Maximum range airspeed in autorotation
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I don't get it. If you want to extend your auto rotation range in a parcel of air, what difference does the wind direction make? If you have a head wind you won't go so far over the ground as if you have a tail wind.
So if you want to extend your range in a head wind, fly at max auto airspeed and min rrpm.
If you want to extend your range in a tail wind, do exactly the same. right?
So if you want to extend your range in a head wind, fly at max auto airspeed and min rrpm.
If you want to extend your range in a tail wind, do exactly the same. right?
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The effect of headwind on range in autorotation is important if you're trying to get somewhere specific. If you don't care where you land, then it doesn't matter.
The tradeoff between increased distance and increased rate of descent can be proven easily with a proper rate of descent vs airspeed graph as was previously shown (except it didn't show the rate of descent below minimum power speed correctly for a helicopter).
But - and this is a very large issue - the other effect is that if you use a higher airspeed to account for the wind at the end of the autorotation, when you start to flare, you'll have significantly more energy with respect to the ground. For example if you use 90 knots instead of 69 knots you'll have 90 x 90 'units' vs 69 x 69 'units' (can't figure out how to do superscripts for a squared sign) of energy - a significant change. You'll also have a shallower rate of descent angle to change to level flight, actually making the job of landing easier.
Adding airspeed when autorotating into the wind has lots of advantages and, as far as I know, no disadvantages.
The tradeoff between increased distance and increased rate of descent can be proven easily with a proper rate of descent vs airspeed graph as was previously shown (except it didn't show the rate of descent below minimum power speed correctly for a helicopter).
But - and this is a very large issue - the other effect is that if you use a higher airspeed to account for the wind at the end of the autorotation, when you start to flare, you'll have significantly more energy with respect to the ground. For example if you use 90 knots instead of 69 knots you'll have 90 x 90 'units' vs 69 x 69 'units' (can't figure out how to do superscripts for a squared sign) of energy - a significant change. You'll also have a shallower rate of descent angle to change to level flight, actually making the job of landing easier.
Adding airspeed when autorotating into the wind has lots of advantages and, as far as I know, no disadvantages.
Shawn
Not sure I agree here, obvious big disadvantage is shortening distance over the ground if you use too much airspeed. Plus If you flared at say 130 kts ( vne in auto) in a 500 then you have an awful lot of airspeed to get rid of and have a lot of rrpm to deal with. Remember it is accleration that kills us. So why increase airspeed to such an extent that you have a monster fwd groundspeed and a monster rate of descent ?
Not sure I agree here, obvious big disadvantage is shortening distance over the ground if you use too much airspeed. Plus If you flared at say 130 kts ( vne in auto) in a 500 then you have an awful lot of airspeed to get rid of and have a lot of rrpm to deal with. Remember it is accleration that kills us. So why increase airspeed to such an extent that you have a monster fwd groundspeed and a monster rate of descent ?
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Back at the mile-square plot of grass known as Spencer Field in NW Florida, we used to demonstrate for the students the difference in range one got on an auto in a TH-57B (B206 B-3) at 75 knots and 105 knots.
We used 75 as a standard auto speed as it gave the students a fairly nice margin for error high or low.
The amount of distance gained in a moderate headwind on final was significant, and most of it was gained during the flare, as Shawn points out. The significantly higher ROD at 105 negated a lot of the advantage of the higher speed over ground in the steady-state part of the auto.
75 and 105 have a ratio of 5:7. If, as Shawn did, one squares those two numbers, you'll find the aircraft's kinetic energy is just about doubled at 105 knots, 25:49, so one can hold Nr with a shallower descent angle while bleeding off airspeed in the flare for a longer time. In fact, you have to... if you want the same energy on touchdown in both cases, barring doing anything else to dissipate energy like sideslipping.
That is a potential disadvantage... being too hot on landing... which is why it's a visual maneuver (manœuvre?) and not flown purely on speeds. But it is good to know if you're over the Sahara forest and the only place to land is just a biscuit past where you know you can get at Vy, that you can quite possibly get there.
chopjock... in a headwind, the wind is negatively impacting your groundspeed, so you want to be exposed to it for a shorter time but also want to make the best progress against it, so you use a higher airspeed. How much higher?
In a tailwind, the wind is positively impacting your groundspeed, so being exposed to it is actually assisting you with distance, so you want to be exposed to it for a longer time, which means a somewhat slower airspeed. How much slower?
This is where our friend ramen noodles comes in and shows us with those truly artful (with entirely fake data) charts. You're trying to maximize the x:y ratio, which is done using a tangent to the bucket curve. As is also shown, the wind component moves the intersection of the glideslope with the x-axis left or right for a tail or head wind, respectively.
SASless... one does indeed get better distance doing an auto downwind... but it's that transition to touchdown where one might have his hands a bit full if trying to put it on a spot.
George... I did the 407 course with Kevin Brandt years ago... that fellow can surely fly autorotations. One can't help but get better at it flying with him.
We used 75 as a standard auto speed as it gave the students a fairly nice margin for error high or low.
The amount of distance gained in a moderate headwind on final was significant, and most of it was gained during the flare, as Shawn points out. The significantly higher ROD at 105 negated a lot of the advantage of the higher speed over ground in the steady-state part of the auto.
75 and 105 have a ratio of 5:7. If, as Shawn did, one squares those two numbers, you'll find the aircraft's kinetic energy is just about doubled at 105 knots, 25:49, so one can hold Nr with a shallower descent angle while bleeding off airspeed in the flare for a longer time. In fact, you have to... if you want the same energy on touchdown in both cases, barring doing anything else to dissipate energy like sideslipping.
That is a potential disadvantage... being too hot on landing... which is why it's a visual maneuver (manœuvre?) and not flown purely on speeds. But it is good to know if you're over the Sahara forest and the only place to land is just a biscuit past where you know you can get at Vy, that you can quite possibly get there.
chopjock... in a headwind, the wind is negatively impacting your groundspeed, so you want to be exposed to it for a shorter time but also want to make the best progress against it, so you use a higher airspeed. How much higher?
In a tailwind, the wind is positively impacting your groundspeed, so being exposed to it is actually assisting you with distance, so you want to be exposed to it for a longer time, which means a somewhat slower airspeed. How much slower?
This is where our friend ramen noodles comes in and shows us with those truly artful (with entirely fake data) charts. You're trying to maximize the x:y ratio, which is done using a tangent to the bucket curve. As is also shown, the wind component moves the intersection of the glideslope with the x-axis left or right for a tail or head wind, respectively.
SASless... one does indeed get better distance doing an auto downwind... but it's that transition to touchdown where one might have his hands a bit full if trying to put it on a spot.
George... I did the 407 course with Kevin Brandt years ago... that fellow can surely fly autorotations. One can't help but get better at it flying with him.
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H500, I can see where you're coming from but if the headwind component was sufficiently strong to require 130kts IAS to achieve best glide then you would be flying into a very strong wind and so your groundspeed would be much lower.
Matthew Parsons 1/2 wind speed approximation is a pretty good rule of thumb and so that might mean a headwind of something like 100kts to require 130kts IAS for best range auto (assuming 80kts best glide in still air for 500D). The groundspeed is therefore only 30kts so not much difficulty flaring that off or controlling Nr.
Possibly the 1/2 wind speed approximation will be a bit inaccurate by 100kts (not important outside a theoretical discussion I would suggest!)and groundspeed might be higher than 30kts but even so it would be simple to flare earlier if required and intercept a familiar profile for final approach.
Matthew Parsons 1/2 wind speed approximation is a pretty good rule of thumb and so that might mean a headwind of something like 100kts to require 130kts IAS for best range auto (assuming 80kts best glide in still air for 500D). The groundspeed is therefore only 30kts so not much difficulty flaring that off or controlling Nr.
Possibly the 1/2 wind speed approximation will be a bit inaccurate by 100kts (not important outside a theoretical discussion I would suggest!)and groundspeed might be higher than 30kts but even so it would be simple to flare earlier if required and intercept a familiar profile for final approach.
So just how many posters here have flown in a 60 kt wind once let alone regularly??? A rather pointless discussion since whatever the windspeed is, your auto speed needs to be more than it to gain ground into wind, even if that means flying faster than your RFM specified range speed.
If your RFM says 90 kts for range and you are in a 90kt wind then you must fly at more than 90kts if the place you want to land is upwind from where the engine fails.
Equally, the comments about being hot on the landing are irrelevant if you have such a strong wind since you will easily achieve zero groundspeed for touchdown with minimal flare.
The only practical lessons here are that increasing speed in auto beyond recommended auto speed will a. get you further and b. increase your RoD giving you less time in the air but you need to make the decision to go for range early if you are reap the full benefits.
If your RFM says 90 kts for range and you are in a 90kt wind then you must fly at more than 90kts if the place you want to land is upwind from where the engine fails.
Equally, the comments about being hot on the landing are irrelevant if you have such a strong wind since you will easily achieve zero groundspeed for touchdown with minimal flare.
The only practical lessons here are that increasing speed in auto beyond recommended auto speed will a. get you further and b. increase your RoD giving you less time in the air but you need to make the decision to go for range early if you are reap the full benefits.
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Crab, it may be a pointless discussion until the day comes when one MUST make best distance to make a safe landing site (clear a forest/hillside/water etc ) post power loss.
On that day surely it becomes the most important knowledge?
Judging by a number of posts the discussion is also of educational benefit to some pilots and so can hardly be pointless.
On that day surely it becomes the most important knowledge?
Judging by a number of posts the discussion is also of educational benefit to some pilots and so can hardly be pointless.
Feathering - exactly my point - when the day comes you will put the nose down until the point you need to reach stays steady in the windscreen or make a new choice of landing site (or crash site) if that isn't achievable - you won't, I would suggest, be worrying about what speed to fly at because you might fly the perfect range auto as per the RFM and not make the LS!
As far as an academic discussion goes, all PPLs learn advanced autos and are shown the effects of speed changes in auto - why this should be new learning to them now is interesting. Why they need to think about autoing in a 60 plus knot wind is also something of a mystery.
As far as an academic discussion goes, all PPLs learn advanced autos and are shown the effects of speed changes in auto - why this should be new learning to them now is interesting. Why they need to think about autoing in a 60 plus knot wind is also something of a mystery.
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The way I see it is that you can glide at a higher KIAS for range but it would be nicer to intercept a normal glidepath at a manageable groundspeed closer to the touchdown point. If I were gliding for range, I would slow her down to normal glide airspeed about 100-150 feet AGL to configure a more familiar landing profile.
I agree there is no single theoretical glidespeed but different glidespeeds for different makes of helicopters. The optimal glidespeed (nil wind) is the one where the rotor disk is level with the horizon in the auto and that is why such airspeed produces the lowest ROD. This level disk attitude is the same as that in a hover (nil wind).
I agree there is no single theoretical glidespeed but different glidespeeds for different makes of helicopters. The optimal glidespeed (nil wind) is the one where the rotor disk is level with the horizon in the auto and that is why such airspeed produces the lowest ROD. This level disk attitude is the same as that in a hover (nil wind).
Reverse flight - I am afraid that just isn't true - the optimum 'glidespeed' is the one that gives you minimum RoD (and therefore more time in the air) and is usually similar to your minimum power speed in normal flight. Your disc attitude doesn't change much in many helicopters through the speed range because of the horizontal stabiliser.
The optimum speed for range is, as Ramen showed earlier, the best ratio of forward speed to RoD.
The optimum speed for range is, as Ramen showed earlier, the best ratio of forward speed to RoD.
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crab:
Sadly, US PPL students get very little exposure to advanced autorotations.
And to all the others who don't believe how easy it is to carry out the maneuvers - about one hour of flight time with a variety of autorotation profiles to get you to land on a spot would make believers out of you. Simple and straightforward to teach and easy to learn.
Sadly, US PPL students get very little exposure to advanced autorotations.
And to all the others who don't believe how easy it is to carry out the maneuvers - about one hour of flight time with a variety of autorotation profiles to get you to land on a spot would make believers out of you. Simple and straightforward to teach and easy to learn.
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Thanks crab, I am aware that your definition of lowest ROD is more orthodox as it refers to the maximum excess power which the "power available" curve has above the "power required" curve.
I threw in the disk attitude idea as a certain helicopter guru instructor at Maroochydore reckons that's a practical approximation to the lowest ROD airspeed in an auto.
Shawn, I understand that full down autos are not even part of the FAA CPL(H) syllabus and they generally don't learn those until they get to CFI stage. Thankfully I was trained here in Australia where we get to perform such varied manoeuvres as autos from 50' AGL as part of our CPL(H) course.
I threw in the disk attitude idea as a certain helicopter guru instructor at Maroochydore reckons that's a practical approximation to the lowest ROD airspeed in an auto.
Shawn, I understand that full down autos are not even part of the FAA CPL(H) syllabus and they generally don't learn those until they get to CFI stage. Thankfully I was trained here in Australia where we get to perform such varied manoeuvres as autos from 50' AGL as part of our CPL(H) course.
Shawn - that's rather shocking - what do the FAA checks consist of, surely there is a PFL of some sort to prove the pilot can actually make a landing area that isn't just under the nose when the engine quits?
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Shawn,
I agree that learning different autorotation profiles is "simple and straightforward" in a training machine. One that is bigger, being employed to earn money, etc. may never be autorotated to a landing by most of the pilots. In that case, an emergency situation is the wrong time to try and fly a profile that you've never seen. If you do get the opportunity to learn other profiles, then obviously you should use those in an emergency, but I'd be surprised if there is much opportunity to get that training.
Cheers,
Matthew.
I agree that learning different autorotation profiles is "simple and straightforward" in a training machine. One that is bigger, being employed to earn money, etc. may never be autorotated to a landing by most of the pilots. In that case, an emergency situation is the wrong time to try and fly a profile that you've never seen. If you do get the opportunity to learn other profiles, then obviously you should use those in an emergency, but I'd be surprised if there is much opportunity to get that training.
Cheers,
Matthew.
Crab:
Is that right? I would have thought the horizontal stab would affect the fuselage attitude rather than the disc attitude.
Your disc attitude doesn't change much in many helicopters through the speed range because of the horizontal stabiliser.
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your right crab, i think reverse flight may have been referring to me. (not a guru though).
i try to show students that if you can maintain a relatively flat attitude in auto (not accelerating or decelerating) you should achieve close to a minimum rate of descent speed, as you say, close to min power attitude not necessarily a horizontal disc as such. it seems to work well in most machines, just a rule of thumb.
i try to show students that if you can maintain a relatively flat attitude in auto (not accelerating or decelerating) you should achieve close to a minimum rate of descent speed, as you say, close to min power attitude not necessarily a horizontal disc as such. it seems to work well in most machines, just a rule of thumb.
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imabell, if you're who I am referring to, you certainly look a lot younger and energetic than 63 - please take that as a great compliment !
Robbo Jock, agreed that we should be talking about the disc, not the fuselage. I think it depends on the rotorhead setup - in a rigid rotorhead, I expect the fuselage to bear a pretty much fixed relationship with the rotor disc, while in a teetering head the disc is entitled to have an attitude/mind of its own.
Robbo Jock, agreed that we should be talking about the disc, not the fuselage. I think it depends on the rotorhead setup - in a rigid rotorhead, I expect the fuselage to bear a pretty much fixed relationship with the rotor disc, while in a teetering head the disc is entitled to have an attitude/mind of its own.
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ROD
Without resorting to theoretical issues, doesn't the rate of descent actually decrease in a high speed autorotation because of the simple fact that you also choose a very low NR in order to get the range? My previoius post of 90kts/90%NR is obviously along the same line of reasoning as the beutiful fake bucket diagrams.
I think a lot of you posters who talk about the rate of descent being higher at higher speed forget the fact that the pilot will raise the collective a) in order to avtually contain an over-RPM condition, b) lower NR to the point where the rate of descent goes down significantly to get the low ROD and thus the range.
I guess the same principle follows in the flare. Unless you have a truly ****ty situation, you don't want to rev up too much in the flare. Raising the collective during it will lower your ROD and slow down you ground speed at the same time.
When I was tought advanced autorotations (yes, at PPL level and at type ratings and PCs, still at PPL) we always flare with a very high collective setting and an NR of less than 100%, and the benefit of zero groundspeed easily outweighs the slightly lower NR you end up with just before touchdown. Couple to that the fact that you don't fall through in the flare and you're home safe.
Can any of you high time instructors verify this? 90/90 coupled with a low NR flare?
On a side point, after my 206 type rating a few years ago I totally fell in love with autorotations. Even the 500 and the 300 are fun and a breeze when your fear leaves you and you can concentrate on trying to master the art of beutiful autorotations!
I think a lot of you posters who talk about the rate of descent being higher at higher speed forget the fact that the pilot will raise the collective a) in order to avtually contain an over-RPM condition, b) lower NR to the point where the rate of descent goes down significantly to get the low ROD and thus the range.
I guess the same principle follows in the flare. Unless you have a truly ****ty situation, you don't want to rev up too much in the flare. Raising the collective during it will lower your ROD and slow down you ground speed at the same time.
When I was tought advanced autorotations (yes, at PPL level and at type ratings and PCs, still at PPL) we always flare with a very high collective setting and an NR of less than 100%, and the benefit of zero groundspeed easily outweighs the slightly lower NR you end up with just before touchdown. Couple to that the fact that you don't fall through in the flare and you're home safe.
Can any of you high time instructors verify this? 90/90 coupled with a low NR flare?
On a side point, after my 206 type rating a few years ago I totally fell in love with autorotations. Even the 500 and the 300 are fun and a breeze when your fear leaves you and you can concentrate on trying to master the art of beutiful autorotations!
RobboJock - quite right, your disc attitude will change whilst your fuselage attitude remains broadly the same thanks to the horizontal stab. I think it is more complex in auto because of the upwash caused by the RoD but I was just trying to highlight that basing your auto speed on disc attitude wasn't as good as using the ASI.
Imabell - yes I see where you are coming from with that.
Perfrej - I don't know that all aircraft will behave the same in auto at reduced Nr - it maybe that the aerofoil section (if symmetrical) will keep efficiency (the Gazelle used to be very good at it and had a specified Nr - 330 - to reduce to for max range) but more modern cambered sections might get less efficient with reduced Nr. Don't forget that as you reduce Nr, you increase the coning angle - effectively reducing the area of the disc. If your flight manual specifies a Nr setting for max range then I would use it but if it doesn't, there may be a good reason.
Imabell - yes I see where you are coming from with that.
Perfrej - I don't know that all aircraft will behave the same in auto at reduced Nr - it maybe that the aerofoil section (if symmetrical) will keep efficiency (the Gazelle used to be very good at it and had a specified Nr - 330 - to reduce to for max range) but more modern cambered sections might get less efficient with reduced Nr. Don't forget that as you reduce Nr, you increase the coning angle - effectively reducing the area of the disc. If your flight manual specifies a Nr setting for max range then I would use it but if it doesn't, there may be a good reason.