking about SEF and RPM droop after engine failure, but didn't explain myself very well. What i'm trying to find out is more related to what Shytorque tells about blades efficiency.
Join Date: Oct 2012
Location: Canada
Age: 53
Posts: 215
Likes: 0
Received 0 Likes
on
0 Posts
What Shy wrote is spot on.
The target Nr is what the manufacturer has determined to be the compromise of most efficient with more limited protection factor. In training we teach to target that number to ensure that your reflexes will give you maximum performance, to be adjusted when you have time. If you aren't getting full droop, you aren't getting best performance. The same problem if you droop too much!
Be aware that different manufacturers use different philosophies. While an old 2 bladed Bell product generally uses a target figure of 91-ish% Nr, the Super Puma used 97% if I remember. During the conversion I was reverting to my old habits and the poor girl just would not perform!
The target Nr is what the manufacturer has determined to be the compromise of most efficient with more limited protection factor. In training we teach to target that number to ensure that your reflexes will give you maximum performance, to be adjusted when you have time. If you aren't getting full droop, you aren't getting best performance. The same problem if you droop too much!
Be aware that different manufacturers use different philosophies. While an old 2 bladed Bell product generally uses a target figure of 91-ish% Nr, the Super Puma used 97% if I remember. During the conversion I was reverting to my old habits and the poor girl just would not perform!
Drooping rotor rpm decreases drag from both main and tail rotor and give more available power for lift(pitch).
Due to OEI there is much less total power = the need is less for tail rotor power(and in Cat A profile you either desends for landing on or accelerates forward so LTE risk should be less).
If MGB/drive train is considered strong enough its allowed to use all available engine power. When engine power is less then power needed the rpm will droop. A 'clever' fuel control/ECU/FADEC will allow the rpm to reduce in OEI to give maximum power in the same time as drag is reduced to leave more power for lift.
Other MGB's/drive trains may have a limitation for input power on single engine that is less then the engines maximum power. In this case the rpm will not drop(or only a very little droop due to fuel control caracteristics).
@pilot and apprentice: I fly the Super Puma, our rotor speed gauge in rpm. Normalt flight gives 268-270 rpm( 265 is considered 'nominal'), and min rpm in one engine flight is 245. At least in the simlutator(AS532) a rpm of 245 give maximum power and lift. I calculate 245 from 265 as 92%, so close the the other helos.
Made a check flight yesterday, and in this cold climate/low level we needed to pull very much power to get a noticable droop att all. The droop came much later and a bit qiucker than in the sim.
Due to OEI there is much less total power = the need is less for tail rotor power(and in Cat A profile you either desends for landing on or accelerates forward so LTE risk should be less).
If MGB/drive train is considered strong enough its allowed to use all available engine power. When engine power is less then power needed the rpm will droop. A 'clever' fuel control/ECU/FADEC will allow the rpm to reduce in OEI to give maximum power in the same time as drag is reduced to leave more power for lift.
Other MGB's/drive trains may have a limitation for input power on single engine that is less then the engines maximum power. In this case the rpm will not drop(or only a very little droop due to fuel control caracteristics).
@pilot and apprentice: I fly the Super Puma, our rotor speed gauge in rpm. Normalt flight gives 268-270 rpm( 265 is considered 'nominal'), and min rpm in one engine flight is 245. At least in the simlutator(AS532) a rpm of 245 give maximum power and lift. I calculate 245 from 265 as 92%, so close the the other helos.
Made a check flight yesterday, and in this cold climate/low level we needed to pull very much power to get a noticable droop att all. The droop came much later and a bit qiucker than in the sim.
Last edited by AAKEE; 22nd Jan 2013 at 22:30.
The reason for drooping the Nr on the S61/Sea King is to utilise the extra energy in the rotor system to help achieve a safe single engine safety speed - once this is passed, the Nr is allowed to rise again (usually giving a climb because 100% is where the rotor is more efficient.
We utilise the same technique for both flyaways (from the hover or forward flight) and for single engine water take-offs.
Unlike some smaller singles, the Nr is not more efficient at 91% and that is why we don't droop it in auto to gain extra range - we use 104% for range auto.
The Nr rises with application of power in the SK but has to be manually set in the S61 - the reasons are to maintain the rotor efficiency and prevent static droop taking Nr below 100% since it gives you more NR to play with following an engine failure and prevents coning (if the rotor slows) reducing the effective size of your disc.
We utilise the same technique for both flyaways (from the hover or forward flight) and for single engine water take-offs.
Unlike some smaller singles, the Nr is not more efficient at 91% and that is why we don't droop it in auto to gain extra range - we use 104% for range auto.
The Nr rises with application of power in the SK but has to be manually set in the S61 - the reasons are to maintain the rotor efficiency and prevent static droop taking Nr below 100% since it gives you more NR to play with following an engine failure and prevents coning (if the rotor slows) reducing the effective size of your disc.
Firstly, I am not an S61 qualified pilot, secondly, what follows does not pertain to the S61, so apologies if it contradicts those S61 percularities.
We need to discuss some issues raised above. "Efficiency" of the rotor system in these conditions should be related to lift v drag ratios, not other factors mentioned above. Drooping and increasing RRPM are merely ways of playing with the lift/drag ratios using the influence of V squared in the lift and drag formula.
Each helicopter will have a most efficient lift/drag rotor speed and angle of attack and the idea of every modern twin is to have that most efficient speed (NR in this case) occur immediately after an OEI event.
The other issue is that at the ideal lft/drag ratio, any further degradation in speed ( droop) will not have a linear effect, it has almost an exponential one. So if you are flying the rotor at exactly the best NR for max lift V drag and you accidentally have to pull a little more, you will be making drag increase at a vastly higher rate than lift increases. I other words, if you are already topping out the good engine when this occurs, it cannot overcome the rapid increase in drag and thus droop occurs. Quickly. As droop occurs, you will sink a bit more as there is now less V squared. As you sink, you apply more pitch and the cycle gets worse quickly.
Accordingly, it is smart modern practice to have the acft operate a little above the NR speed that would represent max efficiency for Lift v Drag. That way if you make a small correction you will droop initially to a better efficiency, not droop to an exponentially worsening one. Obviously if you keep pulling because the initial increase in efficiency did not get you out of jail, then you are back to the case of being susceptible to the exponential increase in drag. But at least you had an initial increase to help you fix your problem and stay away from the ground. If you automatically go to the best ratio, then from that point there is no margin for error and any slight error you do make will result in a quickly worsening situation.
B205s (UH-1H Hueys) for example fly generally at 100% which equates to 6600 RRPM, but the Australian Defence Force used to beep down to 6400 NR RRPM for limited power operations because the rotor is much more efficient in lift v drag at that speed. They had several significant (and fatal) accidents using this technique as any slight error requiring them to apply more collective at a critical point induced a further and accelerating rotor bleed if they were committed to having to retain pitch.
They altered the procedure to ban 6400 RRPM after some 20 years and retained 6600 for limited power operations. As a result, any slight error would result in a bleed toward 6400 but as the rotor was becoming more efficient with the bleed, lift was increasing a lot too, and the pilot could correct the error. At least initially - which is all you need. The result was no limited power accidents recorded since the change ( more than 10 years). It was not the only factor that changed, but I hope you can appreciate the point.
As a GENERAL rule, 100% NR is a little bit more than the modern twin needs for max lift drag efficiency thus providing us with a slight NR buffer to cope with our less than perfect reaction to an OEI situation. Well at least some of us who are not perfect anyway
Some twins even require NR to be increased above 100% for further buffer on efficiency such as the EC135, 145 and even the B412EP. That way if you have an OEI it goes back to about 100% NR which is about most efficient without you having to look inside and try beeping up initially.
Anyway - generalisations will always fail to explain specifics, so I know there will be a ton of replies detailing particular helicopters that behave very differently to the physics laws generalised above, but I hope I have give you a general explanation. And we have not touched upon the human factor part of this buffer and the "reasonable pilot" 2 second reaction for the certification of The OEI procedures.
Finally, the droop has NOTHING to do with LTE. There is a wealth of other threads dealing with this and it would just hijack this thread. LTA (running out of power pedal authority) - Yes. LTE - No.
We need to discuss some issues raised above. "Efficiency" of the rotor system in these conditions should be related to lift v drag ratios, not other factors mentioned above. Drooping and increasing RRPM are merely ways of playing with the lift/drag ratios using the influence of V squared in the lift and drag formula.
Each helicopter will have a most efficient lift/drag rotor speed and angle of attack and the idea of every modern twin is to have that most efficient speed (NR in this case) occur immediately after an OEI event.
The other issue is that at the ideal lft/drag ratio, any further degradation in speed ( droop) will not have a linear effect, it has almost an exponential one. So if you are flying the rotor at exactly the best NR for max lift V drag and you accidentally have to pull a little more, you will be making drag increase at a vastly higher rate than lift increases. I other words, if you are already topping out the good engine when this occurs, it cannot overcome the rapid increase in drag and thus droop occurs. Quickly. As droop occurs, you will sink a bit more as there is now less V squared. As you sink, you apply more pitch and the cycle gets worse quickly.
Accordingly, it is smart modern practice to have the acft operate a little above the NR speed that would represent max efficiency for Lift v Drag. That way if you make a small correction you will droop initially to a better efficiency, not droop to an exponentially worsening one. Obviously if you keep pulling because the initial increase in efficiency did not get you out of jail, then you are back to the case of being susceptible to the exponential increase in drag. But at least you had an initial increase to help you fix your problem and stay away from the ground. If you automatically go to the best ratio, then from that point there is no margin for error and any slight error you do make will result in a quickly worsening situation.
B205s (UH-1H Hueys) for example fly generally at 100% which equates to 6600 RRPM, but the Australian Defence Force used to beep down to 6400 NR RRPM for limited power operations because the rotor is much more efficient in lift v drag at that speed. They had several significant (and fatal) accidents using this technique as any slight error requiring them to apply more collective at a critical point induced a further and accelerating rotor bleed if they were committed to having to retain pitch.
They altered the procedure to ban 6400 RRPM after some 20 years and retained 6600 for limited power operations. As a result, any slight error would result in a bleed toward 6400 but as the rotor was becoming more efficient with the bleed, lift was increasing a lot too, and the pilot could correct the error. At least initially - which is all you need. The result was no limited power accidents recorded since the change ( more than 10 years). It was not the only factor that changed, but I hope you can appreciate the point.
As a GENERAL rule, 100% NR is a little bit more than the modern twin needs for max lift drag efficiency thus providing us with a slight NR buffer to cope with our less than perfect reaction to an OEI situation. Well at least some of us who are not perfect anyway
Some twins even require NR to be increased above 100% for further buffer on efficiency such as the EC135, 145 and even the B412EP. That way if you have an OEI it goes back to about 100% NR which is about most efficient without you having to look inside and try beeping up initially.
Anyway - generalisations will always fail to explain specifics, so I know there will be a ton of replies detailing particular helicopters that behave very differently to the physics laws generalised above, but I hope I have give you a general explanation. And we have not touched upon the human factor part of this buffer and the "reasonable pilot" 2 second reaction for the certification of The OEI procedures.
Finally, the droop has NOTHING to do with LTE. There is a wealth of other threads dealing with this and it would just hijack this thread. LTA (running out of power pedal authority) - Yes. LTE - No.
Thread Starter
Join Date: Jan 2013
Location: Planet earth
Posts: 9
Likes: 0
Received 0 Likes
on
0 Posts
Helmet Fire, what you tell can work with S61 quite well. In fact, you need to find 91-94% Nr, which, as you explained, would be the more efficient rpm number, but Nr can fall in this model to 83% before achiving Vtoss. So you are notpulling the limit with recomended 91% number.
While you're playing with the collective trying to find that magic Nr number, don't forget to look out the window and fly the aircraft. The benefits of drooping are minimal anyway and would only be a concern if you had immediate obstacles to clear.
Join Date: Mar 2010
Location: North America
Posts: 107
Likes: 0
Received 0 Likes
on
0 Posts
Assuming that the prescribed AEO rotor speed for a given helicopter model is 100%, some manufacturers have determined that (at Vtoss and Vy) better OEI climb performance is achieved at a slightly reduced Nr (say 95%). This likely occurs because the rotor power required is reduced, while the engine continues to produce virtually constant horsepower within about +/- 5% of its design operating RPM.
For an OEI continued takeoff following an engine failure, I believe the “most significant benefit” of drooping the rotor below the reference NR (100%) is that it assures that the engine is producing the maximum available power at the associated OEI limit (TGT, NG, Q), because the fuel control is working to increase the NR back to the 100% reference value. This is also a pilot workload reducer since the pilot only has to monitor Nr to be assured that the engine is producing limit power.
For an OEI continued takeoff following an engine failure, I believe the “most significant benefit” of drooping the rotor below the reference NR (100%) is that it assures that the engine is producing the maximum available power at the associated OEI limit (TGT, NG, Q), because the fuel control is working to increase the NR back to the 100% reference value. This is also a pilot workload reducer since the pilot only has to monitor Nr to be assured that the engine is producing limit power.
It is not all about rotor efficiency on the Sea King/ S61 , when conducting a flyaway with one engine in manual control (and therefore limited) and the 'good' engine pulled back, we decay the Nr to 91% to utilise the energy in the rotor to maximise acceleration - in the same way that the energy in the rotor is used to cushion an EOL.
Once above SSE speed, we lower the lever to recover the Nr to 100% and the aircraft climbs because the rotor is becoming more efficient.
Once above SSE speed, we lower the lever to recover the Nr to 100% and the aircraft climbs because the rotor is becoming more efficient.
Join Date: Mar 2010
Location: North America
Posts: 107
Likes: 0
Received 0 Likes
on
0 Posts
Crab,
I understand the process of extracting rotor energy by rapidly increasing collective pitch. The more rapid the NR droop, the greater the power extraction. Would you please explain at what point in the flyaway maneuver and over what time interval (seconds) the power extraction occurs. Am I correct in assuming that the collective pitch is initially lowered at the time of engine failure to contain the NR, then rapidly increased to droop the NR to 91% at some point thereafter when you are trading altitude for airspeed?
HT
I understand the process of extracting rotor energy by rapidly increasing collective pitch. The more rapid the NR droop, the greater the power extraction. Would you please explain at what point in the flyaway maneuver and over what time interval (seconds) the power extraction occurs. Am I correct in assuming that the collective pitch is initially lowered at the time of engine failure to contain the NR, then rapidly increased to droop the NR to 91% at some point thereafter when you are trading altitude for airspeed?
HT
I am also interested in the cushion technique you are talking about crab. How do you know where to stop the rapid application of collective to ensure only 91%? Is it a muscle memory thing to go to a certain position and then adjust slowly from there?
SirK, one of the benefits of the RRPM buffer at AEO is that you don't have to look inside too much to ensure efficient rotor and max power from the remaining engine in an OEI event.
SirK, one of the benefits of the RRPM buffer at AEO is that you don't have to look inside too much to ensure efficient rotor and max power from the remaining engine in an OEI event.
HT and HF - we usually practise the flyaway from a normal transition, usually from 10' using about 5 degrees nose down and hover Tq plus 15%. This means the failure is given at about 40 to 50' and with around 20 kts IAS.
As soon as the failure is initiated the lever is lowered - but the Nr is allowed to decay - whilst the co pilot calls Nr, until 91% is reached and the pilot adjusts the attitude to about 2 degrees nose down (we are practising the technique for an overwater hover at 50' so there is not much height to play with). As 45 Kts is reached (usually with the aircraft between 5 and 15' agl) the lever is lowered to recover the Nr to 100% and the aircraft starts to climb. We then have a choice between continuing to accelerate to 65 kts or climb at 45 kts.
Lowering the lever to climb is counter-intuitive but it works.
When we do a single engine water take off - we advance the Nr to max (108%) and pull to droop the Nr to 91% to unstick the aircraft from the water. As soon as the aircraft is approaching SSE speed, the lever is lowered again to recover Nr as previously mentioned.
They are 2 different ways of using the rotor's energy to assist the flyaway with the first being more gradual than the second.
As soon as the failure is initiated the lever is lowered - but the Nr is allowed to decay - whilst the co pilot calls Nr, until 91% is reached and the pilot adjusts the attitude to about 2 degrees nose down (we are practising the technique for an overwater hover at 50' so there is not much height to play with). As 45 Kts is reached (usually with the aircraft between 5 and 15' agl) the lever is lowered to recover the Nr to 100% and the aircraft starts to climb. We then have a choice between continuing to accelerate to 65 kts or climb at 45 kts.
Lowering the lever to climb is counter-intuitive but it works.
When we do a single engine water take off - we advance the Nr to max (108%) and pull to droop the Nr to 91% to unstick the aircraft from the water. As soon as the aircraft is approaching SSE speed, the lever is lowered again to recover Nr as previously mentioned.
They are 2 different ways of using the rotor's energy to assist the flyaway with the first being more gradual than the second.
Join Date: Apr 2007
Location: Europe
Posts: 4
Likes: 0
Received 0 Likes
on
0 Posts
The anvil
Somepeople say that for going for a walk in the savannah you have to bring with you an anvil. This is because if a lion appears, you can release the anvil and you will be able to run faster.
When I got my glider lincence many years ago, we were told to plan the glide path from down wind to touch down point with the airbrakes halfway out. If you were too short, put them in. If you were too long, put them full out.
So, I think the issue is to operate normaly a little bit penalized in the ratio lift/drag (100%- 102% Nr), but when the things go wrong, in case of a loss of power, you get in a better situation, at least in the lift/drag ratio. This should be around 91% Nr. These are the rpm's that gives you the maximun lift with the minimun drag, because is just what you need in that moment.
This is what I think because I cant prove it.
When I got my glider lincence many years ago, we were told to plan the glide path from down wind to touch down point with the airbrakes halfway out. If you were too short, put them in. If you were too long, put them full out.
So, I think the issue is to operate normaly a little bit penalized in the ratio lift/drag (100%- 102% Nr), but when the things go wrong, in case of a loss of power, you get in a better situation, at least in the lift/drag ratio. This should be around 91% Nr. These are the rpm's that gives you the maximun lift with the minimun drag, because is just what you need in that moment.
This is what I think because I cant prove it.
Join Date: Nov 2005
Location: Saudi Arabia
Posts: 1
Likes: 0
Received 0 Likes
on
0 Posts
RPM droop on Cat A procedures.
The most important part of the issue is to ensure that you have all the available engine power being used. The only way to ensure this is to droop the rotor from the AEO setting. If you are using 102% or 100%, whatever, if the rotor is still at that RPM, you aren't using all available power. The range provided is optimum for aircraft on the heavier end of the scale. Comparing AEO hover & takeoff power settings to OEI continuous & 2.5 minute ranges will give you an idea, prior to takeoff, what you can expect if an engine fails during the takeoff. If you are using less than max continuous OEI power for your takeoff, you will likely have to increase power to achieve some droop.
Yes, but you only have to have a little droop to confirm you are at max available - you don't HAVE to droop over 10% to prove that.