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