high/low inertia rotors

[ashar]

hi everyone!

im learning to fly an alouette III helicopter theses days....and my instructor asked me to find out what type of a rotor is fitted on the heli either a high inertia or low inertia and ive been stuck on it for a couple of days now. can anyone please help me out on what is rotor disc inertia and what makes a rotor high or low inertia

[pilot#476398]

Ask your instructor to demonstrate an autorotation. If you glide like a feather, its high inertia, if you drop like a brick, its low!

[ridethisbike]

I'd say that's debatable.

 

 

I'm going out on a limb and saying that it's a high inertia rotor. There's 3 blades and, therefore, more momentum to keep them going prior to going through windmill brake state. I'm not sure of blade chord or materials used though, so for all intensive purposes, I could be very wrong.

[pilot#476398]

Three blades = high inertia huh? You've never flown an S300 have you!?

[ridethisbike]

Nope. But like I said, I could be wrong. Biggest ship I've flown is a 44. I don't know what makes one system high inertia when compared to another by just looking at it.

[Wally]

Inertia is inertia, simplistic but true. If the autorotatonal NR changes quickly, it's a low inertia rotor; if not, it ain't. That cuts both ways, you can speed a low inertia rotor up easily, but it also slows as easily and usually has a wider operating speed.

Some low inertia rotors do pretty well in an established descent, it's the entry and the termination that demand pilot skill in NR management. You can wind that pig up tight while you're changing heading, for example, which will get you a frowny face on a check ride...

 

Examples, max glide range technique-

High inertia rotor, low green NR and high end of VX/VY speed, terminal flare to check descent and ground speed will be slow in bringing NR up for cushion, you might not see the high green even with plenty of useful energy at the end;

Low inertia system, same NR/speed parameters, NR builds quickly in the flare, could well overspeed without management.

Edited September 4, 2013 by Wally

[500F]

LOW, and HIGH are undefined subjective terms. There is no correct answer to his question unless he defines low and high. Don’t tell him that unless he happens to also be an engineer or physics professor, it is sure to piss him off.

 

Low and high intertia rotor systems are common terms used to describe autorotational characteristics of different rotor systems but there are really no defined parameters.

 

 

 

Inertia = mass times velocity squared. Velocity is the primary factor since it is squared, but since the tip path speed of most helicopters are very comparable @100%Nr, the deciding factor in inertia is normally mass. How heavy the rotor blades, (particularly the final third of the blade) are.

 

The number of blades does not matter, the combined mass does.

.

 

iChris is due any minute with an answer 10x as informative as mine, but that’s the basic idea.

 

[Eric Hunt]

 

Inertia = mass times velocity squared.

 

Close to the mark, it is actually 1/2 I w squared, where I is the Moment of Inertia (the resistance to turning) and the stylised w is "omega", the rate of rotation in radians per second.

Signed,

The Pedantic Police.

[iChris]

hi everyone!

im learning to fly an alouette III helicopter theses days....and my instructor asked me to find out what type of a rotor is fitted on the heli either a high inertia or low inertia and ive been stuck on it for a couple of days now. can anyone please help me out on what is rotor disc inertia and what makes a rotor high or low inertia

 

With respect to rotor inertia, you’re stuck with what the manufacturer gives you. It’s a design trade-off the manufacturer makes between high inertia for good autorotative entry and flare characteristics and low inertia for minimum blade and hub weight.

 

Rotational inertia, resistance to a change in angular velocity, is termed as I=mr². As you can see you get the most from moving “m” (the center of mass) farther out increasing r² (radius). That being the case, adding small tip weights increases blade inertia.

 

The rule-of-thumb in the past for rotor designers was that the kinetic energy stored in a single engine helicopter at normal RPM should supply the power required to hover for at least 1.5 seconds before the rotor speed decays below blade stall. As an example, the Army tested a modified OH-58 with high-inertia blades that well exceeds that (3.0 seconds equivalent hover time).

 

The light R22 design benefits from its minimum blade and hub weight by trading-off some rotor inertia. However, it's a race to get the throttle fully rolled-off before the helicopter reaches the ground during a practice hover-auto.

 

It’s subjective; some testing had shown that the 1.5 seconds was an adequate margin for average pilots. Whether the requirement of 1.5 seconds must also apply to multi-engine wasn’t demonstrated. There doesn’t seem to be any objective numerical line set between low and high. Enstrom Helicopter quotes a high inertia rotor system, does that just mean it’s higher than an R22? What are the quantitative numbers?

 

I'm going out on a limb and saying that it's a high inertia rotor. There's 3 blades and, therefore, more momentum to keep them going prior to going through windmill brake state. I'm not sure of blade chord or materials used though, so for all intensive purposes, I could be very wrong.

 

The term Windmill Break State is often used incorrectly to imply rotor RPM decay.

 

In general terms, the main rotor flow states are, Normal Flow State, Vortex Ring State, Autorotation Flow State, and Windmill Break State.

 

The Windmill Break State represents increasing rotor speeds, not decreasing speeds.

 

At high rates of descent the airflow is almost entirely up through the rotor system. The rotor system is acting like a windmill and is extracting more energy from the air than is required. In this case, some of that energy must be extracted to protect against rotor over-speed by increasing collective pitch.

 

 

"Inertia = mass times velocity squared."

 

 

Close to the mark, it is actually 1/2 I w squared, where I is the Moment of Inertia (the resistance to turning) and the stylised w is "omega", the rate of rotation in radians per second.

 

 

“½mv^{2 “} relates to kinetic energy stored in linear motion (E_k=½mv²) not inertia.

 

“½Iω²” relates to kinetic energy stored in rotational motion (E_k=½Iω²).

 

If we say; I (Inertia) =½mv² then we’re also saying an object at rest (zero velocity) has zero inertia. In the case of kinetic energy, that’s true, an object at rest (zero velocity) has zero kinetic energy, not so in terms of inertia

 

If you look at, E_k=½Iω², an object at rest (zero rotational velocity ω²) has zero kinetic energy; however, a state of inertia remains.

 

Inertia: the tendency of an object to keep moving, or to keep still.

Edited September 7, 2013 by iChris

[rotornut67]

 

With respect to rotor inertia, you’re stuck with what the manufacturer gives you. It’s a design trade-off the manufacturer makes between high inertia for good autorotative entry and flare characteristics and low inertia for minimum blade and hub weight.

 

Rotational inertia, resistance to a change in angular velocity, is termed as I=mr². As you can see you get the most from moving “m” (the center of mass) farther out increasing r² (radius). That being the case, adding small tip weights increases blade inertia.

 

The rule-of-thumb in the past for rotor designers was that the kinetic energy stored in a single engine helicopter at normal RPM should supply the power required to hover for at least 1.5 seconds before the rotor speed decays below blade stall. As an example the high inertia UH-1 design well exceeds that.

 

The light R22 design benefits from its minimum blade and hub weight by trading-off some rotor inertia. However, it's a race to get the throttle fully rolled-off before the helicopter reaches the ground during a hover-auto.

 

It’s subjective; some testing had shown that the 1.5 seconds was an adequate margin for average pilots. Whether the requirement of 1.5 seconds must also apply to multi-engine wasn’t demonstrated. There doesn’t seem to be any objective numerical line set between low and high. Enstrom Helicopter quotes a high inertia rotor system, does that just mean it’s higher than an R22? What are the quantitative numbers?

 

 

The term Windmill Break State is often used incorrectly to imply rotor RPM decay.

 

In general terms, the main rotor flow states are, Normal Flow State, Vortex Ring State, Autorotation Flow State, and Windmill Break State.

 

The Windmill Break State represents increasing rotor speeds, not decreasing speeds.

 

At high rates of descent the airflow is almost entirely up through the rotor system. The rotor system is acting like a windmill and is extracting more energy from the air than is required. In this case, some of that energy must be extracted to protect against rotor over-speed by increasing collective pitch.

 

 

“½mv^{2 “} relates to kinetic energy stored in linear motion (E_k=½mv²) not inertia.

 

“½Iω²” relates to kinetic energy stored in rotational motion (E_k=½Iω²).

 

If we say; I (Inertia) =½mv² then we’re also saying an object at rest (zero velocity) has zero inertia. In the case of kinetic energy, that’s true, an object at rest (zero velocity) has zero kinetic energy, not so in terms of inertia

 

If you look at, E_k=½Iω², an object at rest (zero rotational velocity ω²) has zero kinetic energy; however, a state of inertia remains.

 

Inertia: the tendency of an object to keep moving, or to keep still.

 

"Inertia: the tendency of an object to keep moving, or to keep still."

 

Love that last sentence! That will go in my lesson plans for sure...but the bulk of your post made my head hurt Chris. Reminded me too much of Physics classes in college!!

[ridethisbike]

I was once taught the the transition from powered flight to aerodynamic flight was called windmill brake state. Which is why I said while you "go through" said state.

 

So if windmill brake state is the point at which aerodynamic flight begins, what is that couple second transition called? The upward flow of air isn't instantaneous, afterall...

[nightsta1ker]

Ask your instructor to demonstrate an autorotation. If you glide like a feather, its high inertia, if you drop like a brick, its low!

I think you have some misconceptions about how this works. The R22 has less inertia in the rotor than the S300, but it glides much better than the 300 does. Rotor inertia is determined by the mass of the blade, and manifests itself in how easily it gains or loses momentum. A low mass blade has low inertia, and will therefore be more easily subject to changes in momentum. A high mass blade has more inertia, and will be less easily subject to changes in momentum. In low inertia systems, rotor RPM is gained and lost very easily. Comparatively, the S300 does not gain or lose RRPM as fast as the R22. But that does not affect it's autorotational glide performance. Despite having slightly higher inertia rotors than the R22, the S300 is quite a bit heavier, has a lot more form and parasite drag, and therefore, does not glide as well as the R22. You do have a much more forgiving rotor system though, with overspeeds a rarity and low rotor RPM blade stall almost unheard of, whereas in the R22, both of these have been known to be a common problem.

 

My point here is: Don't mistake rotor inertia with glide performance, they are not the same thing.

[iChris]

I was once taught the the transition from powered flight to aerodynamic flight was called windmill brake state. Which is why I said while you "go through" said state.

 

So if windmill brake state is the point at which aerodynamic flight begins, what is that couple second transition called? The upward flow of air isn't instantaneous, afterall...

 

The important fact is there’s an instantaneous change in the aircraft’s equilibrium as gravitational and drag forces start to take immediate effect. However, the effects may have yet to reached their steady state or maximum magnitude.

 

The transition you’re talking about is the start of the Autorotative State and that’s what it’s called, the Autorotative Flow State. In fact the Autorotative State is termed the boundary between conditions where engine power must be delivered to the rotor to prevent RPM decay and were power must be extracted to prevent rotor overspeed. The Windmill Brake State is experienced at high rates of descent were airflow is almost entirely up through the rotors.

 

I assume you’re referring to the transition from the normal flow state, during power flight, to the aerodynamic flow state termed autorotation. Once the engine quits, the rotor speed will immediately start to decay and lift will diminish as the rotors extract the remaining kinetic energy stored in the rotational motion in an effort to provide lift and overcome drag. However, this effort is short lived and unless the pilot takes action, within the next few seconds, the rotors will stall.

 

Those couple seconds of transition are called, critical. The pilot must make the transition from the normal flow state were engine power was driving rotor speed to an autorotative descent, were the upward flow of air is driving rotor speed. The designers have accounted for this transition phase by insuring the rotor system will provide enough inertia to bridge that gap and make a safe autorotative entry.

 

You’re autorotative entry isn’t complete until the helicopter begins descending and air is flowing up through the rotors. Until this is accomplished rotor speed will continue to decay even with full down collective. However, in some schools of training the first step after an engine failure in forward flight is to not only lower collective, but also to simultaneously apply aft cyclic to recover kinetic energy stored in the aircrafts forward airspeed, thereby delaying the rotor decay. This technique effectively emulates an increase in rotor inertia.

Edited September 7, 2013 by iChris

[iChris]

 

"Inertia: the tendency of an object to keep moving, or to keep still."

 

Love that last sentence! That will go in my lesson plans for sure...but the bulk of your post made my head hurt Chris. Reminded me too much of Physics classes in college!!

 

 

OK...... How about

 

 

1) Inertia: the tendency of an object to keep moving, or to keep still.

 

2) E_k=½mv² and E_k=½Iω² = If you want to store more energy in your rotor system the best way is to turn the rotor faster (increase RPM) and/or add weight to the blades.

 

3) I=mr² = In most cases to increase rotor inertia the best place to add weight is near the blade tips.

Edited September 7, 2013 by iChris

[pilot#476398]

_{So Ashar, have you found the answer yet?}

[Mikemv]

"However, in some schools of training the first step after an engine failure in forward flight is to not only lower collective, but also to simultaneously apply aft cyclic to recover kinetic energy stored in the aircrafts forward airspeed, thereby delaying the rotor decay. This technique effectively emulates an increase in rotor inertia."

 

 

Just this week, Pete Gillies from Western Aircraft and I have been e-mailing about his efforts to take the Cyclic Back concept forward in the industry to reach all helicopter pilots and save lives. USHST has addressed this with a Safety Bulletin/fact sheet.

 

In a nutshell, Pete states that "at the moment of engine failure in any mode of flight, simultaneously apply aft cyclic and bottom the collective". He understands that the amount of aft cyclic varies but states that this positive aft cyclic movement will not hurt the entry into ANY autorotation and will be the MOST positive way of preserving RRPM!

 

Here is a link to Pete's article, read it and absorb to survive!

 

http://ainonline.com/aviation-news/aviation-international-news/2013-05-01/astar-accident-shines-light-autorotation-training

 

 

I promised Pete to assist in getting this message out to all helicopter pilots. It will be addressed in the updates of FAA Handbooks that FAA/USHST are working on.

 

Rubidug, please let Pete know about this post!

 

Sincerely,

 

Mike

Edited September 8, 2013 by Mikemv

[WolftalonID]

Thanks Mike! I love that line, take the cyclic back. What a way to provide simple primacy at a life saving level! This is a good way to teach this.

[eagle5]

Interesting article. Especially this part;

 

The problem, he explained, is that during training helicopter pilots are taught a smooth entry to autorotation. “When we’re taking checkrides we do the best we can to make a smooth coordinated entry so the aircraft goes from normal flight into a nice descent. That is the point–the beginning of the autorotation–where if it is not done the way I’ve described with the cyclic coming back, you’re headed for a severe problem. You lose control of the helicopter and it can never be brought back. Almost all autorotations are begun with the pilot putting the pitch [collective] down smoothly, bringing the cyclic back smoothly and putting the engine at idle. You don’t chop the throttle then expect the student to react. You make a smooth entry. Many pilots think this is the way all autorotations begin.”

 

My instructor used to chop the throttle on me all the time. I would slam the collective down so fast that sometimes it would take him by surprise causing him to brace himself!

 

Athough I was never taught to pull aft cyclic during the entry, I was taught to keep the ship level (not allow the nose to drop)! I guess its the same "basic" principle just from a different angle?

 

I would find it hard to pull aft cyclic upon entering an auto in the 22 since the rpm increases so quickly that I always have to pull up on the collective a second or two after to prevent overspeed,...but I guess that's because I'm not letting the nose drop.

 

I should be at Robinson in November, so I guess I'll see what Tim says about it then?

[Mikemv]

Interesting article. Especially this part;

 

 

My instructor used to chop the throttle on me all the time. I would slam the collective down so fast that sometimes it would take him by surprise causing him to brace himself!

 

Athough I was never taught to pull aft cyclic during the entry, I was taught to keep the ship level (not allow the nose to drop)! I guess its the same "basic" principle just from a different angle?

 

I would find it hard to pull aft cyclic upon entering an auto in the 22 since the rpm increases so quickly that I always have to pull up on the collective a second or two after to prevent overspeed,...but I guess that's because I'm not letting the nose drop.

 

I should be at Robinson in November, so I guess I'll see what Tim says about it then?

Eagle 5,

 

Not the same basic principle! Maintaining the current attitude may kill you especially in high airspeed conditions even in light training helos. This is Pete's point exactly! Move to a B206, 206L, 407 and be above the barber pole or blue line and maintain attitude and have a loss of RRPM and ROD of 4K/fpm and increasing.

 

Tim is in agreement with this and stated so in the article I believe, maybe elsewhere? I have been swimming in interaction lately with both of them so forgive me if I am wrong about Tim supporting this in the article. Pete sent me numerous articles. I know Tim agrees from our IHST,USHST web conferences.

 

If you were in a high cruise descent airspeed in the R22 and switching the radio freq. with your collective hand and the engine quit, CYCLIC BACK will preserve a loss of RRPM and extreme ROD and save your life. Our collective hand is our "working hand" while our cyclic hand is our "controlling hand".

 

The set up training environment auto is not the point but CYCLIC BACK (aft cyclic) and collective down as an initial response will save your life. You could check RRPM (be a pilot-fly the aircraft) to prevent an overspeed or not and not die.

 

Mike

[Mikemv]

Thanks Mike! I love that line, take the cyclic back. What a way to provide simple primacy at a life saving level! This is a good way to teach this.

WolftalonID,

 

We are working on getting this Cyclic Back (aft cyclic) simultaneous reaction/entry principle into the handbooks at revision. Pete is concerned about getting this out to all experienced pilots in the field as I/others concentrate on the training environment. Save everyone, protect the sacred trust that your pax place in you as PIC.

 

The Mosby AS350 pilot never applied aft cyclic and in Sim. reproductions had a ROD of I believe 6K/fpm, and killed everyone (med crew and patient)!

 

Mike

[Mikemv]

For all to view, Pete Gillies on you tube.

 

 

Mike

[eagle5]

I guess they could just add one more thing to the PTS, a high-speed auto entry?

[nightsta1ker]

"However, in some schools of training the first step after an engine failure in forward flight is to not only lower collective, but also to simultaneously apply aft cyclic to recover kinetic energy stored in the aircrafts forward airspeed, thereby delaying the rotor decay. This technique effectively emulates an increase in rotor inertia."

 

 

Just this week, Pete Gillies from Western Aircraft and I have been e-mailing about his efforts to take the Cyclic Back concept forward in the industry to reach all helicopter pilots and save lives. USHST has addressed this with a Safety Bulletin/fact sheet.

 

In a nutshell, Pete states that "at the moment of engine failure in any mode of flight, simultaneously apply aft cyclic and bottom the collective". He understands that the amount of aft cyclic varies but states that this positive aft cyclic movement will not hurt the entry into ANY autorotation and will be the MOST positive way of preserving RRPM!

 

Here is a link to Pete's article, read it and absorb to survive!

 

http://ainonline.com/aviation-news/aviation-international-news/2013-05-01/astar-accident-shines-light-autorotation-training

 

 

I promised Pete to assist in getting this message out to all helicopter pilots. It will be addressed in the updates of FAA Handbooks that FAA/USHST are working on.

 

Rubidug, please let Pete know about this post!

 

Sincerely,

 

Mike

Mike,

 

It shocks me to hear that this is not common knowledge.

 

The one thing I don't get, is where the article says that at higher cruise speeds, you need more aft cyclic when you enter... It does NOT quote Pete as saying it, and I think it's a misconception on the part of the author. 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?

Edited September 9, 2013 by nightsta1ker

[ridethisbike]

Isn't airspeed kinetic energy?

 

Fresh from the robinson course, the kinetic energy formula they gave would agree with you. As would I, even without the formula.

 

KE = 1/2 xMass x Velocity²

 

That V² bit can make a huge difference...

[Mikemv]

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