R22 Corner

[fulldownauto]

If after lowering the lever, RRPM continues to fall, the rotor has started to stall. Left roll is likely. flare will push the disc further into the stall. Judicious forward cyclic to reduce AOA is your only chance.

Say both blades are close to stall, forward cyclic is put in, the angle of attack on the retreating blade is increased, now why would a retreating blade be less likely to stall than an advancing blade? When it does stall, the advancing blade doesn't, but the result of the aft cyclic, the disk has started to tilt aft, and the retreating blade stalls, which would have the same effect as a full rotorstall because the drag is dramatically increased. You can demonstrate rotorstall close to the ground, it's dramatic, 75% and boom, all RPM is gone. Since you're traveling forward, and the retreating blade has stalled, the rotorsystem will blowback and chop the boom.

Quote from SN-24
"When the rotor stalls, it does not do so symmetrically because any forward airspeed of the helicopter will produce a higher airflow on the advancing blade than on the retreating blade. This causes the retreating blade to stall first, allowing it to dive as it goes aft while the advancing blade is still climbing as it goes forward. The resulting low aft blade and high forward blade become a rapid aft tilting of the rotor disk sometimes referred to as "rotor blow-back". Also, as the helicopter begins to fall, the upward flow of air under the tail surfaces tends to pitch the aircraft nose-down. These two effects, combined with aft cyclic by the pilot attempting to keep the nose from dropping, will frequently allow the rotor blades to blow back and chop off the tailboom as the stalled helicopter falls."

Tim Tucker had the RPM in the certification tests they were doing for the R22 down to 73%. I'd like to ask him, but my guess is he didn't have aft cyclic in when recovering. Too bad they didn't have mini video cams back then, would be some very interesting footage.

James

[13snoopy]

Fulldownauto,
To answer your question regarding Gaseous' assertions about retreating blade stall? Retreating blade stall has absolutely nothing to do with blade stall in an auto. Again, this Gaseous fellow is totally misinterpeting a different set of circumstances and applying them to autos. I say his poster name is correct though.

[fulldownauto]

I used the term retreating blade stall. Look at the context. I clearly wasn't referring to high forward speed retreating blade stall.

What we're talking about here, and I can't figure out why this is hard to grasp, is the effect of cyclic movements when the rotorsystem is near stall. That's it! Just a moment in time. Not a sweeping rethinking of how engine failures should be handled.

In the context it was in (read the SN, they mention "This causes the retreating blade to stall first") how could you possibly think I was referring to high forward speed retreating blade stall??

We can have intelligent discussions about theoretical situations without confusing people as to what they know to do and actually thinking people would misinterpret what is said here and apply it to when they have an engine failure. We're all going to do as we're trained and have experienced.

Back to the points. Since my Socratic method hasn't worked, I'll say what I think. I think that any cyclic movement when the rotor is near stall is not good. Keep it centered, and if you've got the collective full down, it's just a matter of waiting till the aircraft descends fast enough for the RPM to start building.

Thanks,

James

[3top]

Hi guys,

FWIW:

a) If I remember right from the last 2 safetycourses, the 1.1 sec figure is only true IF you want to "keep speed + altitude the same as before the power failure".
You loose all power, you immediately will start to slow down and start to sink if you do nothing. So here you already got some time coming towards you.
IF you really wanted to keep v+h you would have to get some forward cyclic and collective INCREASE at the same time, then of course everything is over in 1.1 sec,

b) The suggestion in the safetycourse was " to apply gentle aft pressure on the cyclic to maintain the nose more or less level. Basically don't let the helicopter accelerate."
While "gently" applying aft pressure you also would want to lower the collective .

c) "Slamming down the collective!" This is a no-no!!
Slamming/snapping down the collective does nothing good - it will only get you towards a low-g situation (which in an auto is not as bad as in a regular push-over, but....).
All you will do is go nearly through the roof!
Correct action is to lower the collective SMOOTH and QUICK, but please don't slam it. Give the rotor some time to change the airflow!

back to b) The faster you fly when 0-power hits, the faster Nr will decay. Worse yet the faster you fly the more inertia the helicopter will have (the whole machine, NOT the blades!), which means it will not want to really start to decend, but keep going straight (I think that Newton-guy found that out some time ago...)

So back to the safetycourse (and reality), if you are fast it will take some time to get a decent decent rate. Now as we will need something to convert to maintain Nr, the only thing we have for the moment is speed.
So if you would like to keep up Nr in the comfort range you will HAVE to do a "GENTLE" flare, produced by the "GENTLE" backpressure on the cyclic.

Those who attended the safetycourse will remember the graphs that showed the amount of energy you get from decelerating from higher speeds towards 60 kts - V2(squared) does magic,

However it is very important to not overdo this as you can maintain altitude perfectly well nearly all the way down to 25-30 kts by flaring hard enough. Only you have nothing left to convert if you are at some serious altitude at this point.
The idea is to just hold back enough (decelerate) to keep Nr happy until the decent-rate is good enough to start milking altitude for energy.

With enough practise you can do throttle-chops (not recommended by RHC or me) at 90 kts and never hear the low rpm-horn (98%) until the level-off at the bottom of the auto.

"With enough practise" is the key-word!!

When I do occasional pre-solo checks, all I really care for (in the auto department...) is a correct AUTOMATIC reaction from the student if something goes towards low Nr. And mostly they are surprised. After a couple of real entries they seem to asimilate the idea.

You can't expect a student to be perfect when going for a solo, especially in a R-22 and around 20 hrs total time. All I care for is that they are able to get into a stabilized auto. At the bottom they will do something and most likely they will walk away in the unlikely event.

I don't usually chop throttles, but I will start to roll off power slowly to get the low rpm horn. After a couple of trials the reaction becomes automatic. Smoothly lowering the collective and SIMULTANIOUSLY applying "GENTLE" backpressure.
At this time we are not practising perfect autos (1-2-3 lower collective and roll it off) but reaction to the unexpected.
Mostly I start to roll the throttle towards low rpm on the way back to the hangar a couple of times too, and things get more relaxed all the time. Most students even start to react before the warning horn comes on (just hearing the change in rpm) - collective starts to go down smooth and cyclic starts to apply back pressure ever so subtle.

Remember not all low Nr is because of a power failure: Some students and "done" pilots are nervious and grip the throttle too hard so the governor can't do its job - reaction should be the same.

As mentioned the R-22 is not going to pardon slow reaction time, so it is a basic requirement to get your reactions to any changes in Rotor-sound and/or Engine-sound AUTOMATIC, like putting your foot on the brake pedal if you see something unexpected in front of you on the road. You might not want to brake after all, but you want to be ready - AUTOMATICALLY.

Don't blame the machine for your lack of practise or wrongly learned maneuvers!

d) "Increasing rpm in turns" This happens mostly because of deceleration in the turn. Try it out, concentrate on keeping that 60-65 kts in the turn and your rpm hardly moves! Decent-rate will increase some, but will also come back when you straigten out. While maintaing your speed the nose will go a little low too, but rpm will hardly move.......keep it at or below 30ş bank too!

You don't have to fly the R-22, constantly awaiting desaster, most likely it will never happen. Just spend some quality-time with your instructor practising autos/reactions/etc.
"You shouldn't feel right if you didn't practise at least 1 auto everytime you are out flying! " Autos should become fun to do, not a dreaded necessity.......
("1 auto everytime out" is mainly aimed at low time pilots, I understand that in the commercial world this will mostly not be possible. Also for low timers this should be done with your safety-course equipped CFI!!)
Just plan for some 15 min extra and do some quick-auto-warm-up with your CFI. Then kick him out, get your pax and go fly!!

Cheers,
3top,

PS: I did the "GENTLE" thing repeately and on purpose - It is very important! I don't like big stick moves!! Helicopters want as little movement on the controls as possible and if you do, you actually want to think about "applying pressure" rather than moving - whatever you do, do it smooth, no need to snap the controls anywhere ever and once you're done quit stirring the fuel...........

[3top]

The static weight on the blades wouldn't be the problem.

The problem would be blade grips, spindle bearings, rotor head, mast, trans, etc.

but it ends the same, call it 44

The new R-22 blades are no longer Alu wrapped, but stainless like the R-44 blades, however to maintain the weight, it is rather thin stuff covering the honeycomb!

3top

[Haggis Hunter]

am I right in thinking that the tip weights are roughly 6lbs? remember hearing this somewhere before... HH

[Kyrilian]

Gaseous,
You say "If you've got the lever down and your Nr is still decaying you should come back and haunt your engineer! (or mechanic if you live in USofA). Helicopters should be trimmed to maintain autorotation with the lever down and should do to the point where the rotor stalls."
Agreed!

Another concern beyond misrigging is the possibility of something getting stuck under the collective. Obviously this is something that we don't want to happen, but if you lose an engine and simply cannot get the collective far enough down due to a handheld radio or misrigged collective, turns were my suggestion.

Regarding the aerodynamics, I erred. You are right, the outermost region is the driven region, rather than the driving region. Shame on me. I should (do) know better.

If we agree that the retreating side is the likely area for stall then consider my remark about reverse flow. You are right in your point that the angle of attack at the blades will increase and the stall region will move outward. However, I believe the impact is relatively small and may be offset by blade angle changes caused by a push on cyclic. Airflow impinging the bottom of the disk on the retreating side in an auto is at some acute angle. In theory, if we're doing 70kts and descening at 1500 fpm (pulling number out of my butt here--actual numbers are irrelevant), the disk is seeing airflow coming at approximately 12deg angle from horizontal at the 9 o'clock position if the ship is level. If we pitch up a degree, this inflow angle becomes 13deg. Fair enough? At 80% NR the 75% rotor radius of the blade is moving in plane at roughly 423fps, or 25380fpm relative to the aircraft. Thus, at zero blade angle, where we have 12deg disk inflow, the angle of attack is 4.687deg. The blade is seeing 1500 fpm in the vertical direction and 18296 fpm in the in-plane direction (25380 - 7084). When the disk pitches back a degree, the angle of attack is 4.956deg, a change of only 0.269deg for the 1deg disk pitch up.

Also, consider that when you pull back on the stick you are increasing pitch on the advancing side (primarily--a bit over the nose as well owing to the delta-3) and decreasing it on the retreating side. Therefore pushing forward cyclic will lead you closer to stall on the retreating side. Whether pulling back enough to yield a one degree disk pitch up will give more or less than a 0.269deg reduction in local blade angle, I don't know.

Please realize the above contains many assumptions and I've been quite loose with theory (delta_blade angle is a function of delta_angle of attack for instance). I think it helps to bring out the factors affecting the angle of attack however. I'd still like to delve a bit deeper to feel better about this analysis.

"No one really knows what the dead pilots of all the crashed R22s did last. I suspect some of them did a big flare with low Nr because they thought it would increase Nr. Imagine their surprise when the rotor stalled."

Perhaps they pushed forward

<edited to change "ˇă"s into more meaningful terms>

[bugdevheli]

r22 tip weights?

The last set of 22 tip weight I cut out were 2 pounds per blade. held in place on the front spar by if I remember correctly, 4 quarter bolts.

[Gaseous]

I'm not trying to misrepresent anything. I'm trying to work out the complex physics that goes on at rotor stall. I can find nothing in my text books about it and I am trying to put together the bits of the puzzle.

I think we can probably all agree that the rotor stalls due to the angle of attack becoming too large.

Here are 3 extracts taken from Norman Baileys book. I hope he will not mind me using them. He knows who I am.

Cyclic input alters the plane of rotation. NOT the angle of attack. (Bailey, principles of flight, p40).

forget the red herring of cyclic affecting pitch. do not confuse pitch with angle of attack. forward cyclic will not increase angle of attack on retreating blade

When the disc is flared, a componant of the horizontal airflow will now be opposing the induced flow. The change in direction of the airflow relative to the blade will INCREASE the angle of attack and therefore an increase in total rotor thrust. (Bailey, principles of flight, P100).

It is with this in mind that I suggest that a flare will push the disc towards a stall. If this is true, the converse must be true.

...At the same time, section D at the root becomes stalled and the extra drag generated causes a reduction in the size of the autorotative area and a rpm decrease. (Bailey, p50)

There is a diagram on page 51 which shows the stalled area of the blade is larger on the retreating blade than on the advancing blade.

It is with this in mind that I make my assumption of left roll as stall approaches.

[bugdevheli]

Autorotate a R22

The most frightening statistic that has appeared in these posts is the vast differences of opinion as to what actions to take in an emergency situation. I am informed that the most hazardous part of a test pilots job is testing the autorotational limits on any new design. It would appear that a lot of pilots test these limits for free.

[Lu Zuckerman]

Gotta question.

To: bugdevheli

The last set of 22 tip weight I cut out were 2 pounds per blade. held in place on the front spar by if I remember correctly, 4 quarter bolts.

When you "cut out" the weights was this by drawing with a specified material and were bolt locations on the weights deliniated by drawing. The reason i ask is that if the weights are not properly positioned and the static balance of the blade not verified you could run into several problems:

!) Rotor spanwise imbalance

&

2) A serious change in the pitching moment of the blades

PLEASE EXPLAIN THE TERM "CUT OUT"

If you in fact removed the 2 pound weights did you replace them with heavier weights in the same place? This too can seriously effect the pitching moment and the dynamic stability of the blades
as well as possibly effecting the spanwise balance of the blades.

[the coyote]

budgevheli,

This conversation is intellectual chess on what to do in a situation that in my opinion a pilot should never allow to happen.

I have seen the R22 Nr at 78% doing autos in the training environment, and it recovered promptly by waiting. No flare, turn, push or pull. I never want to see it there again though, and I think the pilot (that was me!) is to blame by letting it get that low. I learnt from that.

I wonder just how close the rotor was to stalling back then, but in my mind this conversation is talking about 75% Nr or less, ie critically low Nr.

And yes, I'm as concerned as you are that someone might take this speculation on board somehow and in confusion deviate from what they have been trained to do when the blood is pumping.

Stick to the training! Pprune is a great source of info, ideas, and increased awareness. How people choose to interpret that and what course of action they ultimately take is up to them eh?

[choppersafari]

How does the R22 MCP Limits graph work?

How does the R22 MCP Limits graph work?

Before taking off in the R22, one of the performance checks is to calculate limit of MCP and 5 Min Take Off power available from the graph in the aircraft.

As the OAT gets higher, and thus air less dense, more MAP is available. (And as pressure alt increases at a constant OAT less MAP is available). I have been told this is to do with the temperature of the burn of the fuel on ignition which could lead to overstressing the drive system and blades at lower temperatures.

Can someone give an informed explanation of whats happening inside the engine and why the graph reads like this please?

[Wildwilly]

choppersafari

My understanding of it, stated as simply as I can put it (and correct me if I'm wrong anyone please) is that it's not that more manifold pressure is 'available' but that more manifold pressure is 'required' in order to obtain your 5 minute maximum (131hp). Cooler, more dense conditions - less MAP required. Hotter, less dense conditions - more MAP req'd.

Lets see what the more technically minded and more experienced amongst us have to say...

[the coyote]

A good test of your instructors knowledge, ask them.

I am talking about an engine that is not boosted (turbo charged).

Call fuel/air mixture "Stuff".

MAP is the absolute pressure of Stuff measured in the intake manifold downstream of the throttle. The more the throttle restricts the airflow into the intake manifold, the more the manifold pressure will drop (as the engine 'sucks' harder) and vice versa. When the throttle is wide open, the MAP can only get as close to the ambient outside air pressure as possible. When the engine isn’t running, the MAP will be the same as the ambient air pressure.

It is common to have a ‘de-rated’ engine that is capable of producing more horsepower than the drive train can accept at lower altitudes. Horsepower is purely a function of the pressure differential inside the cylinder when it goes bang, compared to the ambient pressure outside the cylinder in the crankcase. In order to control this pressure differential, we must introduce the correct mass of Stuff into the cylinder to burn. We can only do this by varying the pressure of Stuff going into the cylinder, as we have no control over the density or the cylinder volume.

In order to limit horsepower to the maximum that the drive train can accept, the manufacturer (and the pilot) must therefore limit the intake manifold pressure to deliver a pre-determined maximum mass of Stuff to the cylinder under the given conditions.

Say full throttle will let 90% of the ambient pressure into the intake manifold (I don't know the real data). It is physically impossible to get 100%, as there is always some restriction of the air. As the pressure altitude increases, the ambient air pressure decreases and 90% of less ambient pressure is less intake manifold pressure. A lower pressure is required in the cylinder after combustion to push against a lower ambient crankcase pressure to deliver the same horsepower. This means less mass of Stuff to start with, and is why the MAP limit on the chart reduces as you climb.

As the OAT rises the density decreases, but the ambient pressure stays the same. This means that less mass of Stuff will go into the cylinder at a given manifold pressure, and less pressure differential (horsepower) will be produced after combustion. So in order to get back to the limit you must get more Stuff into the cylinder, and the only way to do that is to increase the pressure of the Stuff going in. This is why the MAP limit on the chart increases as the OAT rises, or more correctly the air density reduces.

This is also why the chart allows full throttle at altitude, because up there the engine can’t get enough Stuff to produce enough horsepower at full throttle to exceed the drive train limit. Shovel less coal, get less power.

It is all to do with finding the correct amount of Stuff to burn to create the required pressure differential between the inside and outside of the cylinder, given the ambient pressure outside the cylinder to start with.

By the way: Pilots that exceed the limit “because the engine is de-rated anyway” don’t realise the damage they may be doing by overstressing the drive train.

Hope this helps.

[Texdoc]

Great STUFF Coyote

[bugdevheli]

Robinson: Tech Q's (Merged)

Answer for Lu Zuckerman. Cut out, as in take saw, cut off outer skin to expose internal tip weights. Remove said tip weights and weigh. Purpose. To determine where and what loads are passed through a R22 blade. Objective. To construct a blade for a homebuilt machine that outperforms r22 blade both in terms of structural integrity and inirtia value whilst decreasing the overall blade weight. Regards Bug.

[Lu Zuckerman]

This is how the big guys calculate in-flight horse power.

On large round engines this is how they calculate horsepower at any stage of flight to include take off.

First the engine must be equipped with a BMEP gage. BMEP is the Brake Mean Effective Pressure inside the cylinder.

To calculate horsepower the flight engineer will use a formula known as PLANK.

P= Prerssure or BMEP
L=Length of the piston stroke
A=Area of the piston
N=Number of cylinders
K=A "K" factor for that particular engine type

In a chain multipication of each of the elements the flight engineer can determine engine horsepower output.

At least that is what I remember from mechanics school back in 1949

[Dave_Jackson]

Lu,

'PLANK' is the old method. They now use 'GIRDER'

It was changed when they went from wooden blades to metal ones.



Dave

PS. Since you have the ire and ear of Heilport, would you do me a favor? Please ask him how to search for some past technical information on a thread that is 433 posts long.

[Lu Zuckerman]

To: Dave Jackson

PS. Since you have the ire and ear of Heilport, would you do me a favor? Please ask him how to search for some past technical information on a thread that is 433 posts long.

If I did it might be the straw that breaks the camels butt. I think you have already asked the question.