high/low inertia rotors

[rotornut67]

Very enlightening, but I was of the assumption that this was common knowledge. But I was wrong. I was flying with a student for the first time last week who is currently working on his commercial ticket that had never been "taught" the reason behind applying aft cyclic upon entering an auto or as part of low rotor rpm recovery!! When I demonstrated the first low rpm recovery in forward flight he asked "why did you come back with the cyclic?" he had been taught to roll on throttle and lower the collective but never to come aft to drive up the rotor rpm. An hour of ground school later I think the lightbulb came on. This kid has over 150 hrs in Robinsons by the way. That little omission from his flight training should have been caught long ago.

[nightsta1ker]

nightsta1ker,

 

"With high forward speed, there is much more potential energy, and it is easier to get a build in RRPM with even a smaller amount of aft cyclic at cruise speeds than it is at lower speeds. Would you agree?"

 

I mostly agree with your statement, "easier" if the timing is correct, however, the time to apply Cyclic Back (aft cyclic) is critical to not have a massive decay in RRPM as I mentioned about the B206/407 max airspeed in autos. Ask, at the time of engine failure, how much delay in reversing the airflow to a driving inflow and loss of RRPM during this time equates to how fast you can regain that lost RRPM if ever?

 

 

I have been discussing this in my Seminars and giving the example of an R22/44 or S300 (the common training a/c) being at 3,000'agl and nearing the destination airport and pushing the nose down and gaining speed to near Vne in exchange for ROD (at this point us old guys really have to pee). In this disc tilted well forward condition, engine failure will bring on a massive loss of RRPM if immediate aft cyclic is not applied to get the driving inflow established.

 

Now ask yourself how many times you have practiced an auto from this condition (near Vne airspeed) with a surprise chop of the throttle? Probably never and rightfully so for safety's sake. This condition and Cyclic Back (aft cyclic) needs to be part of everyones training even if only discussed in cruise as a "What if, now?" discussion. And if we add to the scenario that during the high speed cruise descent, approach control tells you to switch to tower and your hand is off the collective and the engine quits, what loss of RRPM would we experience before we could (if we ever could) accomplish the three aerodynamic transitions? Oh, more to the scenario, this is a pilot's first solo cross country!

 

Mike

Yes of course you are right about timing. But given the nature of a power failure, particularly a sudden one, the nose will drop, and a pilot's instinct is generally to maintain the previous attitude of the aircraft when it unexpectedly changes (this being a large problem when it relates to low-g mast bumping scenarios). I find, even during throttle chops (which I detest doing as they scare the crap out of me, at least in the R22) that simply keeping the ship level during entry is sufficient to maintain RRPM on entry for a few extra moments while you get the collective down, ESPECIALLY if airspeed is high. Sure, the disc angle is farther forward initially, but the potential energy from airspeed is so much greater, that simply keeping the ship level is sufficient to maintain or even build RRPM, particularly in a low inertia rotor system. You would have to let the nose really drop in order to see rapid decay. And if the pilot has the where-with-all to pitch up slightly, all the better. In fact, I have had some close calls with student's nearly overspeeding the rotors because they entered the auto with high airspeed and pitched back too far on entry. I realize that a real world scenario would warrant and perhaps even demand this, but for the purposes of instructing, care needs to be taken in avoiding damaging the aircraft, which seems pretty easy to do.

[pilot#476398]

For all to view, Pete Gillies on you tube.

 

 

Mike

 

I couldn't read his badge, but it looked like a HeliExpo one? One thing is for sure that 8min shpeel was certainly more interesting than that Rotor Rootor Auto class I took at this last HeliExpo Safety Challenge!

 

So It seems we should enter autos like a "quick stop",...minus the "stop" of course! And maybe practice hand off collective entries?

[Mikemv]

Yes of course you are right about timing. But given the nature of a power failure, particularly a sudden one, the nose will drop, and a pilot's instinct is generally to maintain the previous attitude of the aircraft when it unexpectedly changes (this being a large problem when it relates to low-g mast bumping scenarios). I find, even during throttle chops (which I detest doing as they scare the crap out of me, at least in the R22) that simply keeping the ship level during entry is sufficient to maintain RRPM on entry for a few extra moments while you get the collective down, ESPECIALLY if airspeed is high. Sure, the disc angle is farther forward initially, but the potential energy from airspeed is so much greater, that simply keeping the ship level is sufficient to maintain or even build RRPM, particularly in a low inertia rotor system. You would have to let the nose really drop in order to see rapid decay. And if the pilot has the where-with-all to pitch up slightly, all the better. In fact, I have had some close calls with student's nearly overspeeding the rotors because they entered the auto with high airspeed and pitched back too far on entry. I realize that a real world scenario would warrant and perhaps even demand this, but for the purposes of instructing, care needs to be taken in avoiding damaging the aircraft, which seems pretty easy to do.

What works for you in a particular airframe, WORKS in the safety of the training environment. Good job!

 

Now let me ask what would the RRPM loss, recovery and ROD stabilization be if that same airframe was close to MGW at a high DA? Think higher pitch setting at the time of engine failure, less dense air to drive the rotor, etc.

 

I think Pete's point is that just applying aft cyclic to maintain the prior attitude may not be sufficient to preserve RRPM. He is trying to reach all the pilots in the field that do not recognize the importance of Cyclic Back in heavier, faster aircraft.

 

Cyclic Back (aft cyclic) is something we can all bring to our personal survival, training and passenger protection.

Edited September 9, 2013 by Mikemv

[iChris]

I was flying with a student for the first time last week who is currently working on his commercial ticket that had never been "taught" the reason behind applying aft cyclic upon entering an auto or as part of low rotor rpm recovery!!

 

When I demonstrated the first low rpm recovery in forward flight he asked "why did you come back with the cyclic?" he had been taught to roll on throttle and lower the collective but never to come aft to drive up the rotor rpm.

 

An hour of ground school later I think the lightbulb came on. This kid has over 150 hrs in Robinsons by the way. That little omission from his flight training should have been caught long ago.

 

“Gillies reiterates how to deal with engine failure in a helicopter: “Priority number one: engine quits, cyclic back, pitch down. Priority number two: pick a place to land. Priority number three: make that spot. What you don’t have to worry about is the airspeed. You’re flying a rotary-wing aircraft and rotor rpm is the only thing that counts. Airspeed means nothing. The helicopter will autorotate just fine backwards or sideways, but we’re always emphasizing straight-in or 180-degree autorotations. The real world? Cyclic back, pitch down, pick a place to land. If you have to back up, go sideways, turn around, look this way or that way while you’re coming down, fine. Pick a place to land and don’t miss it.”

by MATT THURBER Article

 

 

Even good articles don’t always further the student’s understanding but lead to more confusion. Airspeed is the main reason “priority number one” (cyclic back) works and without enough airspeed, all you’ll have at the bottom to arrest your descent is the small pool of energy stored in rotor RPM. I think Matt Thurber may have quoted Pete out of context.

Edited September 10, 2013 by iChris

[Mikemv]

Here is a link to our IHST/USHST created safety bulletin (formerly a fact sheet).

 

It was created by Shawn Coyle and reviewed by our peer group.

 

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

 

Read it, download it, print it out, distribute it, discuss it with your CFIs and peers.

 

Pete Gillies is addressing the entry into an auto and concerned about the preservation (non critical loss) of RRPM. Once the three aerodynamic transitions are accomplished and you are in a "Steady State" autorotation fly the aircraft to the spot. Consider Shawn's info on energy and airspeed.

 

Be a pilot, have the knowledge (proficiency maybe?), do your best and fly the helo all the way to the ground.

 

Autos in a setup/training environment and actual engine failures are different. Here is a link to how to train for practice forced landings.

 

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

 

 

And a link to all the Safety Bulletins with more to be listed soon.

 

http://www.ihst.org/Default.aspx?tabid=3089&language=en-US

 

Mike

 

Edit for the link to the new (May 2013) autorotational training AC

 

http://www.faa.gov/regulations_policies/advisory_circulars/index.cfm/go/document.information/documentID/1021176

Edited September 10, 2013 by Mikemv

[rubidug]

Thanks again Mike and Pete for spreading the 'word'. We now have a 'collective' of aft cyclic fans (sorry that was bad).

 

Pete is an amazing character! His passion for flying, people, and saving lives is obvious. He is not here to make money or friends or fame, he is here to spread the word and save lives, plain and simple.

 

I am very fortunate in that I get to converse and work with Pete on a daily basis. Not too many people on the planet get to train with TWO instructors that have over 20,000 hours each, not to mention a plethora of real-world experience. Look at the pictures on the wall. These guys were doing this when I was in diapers, and continue with that passion today. Awesome.

[Rupert]

The later Bell 47's, the 205 and the 206 have notably high inertia rotor systems. In fact, they have weights in the blade tips intended to increase rotor inertia. One can recover lost NR easily in a high inertia system if maintenance has rigged the collective to allow enough negative pitch. Some flight school operators rig their high inertia systems towards the top, meaning not much negative pitch, because they have concerns about too high NR in practice autos, if the pilot doesn't pay attention.I prefer all the negative pitch I can get.

 

One tends to see low inertia systems associated with governed and twin-engined aircraft, but not always. Designers can reduce overall weight of the aircraft by taking weight out of the rotor system.

 

The EC 135 has the lowest inertia rotor system of my experience. Even with the instructor and student coordinating the throttle for practice autos, you'll get a low rpm audio. Not pleasant.

 

For flight schools, high inertia systems make for much easier autos, with more time to understand the variables. Fewer accidents, I think, with high inertia systems.

[iChris]

One can recover lost NR easily in a high inertia system if maintenance has rigged the collective to allow enough negative pitch. Some flight school operators rig their high inertia systems towards the top, meaning not much negative pitch, because they have concerns about too high NR in practice autos,

 

Most collective pitch ranges are in the positive; moreover the pitch range is specific to the overall design goals for each helicopter. Guidelines are already set in the maintenance manual. In general, a low inertia system has the ability to recover RPM faster than a high inertia system. A few collective pitch ranges are listed below:

 

R22 Beta [+1.5 +14.5]; 300C [+3 +15]; Bell 412 [0 +16]; MD500E [0 +14.3];

CH-53E [-1.4 +19.6]; BK117 [-1.8 +13.3]; BO105CB [-0.2 +15]; UH-60A [+9.9 +25.9]

 

The minimum collective setting normally corresponds to a target range of rotor speeds in autorotation at the extreme combinations of low gross weight and low-density altitude in the design envelope. The example below gives maintainers a target for autorotation Nr that the Bell 407 should meet within 2%. Adjustments are made at the pitch links to made corrections as needed. Other helicopters have similar charts and rigging adjustments outlined in their maintenance manual.

 

Low gross weight, low temperature/low density altitude result in lower autorotation RPM when compared to high gross weight, high temperature/high density altitude.

 

Forward flight, hover, and other performance considerations influenced by rotor disk area, rotor rotational speed, rotor radius, and overall disk load sometime trade-off optimal autorotation characteristics for other optimal design goals.

 

Edited September 17, 2013 by iChris

[rubidug]

How is this possible?

 

UH-60A [+9.9 +25.9]

 

 

My RC heli's are all 3D (upside down hover capable). My pitch is dialed in at +12, -12. Beyond that, there is too much drag, and blade stall just slows down RRPM. Most pilots shoot for 10-12 degrees.

 

26 degrees is more than double! What gives?

[iChris]

How is this possible?

 

UH-60A [+9.9 +25.9]

 

 

My RC heli's are all 3D (upside down hover capable). My pitch is dialed in at +12, -12. Beyond that, there is too much drag, and blade stall just slows down RRPM. Most pilots shoot for 10-12 degrees.

 

26 degrees is more than double! What gives?

 

You need to think large multiengine helicopters designed for high gross weight and high forward speed. You’re still thinking RC helicopters and light single engine helicopters.

 

The average angle of attack over the rotor disc determines the rotor thrust. The collective pitch controls the average value, which also must be high enough to compensate for the inflow velocity through the rotor disc at high forward speed and high gross weight. At hover, the inflow is only the induced velocity, but in level forward flight a component of forward velocity perpendicular to the rotor disc is also coming through the rotor, this additional inflow is due to the rotors forward tilt.

 

The induced velocity is decreasing as speed increases while at the same time the forward flight component perpendicular to the rotor disc is increasing thereby decreasing the average angle of attack. Therefore the collective pitch needed to overcome the effects of the inflow (plus the effects of parasite drag) is changing in a way similar to the old power required curve. Those collective angles are not uncommon to large multiengine helicopters. It depends on each manufactures design philosophy.

 

 

Edited September 18, 2013 by iChris

[rubidug]

 

You need to think large multiengine helicopters designed for high gross weight and high forward speed. You’re still thinking RC helicopters and light single engine helicopters.

 

 

So, if I put another engine on it, and add a bunch of weight, the blades wont stall past 12 degrees? That doesn't make sense to me...

 

BTW, it has plenty of excess power. Compared to a full scale heli, it would probably have more than double the available power. SO, the engine is irrelevant. Power to weight ratio is very different.

 

Sorry~ Still confused. Will be taking this up with my instructors for clarification

[iChris]

 

So, if I put another engine on it, and add a bunch of weight, the blades wont stall past 12 degrees? That doesn't make sense to me...

 

Sorry~ Still confused. Will be taking this up with my instructors for clarification

 

Also, lets not forget the difference between blade pitch angle and blade angle-of-attack. The induced velocities through the rotor disc modify the relative wind into a resultant component at the blade; therefore, a blade pitch angle (angle of incidence) of 23 degrees could yield a blade angle-of-attack of 11 degrees.

 

Blades stall at a given angle-of-attack (AOA), the actual pitch angle (angle of incidence) may vary.

 

You’re hung-up on a purely mechanical range the manufactures designed into their rotor hub. In general, the maximum collective pitch is required at the end of an autorotation after the flare, to allow all the available rotor energy to be used as the rotor slows down during the final collective pull.

 

SEE Page 2-10 Angle of Incidence & Angle of Attack

Helicopter Flying Handbook Chapter 02: Aerodynamics of Flight

 

Edited September 20, 2013 by iChris

[aeroscout]

Good stuff iChris.