Maximum range airspeed in autorotation
 

Hello to everyone.
I have a question regarding influence of the wind in an autorotation. Is there a chance that changing the maximum range airspeed in the autorotation during headwind component of 20kts by adding 20 kts to the indicated airspeed would have any effect on the maximum range in the autorotation?
In other way, the maximum range airspeed is 69kts, we add 20kts to this value and now have 89 kts IAS. Shall we go further or less than just maintaining 69kts?

Would appreciate detailed explanation.

Thanks

Maximum range airspeed in autorotation
 

About 65mph, no wind conditions, in an Air & Space 18A gyroplane.

thanks for the answer, but I am interested on the influence of the wind on the maximum range airspeed for the helicopter Bell206b III.

Rokija,

If you add speed in auto you'll increase the range of the auto. A negative side effect will be increased rate of descent. Seems like the benefit (increased distance covered over the ground) might be canceled by the cost (increased RoD)...true, but when you get to the flare, you'll get a significantly longer flare (slower development) and this will stretch the glide and you'll realise your additional range.

Give it a try one day. Enter auto at the same point (altitude, point over the ground) at 69 and again at 89 kts. Start your flare at about the same height. The extra energy you've got to dissipate in the flare ends up extending your glide a fair bit.

In terms of wind, forget it. Whether you have no wind or 20 knots headwind, you'll get better range in auto with a slightly higher speed.

Hope this helps.

HP

As a general rule, the best glide angle in auto is done at a speed about mid point between the best rate of climb and the best range speed. If you have a headwind, add about 1/2 to 2/3 of the wind speed to the glide speed.

Why is best glide related to best range? Because both measure the least energy used per mile of distance traveled, since fuel burn is the same as altitude lost. Why is autorotation best glide speed slower than best range? Because the freely turning rotor trims to a lower, and worse rpm at the higher speed, so we take a bit of airspeed off.

For least rate of descent in auto fly at best climb speed, because that is the speed for the least fuel burned per minute to stay up, or the least altitude lost per minute.


Example: If the best range speed is 125 knots, and best climb speed is 75 knots, best autorotative glide is probably 110 or so, and min rate of descent in auto is about 75. If there is a 20 knot headwind, best glide is probably about 120 knots.

Try looking in the flight manual, the manufactures spend a lot of time and money working it out for you !

Fairly theoretical issue I think.
Looking inside the cockpit to keep values at a special point will kill you in the real emergency cause you won´t see the wires in your approach path or the ditch in the landing area, which will make you topple - unless it is to late.
There have been enough accidents in training - especially when training off airport!
With the height we normally operate in you won´t have much time anyway (o.k., the old Bell206 sailbird will travel further them most other helicopters)
I think it´s better to get used to achive a steady autorotation shortly after entering, controlling rrpm changes by ear or maximal short glimpse, giving you the time to look out to see, where your safe landing area might be and if you can make it or have to choose another one.
Better have one nearer and weave of some height to make it as going for a far away landing spot.
If you really have to stretch the auto, increase in airspeed with a droop in rrpm can do the job - but you need extra capacity in the cockpit - and on the controls, cause all values change while doing so.....
Just my 2 cents worth.
Greetings Flying Bull

H 500 has put it in a nutshell. The manufactuer gives you the speeds to keep the blades turning fast enough to enable one to carry out a landing without power. Wind has nothing to do with an auto apart from the fact of where you are going to land.
Remember Newtons law? For every action there is an equal and oppisite reaction. Pretty much means you cannot get something for nothing.

If you really need the max range, don't forget you can raise the lever and glide at min power off rrpm as well. You will recover rrpm during the flare. :ok:

Thank you all for answering my question. From my personal experience higher airspeed in wind condition does have effect on the max range but how to explain in aerodynamic terms that situation because every chart regarding max range speed in the autorotation doesn`t include wind correction. It just says the max range airspeed is 69kts (Bell206B) and every speed above or belove cannot provide better range.

Info given to me from a Bell test pilot many moons ago,
Desired airspeed for minimum decent 52 KIAS
Airspeed for max glide 69 KIAS.

The 206 was easy the bigger machines and the 4 blade machines had some interesting figures for weight/RRPM/airspeed, but as said above go by the FM, the info I got was from a test pilot and interesting to work with during test flying

It would appear that 52 knots and 69 knots, more than just being secret info, found their way to the RFM.:)

There was reference in a previous post about weight. Doesn't weight effect the sink rate but not the glide distance?

Assuming the wind is not fixed in velocity, won't its influence on ground speed be constantly changing. Rather than trying to make a series of calculations with your head inside, wouldn't it be better to just look outside as to how the spot is moving on the windscreen and use airspeed, rotor rpm and trim to achieve the best possible glide path. Last time I was at Bell, Kevin Brandt did the whole session of emergencies in the 407 with the airspeed indicator covered. That exercise really reinforced the spot on the windscreen concept.

Finally, an instructor from Canadian Helicopters and friend, Tim Simmons, told me some years back that it is far better to make a good auto to a bad spot than a bad auto to a great spot. That one has stuck with me.

It seems to me that headwind component cannot be dismissed from a discussion about power-off range speeds.

For example; If for a given acft the power-off range speed is 69kts and it is autorotating at 69kts into a 69kt headwind then groundspeed would be zero so distance covered would also be zero.

Therefore an IAS greater than 69kts must give a greater distance covered, ie more than zero.

If you take this concept a step further and say the headwind is now reduced to 68kts then the groundspeed is 1kt, would you achieve a greater range if you increase IAS to 70 kts thereby giving you a groundspeed of 2kts? Or 71 kts giving a groundspeed of 3kts? etc, etc.

90/90
 

Has no one tried the good ole 90/90 rule? 90% NR, 90kts. It really adds range to an auto in the 206. You find yourself with the left hand pretty high and as long you watch that NR you will go really far.

rokija

The way to explain it is to use a plot of rate of descent (y axis) vs airspeed (x axis). You can simply get a few points each time you fly by doing an auto at a speed and note the rate of descent. If you get about 4 points from min RoD speed to max auto speed, the shape should be about right to find the best glide speed in any wind.

Just plot the 0 knots point about 30 knots to the right of the Y axis, to make room for the tail wind.

Here are some examples:
http://mysite.verizon.net/res11h5lk/...res/minRoD.jpg

http://mysite.verizon.net/res11h5lk/...es/0ktwind.jpg

http://mysite.verizon.net/res11h5lk/...30KtHdWind.jpg

http://mysite.verizon.net/res11h5lk/...kttailwind.jpg

Tickles,

Using your logic....the best glide distance would be found "downwind" would it not?

rokija,
Don't confuse "airspeed" and "groundspeed". Performance speeds are all based on flight within an airmass, or more simply flight with ZERO wind. Your 69 knots max glide distance airspeed (if that is what the flight manual calls for) is probably the speed for the best range you can get in autorotation at the proper rotor rpm. It is up to the pilot to understand that with a 10 knot headwind you will not cover as much ground at 69 KIAS as you would with zero wind or with a 10 knot tailwind. If the speeds were derived correctly, any increase above that 69 knots will not increase your glide distance as your rate of descent will increase giving you a steeper descent angle. Slowing the acft down will also give you a steeper angle, but to a point (min rate of descent airpseed) will give you a progressively slower rate of descent.
Next time you are out flying on a calm day, try it out. Get up to 2000 feet AGL or so, enter an auto at max range airspeed with rotor rpm at the proper setting (usually bottom of the green) and stabilize. Look at your glide angle then slowly accelerate a few knots at a time and see what happens to your "circle of action". It should move closer to the acft. If you decelerate back to Min ROD, the circle of action will again move further out. You will also see a change of your circle of action with a change of rotor rpm. Once you see how far the acft glides on a no wind day, just understand that with a headwind you won't go as far and with a tail wind you will glide further, but the bottom part of a tailwind autorotative landing is not the most desireable way to land.
Cheers
Mark

I've added 1/2 the headwind component as a rule of thumb, but you have to be cautious with having a higher airspeed prior to the landing as it will change the profile and could make the landing more difficult.

Probably even more important is to develop the judgement to pick a landing spot that is possible using the standard autorotation speeds. Maintaining one technique, but considering wind in selecting a landing spot should be easier than learning many different autorotation profiles.

SASless, yes, at an IAS factored for tailwind component as 'Noodles very neat graphics.

mfriskel, your airmass observations only hold true for min ROD/endurance speeds. For range you must make a correction for wind.

To those who say its more important to be able to judge/control touchdown point than to achieve max range I suggest that a skillful pilot should be able to do both.

Ramen Noodles is spot on.

The calculations he sets out are exactly the same calculations that are done for glider performance (albeit with a much better glide ratio). The curve is called a polar, and can be calculated for any aircraft that can glide. As a general rule:

- If you are in a head wind or a sinking airmass, then you should increase your airspeed to get the best range over the ground.

- If you are in a tail wind or a lifting airmass, then you should decrease your airspeed to get the best range over the ground.

Daniel

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?:confused:

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.

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 ?

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.

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.

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.

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.

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.

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).

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.

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.

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.

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?

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.

Crab:
Is that right? I would have thought the horizontal stab would affect the fuselage attitude rather than the disc attitude.

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.

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.

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!

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.

Right you are!
 

Definately! I agree. I guess the green arc goes, or maybe some machines have a specific arc for auto conditions...