RPM droop on Cat A procedures.
When operating Category A, there should not be a need to top the engine as the flyways are designed to work within the engine power limits. They are what determines the weight.
As I said, I am not S61 qualified, but I have not yet flown an acft that Category A operations are requiring the rotor droop - all the twins I have flown predicate the Category A weights on 2.5 min OEI engine power, so it is good to learn about the s61 technique crab describes.
Drooping the RRPM on those twins that I have flown is generally a result of either a mishandling of the procedure, OR operating above Category A weights for the given situation (weight, power, p.a., temp, humidity, wind, HLS size, etc).
Category A requires a reasonable pilot to do the operations safely, and the reasonable pilot has a 2 second reaction to lower the collective. This will mean a bit of droop for the reasonable pilot which will ensure that the rotor is becoming more efficient (initially) and the engine is beginning to give you all it has. But if you then fly the published procedure, you should be assured of a safe landing/flyaway without any significant droop - if you are operating within the Cat A weights.
I think what is being talked about above is the situation where you are operating above Cat A and have an OEI event. How far do you pull?
The answer for me has always been
Pull to the 2.5 min limit.
If that is not working, pull to the bleed, maintain just on the bleed
If that is not working, maintain just on the bleed (say 95 to 100) to ensure max engine power and max rotor efficiency until you have to save the acft, then cushion on with what ever you want to pull to save the acft.
But none of that should be necessary with Category A which is what the thread is titled. No twins I have flown bleed for Category A, all do for weights above.
As I said, I am not S61 qualified, but I have not yet flown an acft that Category A operations are requiring the rotor droop - all the twins I have flown predicate the Category A weights on 2.5 min OEI engine power, so it is good to learn about the s61 technique crab describes.
Drooping the RRPM on those twins that I have flown is generally a result of either a mishandling of the procedure, OR operating above Category A weights for the given situation (weight, power, p.a., temp, humidity, wind, HLS size, etc).
Category A requires a reasonable pilot to do the operations safely, and the reasonable pilot has a 2 second reaction to lower the collective. This will mean a bit of droop for the reasonable pilot which will ensure that the rotor is becoming more efficient (initially) and the engine is beginning to give you all it has. But if you then fly the published procedure, you should be assured of a safe landing/flyaway without any significant droop - if you are operating within the Cat A weights.
I think what is being talked about above is the situation where you are operating above Cat A and have an OEI event. How far do you pull?
The answer for me has always been
Pull to the 2.5 min limit.
If that is not working, pull to the bleed, maintain just on the bleed
If that is not working, maintain just on the bleed (say 95 to 100) to ensure max engine power and max rotor efficiency until you have to save the acft, then cushion on with what ever you want to pull to save the acft.
But none of that should be necessary with Category A which is what the thread is titled. No twins I have flown bleed for Category A, all do for weights above.
Join Date: Mar 2010
Location: North America
Posts: 107
Likes: 0
Received 0 Likes
on
0 Posts
HF,
FYI, the S-61N FAA Flight Manual Supplement 15 says to do the following during Category A takeoff with an engine failure occurring after the CDP:
Hover at 10 ft. Move engine control levers fully forward to produce at least 103% Nr but not to exceed 106% Nr with matched torques. Rotate not more than 20 degrees nose down and accelerate forward at constant height using up to twin engine takeoff power. As an airspeed indication is achieved, rotate to a 30 kt climbout attitude and readjust collective to attain takeoff power. If an engine fails after the 70 ft height CDP, rotate to not more than 20 degrees nose down and accelerate to Vtoss. During rotation allow Nr to droop to at least 94%, but not less than 91% Nr. Apply collective and cyclic as required to attain a level attitude with 100% Nr and 2-1/2 minute power at Vtoss.
HT
FYI, the S-61N FAA Flight Manual Supplement 15 says to do the following during Category A takeoff with an engine failure occurring after the CDP:
Hover at 10 ft. Move engine control levers fully forward to produce at least 103% Nr but not to exceed 106% Nr with matched torques. Rotate not more than 20 degrees nose down and accelerate forward at constant height using up to twin engine takeoff power. As an airspeed indication is achieved, rotate to a 30 kt climbout attitude and readjust collective to attain takeoff power. If an engine fails after the 70 ft height CDP, rotate to not more than 20 degrees nose down and accelerate to Vtoss. During rotation allow Nr to droop to at least 94%, but not less than 91% Nr. Apply collective and cyclic as required to attain a level attitude with 100% Nr and 2-1/2 minute power at Vtoss.
HT
Interesting. Another difference is that it climbs out at 30kias, and not Vtoss, only going to Vtoss with an OEI are CDP. Again as a generalisation, the twins I have flown (and they are limited in number ) generally climb out at Vtoss.
Is that to achieve high climb angles for the S61? Or another reason? Is Vtoss a variable speed in the S61, or is it fixed? Or is it about 30?
Is that to achieve high climb angles for the S61? Or another reason? Is Vtoss a variable speed in the S61, or is it fixed? Or is it about 30?
Join Date: Mar 2010
Location: North America
Posts: 107
Likes: 0
Received 0 Likes
on
0 Posts
HF and AnFI,
The S-61N takeoff safety speed (Vtoss) is fixed at 57 kt. The best rate of climb speed (Vbroc) is 67 kt at sea level. The takeoff CDP is 30 kt/70 ft.
During a normal takeoff (with no engine failure) at the CDP, the pilot applies cyclic to rotate the helicoter not more than 20 degrees nose down and accelerates to Vbroc and continues the climb to desired altitude.
Sorry, I omitted the last part of the normal takeoff in my previous post, and that obviously caused confusion.
HT
The S-61N takeoff safety speed (Vtoss) is fixed at 57 kt. The best rate of climb speed (Vbroc) is 67 kt at sea level. The takeoff CDP is 30 kt/70 ft.
During a normal takeoff (with no engine failure) at the CDP, the pilot applies cyclic to rotate the helicoter not more than 20 degrees nose down and accelerates to Vbroc and continues the climb to desired altitude.
Sorry, I omitted the last part of the normal takeoff in my previous post, and that obviously caused confusion.
HT
Thanks, that makes more sense. Not so different, only the bleed as the power limiter, not using a TQ or N1 or Temp as do most of the machines I have flown. again, that is a limited number of types!
I alsomade an error in my call about climbing out at Vtoss, I mean't Vy, sorry.
I alsomade an error in my call about climbing out at Vtoss, I mean't Vy, sorry.
Join Date: Aug 2001
Location: Cornwall
Age: 75
Posts: 1,307
Likes: 0
Received 0 Likes
on
0 Posts
Vtoss and Vy
Vtoss is the Take Off Safety Speed and is the slowest speed at which a rate of climb of 100 ft/min can be maintained and is applicable between TDP and 200 feet ATS (Above the Take Off Surface)
Vy is Best Rate of Climb speed and is applicable for the climb segment between 200' and 1000' ATS during which the min RoC is 150'/min.
This climb profile is set out in the Cat A certification and is an artificial template against which all certified aircraft are measured. Sometime you may need to climb to MSA at Vtoss if you are in an extremely obstacle-rich environment like a city full of sky-scrapers - and cranes!
G
Vy is Best Rate of Climb speed and is applicable for the climb segment between 200' and 1000' ATS during which the min RoC is 150'/min.
This climb profile is set out in the Cat A certification and is an artificial template against which all certified aircraft are measured. Sometime you may need to climb to MSA at Vtoss if you are in an extremely obstacle-rich environment like a city full of sky-scrapers - and cranes!
G
Last edited by Geoffersincornwall; 30th Jan 2013 at 10:49. Reason: addiions
Join Date: Dec 2001
Location: Philadelphia PA
Age: 73
Posts: 1,835
Likes: 0
Received 2 Likes
on
2 Posts
Vx is impossible to define for helicopters - sometimes it might be zero, other days it's closer to Vy, but the airspeed used needs to be something that is repeatable and usable (i.e. can be flown using the airspeed indicator...)
Join Date: Dec 2008
Location: N/A
Posts: 845
Likes: 0
Received 0 Likes
on
0 Posts
Shawn you don't really mean that do you?
IMPOSSIBLE is a little strong. Would it be particularly hard to chart - enter the chart at the weight - move horizontally to the wind value - draw tangent - read airspeed.
It's not going to be perfect - but it might give a pilot more of a clue as to whether his V'best bet' is nearer zero or near Vy.
Vtoss is not often Vbb it's just a crude approximation which has some other desirable attributes (like being simpler to explain!)
Could save a lot of pointless effort accelerating to eg 57kts when infact that may be a MUCH poorer climb angle than for example; vertical at Vbb. In a 30kt breeze, it would be perfectly easy to judge a vertical climb (for example)
why "NEEDS to be"? Pilots can judge some things - like best angle of climb. It would be theoretically VERY easy to judge a vertical climb by eye, but it would be VERY hard to predefine an Indicated Airspeed that could achieve that well.
Anyhow there are 'other factors'...
Vx is impossible to define for helicopters
It's not going to be perfect - but it might give a pilot more of a clue as to whether his V'best bet' is nearer zero or near Vy.
Vtoss is not often Vbb it's just a crude approximation which has some other desirable attributes (like being simpler to explain!)
Could save a lot of pointless effort accelerating to eg 57kts when infact that may be a MUCH poorer climb angle than for example; vertical at Vbb. In a 30kt breeze, it would be perfectly easy to judge a vertical climb (for example)
the airspeed used needs to be something that is repeatable and usable (i.e. can be flown using the airspeed indicator...)
Anyhow there are 'other factors'...
Avoid imitations
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 14,576
Received 433 Likes
on
228 Posts
I think Shawn is totally correct, simply because the term "just cuff it" has never appeared in a Class A performance chart. 
Vtoss couldn't be simpler to explain; it is the lowest speed at which, with one engine inoperative, the aircraft is able to climb. When providing the first segment climb to 200ft, the manufacturer will stipulate the speed required (it may be the speed used for Vtoss which, unlike Vy, will change with the mass of the helicopter).
The best angle of climb is achieved with zero speed and for that reason, it is rarely if ever provided. As Shawn has said, any speed used in the CAT A procedures must be achievable using the airspeed indicator.
We have been waiting for the definitive explanation to the question posed at the start of this thread; I thought HeliTester was going to provide it but he/she didn't so I might hazard a guess:
When the helicopter reaches TDP, unless it is being flown in an aeroplane type runway manoeuvre (i.e. a level acceleration until V1 is reached), it will require an attitude change. If the engine fails at (or just before) TDP the inertia that is stored in the rotor - due to the beeped-up speed - is used to achieve the attitude change without too much height loss. As soon at the resulting (stable) attitude has been achieved the drooped rotor is permitted to recover to the most efficient setting, and the climb commenced.
For helidecks and for elevated heliports, the rotation-and-droop should result in deck-edge miss.
Jim
The best angle of climb is achieved with zero speed and for that reason, it is rarely if ever provided. As Shawn has said, any speed used in the CAT A procedures must be achievable using the airspeed indicator.
We have been waiting for the definitive explanation to the question posed at the start of this thread; I thought HeliTester was going to provide it but he/she didn't so I might hazard a guess:
When the helicopter reaches TDP, unless it is being flown in an aeroplane type runway manoeuvre (i.e. a level acceleration until V1 is reached), it will require an attitude change. If the engine fails at (or just before) TDP the inertia that is stored in the rotor - due to the beeped-up speed - is used to achieve the attitude change without too much height loss. As soon at the resulting (stable) attitude has been achieved the drooped rotor is permitted to recover to the most efficient setting, and the climb commenced.
For helidecks and for elevated heliports, the rotation-and-droop should result in deck-edge miss.
Jim
Join Date: Mar 2010
Location: North America
Posts: 107
Likes: 0
Received 0 Likes
on
0 Posts
I’m not sure what JimL wanted me to say, but Geoffers already wrote the following which defines the OEI climb performance minima established by FAR/JAR-29 when taking off at Category A weights.
OEMs establish Vtoss and Vy to meet the above criteria by comparing the available engine power with the measured power required to fly at all airspeeds between 0 kts and Vmax. The rotor droops and attitude changes associated with transitioning from AEO flight to OEI flight are established to optimize performance during that dynamic period, and vary from model to model. It has been my experience that they are established empirically (rather than analytically as suggested by several other posters), and for any given model can be different for different takeoff profiles (airfield vs. confined area).
Koamill started this thread by talking about S-61 Category A takeoff maneuvers, which I presume are the certified airfield maneuvers (I don’t think the S-61 has a certified Category A confined area takeoff procedure at this time). The S-61 Category A airfield takeoff does not use a level acceleration to V1 (TDP) like most modern helicopters. Instead it climbs to the 70 ft TDP at 30 kt and because of that, the attitude changes after engine failure at TDP are similar to the dynamic nature of the confined area maneuvers that JimL describes.
Vtoss is the Take Off Safety Speed and is the slowest speed at which a rate of climb of 100 ft/min can be maintained and is applicable between TDP and 200 feet ATS (Above the Take Off Surface)
Vy is Best Rate of Climb speed and is applicable for the climb segment between 200' and 1000' ATS during which the min RoC is 150'/min.
This climb profile is set out in the Cat A certification and is an artificial template against which all certified aircraft are measured.
Vy is Best Rate of Climb speed and is applicable for the climb segment between 200' and 1000' ATS during which the min RoC is 150'/min.
This climb profile is set out in the Cat A certification and is an artificial template against which all certified aircraft are measured.
Koamill started this thread by talking about S-61 Category A takeoff maneuvers, which I presume are the certified airfield maneuvers (I don’t think the S-61 has a certified Category A confined area takeoff procedure at this time). The S-61 Category A airfield takeoff does not use a level acceleration to V1 (TDP) like most modern helicopters. Instead it climbs to the 70 ft TDP at 30 kt and because of that, the attitude changes after engine failure at TDP are similar to the dynamic nature of the confined area maneuvers that JimL describes.
Thread Starter
Join Date: Jan 2013
Location: Planet earth
Posts: 9
Likes: 0
Received 0 Likes
on
0 Posts
Well all this questions came to my head because of sentences as "with light helo, it would be necessary to raise coll to droop Nr" on the S 61 OEI helideck takeoff, but as Polaris said, you can find similar sentences on the AW 139; this one, "Continue the acceleration using collective to droop NR to minimum of 90% and to set 2.5min power", is from 139 clear area takeoff, with engine failure at/after TDP. It seems that in both model Nr needes to be drooped to get beter performances... And that, i think, is the main question, what improve in performance you get drooping Nr.
Join Date: Mar 2010
Location: North America
Posts: 107
Likes: 0
Received 0 Likes
on
0 Posts
Koamill writes:
I think there is not a simple answer to your question, because I believe that the optimum Nr for OEI climb is a function of Weight, altitude, temperature, and airspeed. I’m not aware of any Flight Manual performance charts that show OEI ROC as a function of Nr (albeit I haven’t seen the Flight Manuals for all the different models). If a manufacturer determines that reducing the normal AEO Nr produces better OEI climb performance overall, then the OEI climb performance charts will be published for the reduced Nr. When flying Category A (Performance Class 1) procedures, the pilot is not at liberty to deviate from the Certified procedure. Presumably the manufacturer has already experimented with different Nr’s and published Flight Manual performance data and OEI emergency procedures accordingly.
It seems that in both model Nr needs to be drooped to get better performances... And that, I think, is the main question, what improve in performance you get drooping Nr.
I think there is not a simple answer to your question, because I believe that the optimum Nr for OEI climb is a function of Weight, altitude, temperature, and airspeed. I’m not aware of any Flight Manual performance charts that show OEI ROC as a function of Nr (albeit I haven’t seen the Flight Manuals for all the different models). If a manufacturer determines that reducing the normal AEO Nr produces better OEI climb performance overall, then the OEI climb performance charts will be published for the reduced Nr. When flying Category A (Performance Class 1) procedures, the pilot is not at liberty to deviate from the Certified procedure. Presumably the manufacturer has already experimented with different Nr’s and published Flight Manual performance data and OEI emergency procedures accordingly.
Join Date: Jul 2007
Location: New Zealand
Posts: 14
Likes: 0
Received 0 Likes
on
0 Posts
Re Original Question
Gents (and any Ladies hiding out there)
Re Vtoss and Vy, Geoffers has it spot on.
Re the original question (why droop NR during OEI flyaway?), Crab is spot on. The aim of the published Cat-A flyaway procedure is to achieve Vtoss as quickly as possible, so that a safe climb gradient can be achieved. Drooping NR to (or at least towards) the RFM limit will provide the most efficient acceleration to Vtoss without inducing a significant ROD.
Re the debate about rotor efficiency at reduced NR, I defer to Shawn Coyle!
Re Vtoss and Vy, Geoffers has it spot on.
Re the original question (why droop NR during OEI flyaway?), Crab is spot on. The aim of the published Cat-A flyaway procedure is to achieve Vtoss as quickly as possible, so that a safe climb gradient can be achieved. Drooping NR to (or at least towards) the RFM limit will provide the most efficient acceleration to Vtoss without inducing a significant ROD.
Re the debate about rotor efficiency at reduced NR, I defer to Shawn Coyle!
Join Date: Dec 2008
Location: N/A
Posts: 845
Likes: 0
Received 0 Likes
on
0 Posts
I don't think Shawn has or would disagree that Vx gives a better obstacle clearance than [the manufacturers] Vtoss. (would he?)
As for some advantages of using Vtoss, like; simplicity, having a fixed target speed, picking a speed which can reasonably be applied to all cases ... I don't disagree - its a merit of a fixed Vtoss
As JimL rightly points out: the manufacturer gives a speed which is USED as Vtoss.
It obviously is not going to actually be: "the lowest speed at which, with one engine inoperative, the aircraft is able to climb [>100fpm]" for the conditions of weight and wind on the day - is it?
As Shawn says the speed NEEDS ('Desirable' I can see) to be a number on the airspeed indicator [that pilots can remember? ]
Thax: re Geoffers "Vtoss is the Take Off Safety Speed and is the slowest speed at which a rate of climb of 100 ft/min can be maintained" - ok sure - but people do not use that definition of the speed as their Vtoss do they? They use a fixed predetermined speed for their Vtoss instead.
.... and if you wanted to miss obstacles like Geoffers mentioned in his obstacle rich environment the Best Angle - by definition will get you the most height for the least distance gone - if you could determine its speed.
[ Crab: Never on the facts always a slur of some kind - what a guy - it's no fun having a discussion with a bloke who is so rude without facts or logic - why do you do it? Training?
I think you'll find SC was in fact agreeing with me:
"Vw ? .. and Vx is "best angle of climb speed" which is also not Vtoss
But you might prefer to use it if given the choice
it's just hard to pin down beforehand "
SC says IMPOSSIBLE to pin down, I say HARD same thing really - impossible is often hard, miracles often take longer. If you want to base your bullying logic on the difference between HARD and IMPOSSIBLE fine I am happy to defend my word HARD - but it's hardly worth the argument is it?
What'll it be ? Logic or slur? You do better with the slur generally
]
As for some advantages of using Vtoss, like; simplicity, having a fixed target speed, picking a speed which can reasonably be applied to all cases ... I don't disagree - its a merit of a fixed Vtoss
As JimL rightly points out: the manufacturer gives a speed which is USED as Vtoss.
It obviously is not going to actually be: "the lowest speed at which, with one engine inoperative, the aircraft is able to climb [>100fpm]" for the conditions of weight and wind on the day - is it?
As Shawn says the speed NEEDS ('Desirable' I can see) to be a number on the airspeed indicator [that pilots can remember? ]
Thax: re Geoffers "Vtoss is the Take Off Safety Speed and is the slowest speed at which a rate of climb of 100 ft/min can be maintained" - ok sure - but people do not use that definition of the speed as their Vtoss do they? They use a fixed predetermined speed for their Vtoss instead.
.... and if you wanted to miss obstacles like Geoffers mentioned in his obstacle rich environment the Best Angle - by definition will get you the most height for the least distance gone - if you could determine its speed.
[ Crab: Never on the facts always a slur of some kind - what a guy - it's no fun having a discussion with a bloke who is so rude without facts or logic - why do you do it? Training?
I think you'll find SC was in fact agreeing with me:
"Vw ? .. and Vx is "best angle of climb speed" which is also not Vtoss
But you might prefer to use it if given the choice
it's just hard to pin down beforehand "
SC says IMPOSSIBLE to pin down, I say HARD same thing really - impossible is often hard, miracles often take longer. If you want to base your bullying logic on the difference between HARD and IMPOSSIBLE fine I am happy to defend my word HARD - but it's hardly worth the argument is it?
What'll it be ? Logic or slur? You do better with the slur generally
]
Join Date: Mar 2010
Location: North America
Posts: 107
Likes: 0
Received 0 Likes
on
0 Posts
Vtoss and Climb Gradient
AnFi,
If you are loaded to the maximum Category A weight for altitude and temperature, and the WAT curve is based on the first takeoff segment (between TDP and 200 ft AGL), then yes, the manufacturer’s specified Vtoss is the lowest speed at which the aircraft is able to climb OEI at 100 fpm. However, the climb angle at Vtoss will improve with increasing headwinds, and the flight manual will provide a 1st takeoff segment OEI climb gradient chart (mean height gained in 100 ft horizontal distance) showing climb gradient as a function of weight, altitude, temperature, and headwind for the specified Vtoss.
Some helicopters have a selectable Vtoss, so if you have an obstacle in your path that you need to clear, you can select a lower Vtoss along with the reduced weight that goes along with that Vtoss. You can determine the required climb gradient from the takeoff site geometry, then enter the OEI climb gradient chart with the altitude, temperature, headwind, and required gradient to determine the combinations of Vtoss and gross weight that will produce the required gradient.
HT
As JimL rightly points out: the manufacturer gives a speed which is USED as Vtoss.
It obviously is not going to actually be: "the lowest speed at which, with one engine inoperative, the aircraft is able to climb [>100fpm]" for the conditions of weight and wind on the day - is it?
It obviously is not going to actually be: "the lowest speed at which, with one engine inoperative, the aircraft is able to climb [>100fpm]" for the conditions of weight and wind on the day - is it?
Some helicopters have a selectable Vtoss, so if you have an obstacle in your path that you need to clear, you can select a lower Vtoss along with the reduced weight that goes along with that Vtoss. You can determine the required climb gradient from the takeoff site geometry, then enter the OEI climb gradient chart with the altitude, temperature, headwind, and required gradient to determine the combinations of Vtoss and gross weight that will produce the required gradient.
HT
Join Date: Dec 2008
Location: N/A
Posts: 845
Likes: 0
Received 0 Likes
on
0 Posts
Thanks HeliTester.
It is interesting that some types do give a variable Vtoss to accommodate what I have been saying 'bout climb gradient and weights - ie. a fixed predetermined Vtoss has advantages but the best obstacle clearance is not one of them
(CRAB the hard and impossible is me just trying to be polite - it is OBVIOUSLY NOT IMPOSSIBLE it isn't really all that hard either - never the facts Crab always slur - what EXACTLY was I wrong about which Shawn 'corrected? Put up, shut up or appologise - pah!)
It is interesting that some types do give a variable Vtoss to accommodate what I have been saying 'bout climb gradient and weights - ie. a fixed predetermined Vtoss has advantages but the best obstacle clearance is not one of them
(CRAB the hard and impossible is me just trying to be polite - it is OBVIOUSLY NOT IMPOSSIBLE it isn't really all that hard either - never the facts Crab always slur - what EXACTLY was I wrong about which Shawn 'corrected? Put up, shut up or appologise - pah!)
