RPM droop on Cat A procedures.

[Koamill]

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!!!:ugh::ugh::ugh:

[Thomas coupling]

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??

[ShyTorque]

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! :p

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.

[Koamill]

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.

[pilot and apprentice]

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!

[AAKEE]

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.

[[email protected]]

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.

[helmet fire]

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 :8

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.

[Koamill]

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.

[Sir Korsky]

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.

[Koamill]

Don't worry Sir Korsky, in this S 61 sim, you can only fly on night mode, and most of these take off are trained over rigs offshore...!!!

[HeliTester]

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.

[[email protected]]

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.

[HeliTester]

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

[helmet fire]

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.

[[email protected]]

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.

[pilot and apprentice]

AAKEE, I stand corrected. Been a while. :-)

[surfcopter]

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.

[jim_reed]

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.

[[email protected]]

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.

[helmet fire]

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.

[HeliTester]

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

[helmet fire]

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?

[AnFI]

Vtoss is based on 'best angle of climb speed'

Best angle of climb speed is unlikely to actually be Vtoss - since it will depend on the wind and the weight.

[HeliTester]

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

[helmet fire]

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.:O

[Geoffersincornwall]

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

[AnFI]

... 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

[Shawn Coyle]

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...)

[AnFI]

Shawn you don't really mean that do you?Vx is impossible to define for helicopters 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)

the airspeed used needs to be something that is repeatable and usable (i.e. can be flown using the airspeed indicator...) 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'...

[ShyTorque]

I think Shawn is totally correct, simply because the term "just cuff it" has never appeared in a Class A performance chart. ;)

[JimL]

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

[HeliTester]

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.


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.

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.

[Koamill]

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.


:confused::confused:

[HeliTester]

Koamill writes:

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.

[Thax]

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!

[AnFI]

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:rolleyes:]

[HeliTester]

AnFi,


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?
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

[AnFI]

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!)