Maximum Glide Configuration

[Bootcamp]

I have an ongoing debate with my IP regarding the maximum glide configuration in an autorotation in a Bell 206/OH-58.

 

I believe the maximum glide configuration is achieved at minimum RPM (above stall), approximately 90%.

 

He states (backed up with the -10 for the OH-58), that it is 100%.

 

I haven't been able to find a free copy of the B-206 RFM online to support my argument and most of my experience has been with Robinson helicopters. Most of my Google searches turn up 90%, but none of them from official documents.

 

Can anyone provide any insight?

 

Thanks.

[aeroscout]

Sounds like a question for...iChris !!!

[Astro]

My 206 manual doesn't have specific rpm for autos like the Robbie. It just says,

 

"Collective pitch - Adjust as required to maintain rotor RPM, 90% to 107%.". "Rotor RPM maintained at the high end of the operating range will provide maximum rotor energy to accomplish the landing; but will cause an increased rate of descent"

 

Considering how similarly the 206 flies to the 44 I'd go with 90%, but nothing beats just trying it out in the air.

[apacheguy]

Not trying to point out the obvious here, but your entry speed will make a huge difference in the distance you glide. Entering slower will shorten the glide immensely.

[Eric Hunt]

Go for 70 kt (manual says 69, but 70 is an easier number) and pull the RRPM down to 90%. Makes a noticeable difference to the glide range, maybe 10% extra of the normal distance.

The horn will bleat at you on and off, just play the lever a little to make it go away.

 

Allow at least 100' altitude to revert to normal 60kt/100% before the flare - start by raising the attitude as you gently lower the lever. If you dump the lever first, the ROD will go up and you will use more than 100' to recover to normal flare configuration.

 

For a power terminated auto, you obviously can't start to wind on the throttle while the revs are at 90% because it will totally change the profile. You will need to come out of range earlier, to allow time to get the throttle on gently and be fully open before the flare starts.

Edited July 11, 2014 by Eric Hunt

[Bootcamp]

Thank you all for the detailed answers.

 

Does anyone have any references (or Bell test pilots) I can cite?

 

Also, any Army OH-58A/C guys out there with insight as to why the -10 suggests 100%?

[helonorth]

It has always been drilled into my head that a lower rotor RPM gives you the best glide. Why don't you try it for real on your next flight? That will give you a definitive answer.

[Bootcamp]

It has always been drilled into my head that a lower rotor RPM gives you the best glide. Why don't you try it for real on your next flight? That will give you a definitive answer.

 

I asked...denied. :-(

[Gomer Pylot]

Bell instructors teach 90%Nr and higher airspeed for max glide distance. It makes a huge difference between using 100%Nr and ~60 kts and 90%Nr and ~70kts, or a little more, almost the difference between touching down at the approach end of a runway and at the departure end. My company has Bell come down and give a course, including classroom and flight instruction. A big part of the flying is demonstrating different autorotation techniques, and which to use for different situations. Any pilot needs to know all the techniques, because one never knows when the engine failure will occur, or what landing areas will be available where. I have no idea at all why the Army -10 for the OH58 prescribe 100% Nr, other than either complete ignorance or lack of trust in aviators. 100% Nr certainly gives you the most kinetic energy in the rotor system for touchdown, but sometimes that isn't the most important factor, and certainly won't give you the maximum glide distance. I've seen it many times, to full touchdowns, both in civilian and Army aircraft. When I flew in the Army, we did full touchdown autos in the OH58 & UH-1, and depending on the IP, we could experiment with autos, finding what worked where, and one of my primary instructors demonstrated backwards autos in the TH55. Things change, and I doubt that sort of thing is done these days, but back then things were much looser, and we could get by with a lot more. After VietNam finished, things started tightening up, and probably have continued to do so. In any case, I assure you that pulling the rotor RPM to the very bottom of the green and increasing the airspeed to the max glide airspeed, which is in the RFM, will give you the best glide distance, in any helicopter.

Edited July 12, 2014 by Gomer Pylot

[akscott60]

The -10 is the bible for the Army. Just accept it.

 

Edited July 12, 2014 by akscott60

[Spike]

Push the cyclic forward. Raise the collective to gain the sight picture. Forget about the numbers…..

 

This procedure only allows you to get so far. If your sight picture can’t give you what you need, then make the best of a normal auto…….

 

Autorotation’s 101; It’s much more survivable to do a good auto to a bad spot then to do a poor auto to a good spot……..

[Wally]

Autorotation’s 101; It’s much more survivable to do a good auto to a bad spot then to do a poor auto to a good spot……..

Amen, brother. Hear him!

[Mikemv]

Spike,

 

"Push the cyclic forward. Raise the collective to gain the sight picture. Forget about the numbers….."

 

 

Is this applied to a specific condition of flight or all conditions? It is a general statement and does not apply to many conditions of flight!

 

What about high speed cruise?

 

What about out of ground effect hover at 3000' AGL?

 

What about 40kts. at 200'?

 

Endless other conditions of airspeed and altitude.

 

I ask this for every readers benefit as the NTSB & IHST/USHST have identified lack of autorotational training addressing the above conditions as causal factors in accidents/fatalities.

 

Some resources are found here:

 

http://www.ihst.org/Portals/54/insights/energy.pdf

 

http://www.ihst.org/portals/54/QR4.pdf

 

Respectfully,

 

Mike

Edited July 12, 2014 by Mikemv

[Gomer Pylot]

The subject of the thread is how to attain maximum autorotative glide distance, not how to perform an autorotation under all circumstances.

[Mikemv]

The subject of the thread is how to attain maximum autorotative glide distance, not how to perform an autorotation under all circumstances.

I understand that but ask from what mode of flight? From a flight training set up in the pattern? From high speed cruise, etc.

 

If we start in trim at max glide airspeed and the OP asks the question about RRPM, then minimum green will produce the longest glide for a given GW & DA. This has been stated above by others.

 

I wanted to point out that pushing the cyclic forward and lifting the collective was not a suggested fix all?

 

Getting to maximum glide airspeed and bottom of the RRPM green will provide that, but how do we get there? Not by always pushing the cyclic forward and pulling up on the collective!

[aeroscout]

Not to stray from the topic, but if max glide distance is desired, you might have to consciously turn with the wind.

At some point you have to decide if you want to turn back into it.

But for the sake of the subject I suppose we can assume no wind.

[280fxColorado]

For what it's worth, in Enstrom autos-

 

Best glide distance: 80MPH, RPM bottom of green arc (332 or 334).

 

Best (lowest) rate of decent: 58MPH, "RPM in the green."-POH. Experience has shown lowest ROD @ bottom green too. Remember to bottom out collective prior flare to build RPM back to the top.

 

As others have stated, just try and see. In general, a faster airspeed with some collective pulled (lower RPM) will give you a longer glide. You can easily feel and see the result.

 

Shawn Coyle's book, Cyclic and Collective, provides an outstanding discussion on optimizing autorotations in various configurations.

Edited July 12, 2014 by 280fxColorado

[280fxColorado]

You often hear that phrase, "A good auto to a bad spot is better than a bad auto to a good spot." I generally agree with that, but just to play Devil's advocate, what about a textbook maneuver "good auto" to the tops of a 150 foot oak forest versus a skid spreading vertical descent auto into a small clearing in the trees? Or, a "good auto" into a deep lake versus a tailwind riding 88% RRPM overstretched max glide to get to the shoreline? Or a "good" auto to the roof of a Walmart or the full parking lot versus a unusually slow/steep/high-ROD/crosswind-flare auto into the empty baseball field that was behind you? So many variables "when it happens," it's hard to speak in absolutes.

[Mikemv]

A "good" auto is always good, that is an absolute.

 

No one knows the outcome of a "bad" auto, also an absolute.

 

I lost a friend that stretched an auto in an AS350, reached a level open area with loss of controllability, rolled over as he hit hard, burst into flames as the airframe broke up and 3 people died!

 

If you had asked him on the way down how it would turn out as a "bad" auto, he probably would have said "better than a good auto to the trees".

 

In training, we practice to complete good autos. How many autos have we practiced with the rotor below the green with a high rate of descent or low airspeed?

 

I hope no one accepts doing a "bad" auto the first time it becomes a choice for them.

 

Mike

[Wally]

A couple thoughts:

 

Faster is not necessarily longer or "barber poles" wouldn't be on airspeed indicators. Besides NR issues in a high speed glide, at some point the conversion of altitude to airspeed results in sacrificing to much energy for speed.

 

It would be better not to have to decide between the tops of any kind of considerable forest of trees and a vertical drop into a hover hole. I know pilots who have accomplished both, but they had a particular technique for each, beyond the range of what's wise to be discussed here.

 

A successful auto to an open body of water without previous training is a serious challenge, much less exiting a non-float equipped cabin afterwards. So, if your scenario is predicated on 88% being just under the NR green arc and a manageable tailwind to shallow water, that is what I'd preferentially do: for the altitude reference off the nearer beach which will make a more survivable landing, and to have less chance of drowning, trapped, in a sinking aircraft.

 

WalMart roof or baseball field? Baseball field, less bystanders. But I'd have a survivable plan before being put in that spot. Survival is the best you'll get, sometimes. Don't expect perfect, that will kill you.

[280fxColorado]

Mike, completely agree. I just think the old saying is too vague and begs the question, "What IS a 'good' auto?" AND needs the qualification "usually."

 

Wally, thanks for the input and clarification on "faster." Only as fast as the manufacturers precisely determined max range speed. Not suggesting anyone high tail it straight into the ground!

[Jaybee]

You often hear that phrase, "A good auto to a bad spot is better than a bad auto to a good spot." I generally agree with that, but just to play Devil's advocate, what about a textbook maneuver "good auto" to the tops of a 150 foot oak forest versus a skid spreading vertical descent auto into a small clearing in the trees? Or, a "good auto" into a deep lake versus a tailwind riding 88% RRPM overstretched max glide to get to the shoreline? Or a "good" auto to the roof of a Walmart or the full parking lot versus a unusually slow/steep/high-ROD/crosswind-flare auto into the empty baseball field that was behind you? So many variables "when it happens," it's hard to speak in absolutes.

 

Been a while since I read it, but if I remember correctly the gist of it was landing on top of brush or into trees was more survivable.

 

http://www.bladeslapper.com/m/how_to_crash.pdf

 

In any event, this covers a multitude of situations and gives lots of food for thought.

[Spike]

Experience teaches us, the theoretical world is a perfect world and if practiced, it will get us into the ballpark when the real preverbal poop hits the fan. Otherwise, everything else needs to be instinctive….

 

Once I entered the auto during my engine failure event, it was clear if I didn’t extend, I was going to end up in a small grove of trees. I immediately increased the airspeed and raised the collective. How much I couldn’t tell you, but I knew I was in the ballpark because it gave me the sight-picture I needed to clear the trees. Plus, once in the extended glide, I had recognized I pulled the RPM’s down too far because I could see the blades more clearly through their plane of rotation and the vibrations increased significantly. Luckily, by this time, I had cleared the trees which meant I could reestablish the “normal” auto profile which allowed me to lower the collective (to the floor) and pitch the nose up just in time for the flare…..

 

IMO, if you have the time to establish the “numbers”, then you should be golden. During the above event, it took about 8 seconds from the engine quitting to ground contact. Subtract .75 of-a-second for my brain to realize the problem and an additional .75 of-a-second to react. This leaves (about) 6.5 seconds to enter the auto, realize the need to extend, gain the extended glide then to reestablish the normal glide, flare, level and pull. Ergo, my concern wasn’t the numbers on the panel. It was about surviving the moment through muscle memory and sight……

 

@Mike,

 

I 100% agree; every situation will be different. However, I assume, the circumstance brought forward by the OP required an extended glide….. If a pilot cannot immediately determine the conditions necessary for a survivable auto, then God help him………

Edited July 17, 2014 by Spike

[Rupert]

Let's keep this to two-bladed rotor systems. I don't know how much it applies to three or more blades.

 

During the 1960's, if one transitioned into Hueys at Hunter Stewart, the Army gave each student an hour with a Grey Eagle who demonstrated extended-glide autorotations.

 

High speed and low NR.

 

At the end, he extended the glide a little further by trading airspeed for distance, but at 60 kts he flared and regained his NR. Don't go below 60 kts without regaining NR.

 

In the late 1980's while attending Bell Factory school in a Long Ranger, we had an actual engine failure while practicing extended glide autos. Go figure. We made it. Hit hard. No damage to aircraft.

 

Interestingly, I had the same thing happen in the 1970's in an FH1100 during a practice auto. Taught me to check the N1 after rolling off the throttle on entry.

[iChris]

I have an ongoing debate with my IP regarding the maximum glide configuration in an autorotation in a Bell 206/OH-58.

 

I believe the maximum glide configuration is achieved at minimum RPM (above stall), approximately 90%.

 

He states (backed up with the -10 for the OH-58), that it is 100%.

 

I haven't been able to find a free copy of the B-206 RFM online to support my argument and most of my experience has been with Robinson helicopters. Most of my Google searches turn up 90%, but none of them from official documents.

 

Can anyone provide any insight?

 

Thanks.

 

Looks like you’ve gotten hung-up debating a well know generalization. In general, lowering the rotor RPM at a given forward speed during the autorotative state will stretch the glide distance; however, it’s not a given fact in all flight conditions.

 

Just like there’s an optimum forward airspeed, there is also an optimum autorotative rotor speed that can stretch the glide. This is rotor speed resulting in the most blade elements working at their best angles of attack for maximum lift-to-drag ratios.

 

The average in autorotation was normally around 5°. However, many helicopters were designed to operate at lower angles than optimum under normal conditions to allow margin for adequate capability under more demanding conditions.

 

This means that in conditions such as a steady autorotation at low gross weight and altitude, the rotor efficiency can be increased somewhat by lowering the rotor RPM until the rotor is operating at the optimum average blade element angle of attack.

 

The average blade element angle of attack is related to what is called the blade-loading coefficient (C_t/σ). It’s a concept that lets us see how far from stall the rotor is operating, similar to the coefficient of lift for an airplane's wing.

 

In the past, most rotors were said to be at the peak of their efficiency in forward flight when C_t/σ was around .08. If under conditions the value were less than this, slowing the rotor (increasing C_t/σ) would give better auotrotative performance (efficiency) and if the conditions place the coefficient above the optimum, the rotor speed should be increased (lowering C_t/σ).

 

As you can see from the figures below you need to know something about the design of the helicopter, information from the manufacturer, or RFM to get the rest of the story. Note from the OH-58A/C glide characteristics below, the Army manual limits itself to glides at 100% Nr at various airspeeds. It does not take into account or address changes in rotor RPM.

 

Also note the example helicopter chart at 24,000# gross; an increase in RPM stretches the glide as opposed to the decrease in RPM needed to stretch the glide at 16,000# gross.

 

Sometimes these manuals only address the basic or overall safest procedures applicable to average pilot skill levels. More advance procedures are often not included in the RFM.

 

Click photo to enlarge

 

 

 

Edited July 18, 2014 by iChris