Blade pitch stall margin
 

The Robinson R22 flight manual states the blade pitch is near stall at higher gross weight. Also the NTSB special report on the R22 accident history dicusses the high blade pitch of the Robinson as a possible cause of the several dozen rotor separation incidents.
My question is: what is the blade pitch of a R22 in hover and cruise and what is the blade pitch of other helicopters. Is it normal to run close to blade stall?

Using Prouty's calculations, the Robinson R22 Beta would have a collective (root) pitch of 14.5-degrees in hover at GW, if it used a 0012 airfoil. It actually uses a 63-015 airfoil, and therefor the pitch should be slightly less. The blades have -7-degrees of twist. The blade loading in hover at GW is 91-lb/sq-ft.

For comparison, the Schweizer/Hughes 300C uses an 0015 airfoil and it's blades have -8.7-degrees of twist. The blade loading in hover at GW is also 91-lb/sq-ft.

Which pitch is which?
 

The Robinson has a seven-degree negative pitch in the blades. Prior to start of the rigging process assuming everything is normal if the blade root has a pitch angle of seven degrees the tip has a setting of 0-degrees. The blade pitch settings are set with the protractor at approximately 75% span and under the conditions stated above the pitch angle would be 1.75 degrees.

From this point forward everything gets confusing. I have been working in various aspects of helicopter maintenance since 1949 and I have never seen a rigging procedure as confusing as that of the R-22 and by extension the R-44.

One point of confusion is that the cyclic stick has limited travel in all directions with these limits established by the cyclic control stop plate. This is contrary to FAA guidelines but I won’t go into that. The rigging procedures start with the cyclic in its neutral position relative to fore and aft and lateral control. This means that the travel for fore and aft is the same relative to neutral. However when the cyclic is moved forward the required pitch setting for the basic R-22 is 8.3 to 8.8 degrees. I have never been associated with a helicopter that had a range in the pitch setting but there may be more than the R-22. Back on track, the aft pitch setting is 8.5 to 9.0 degrees.

If the desired aft range can’t be achieved the mechanic must adjust the push pull tubes attached to the swashplate. This will change the setting for forward pitch and the mechanic is told the following.

NOTE: If adjustment is required to obtain aft cyclic control blade angles, the forward cyclic must be rechecked.

Two points to consider: 1) The mechanic is not told what to do in order to change the forward cyclic angle because if he does he will also change the aft cyclic angle and the whole process starts all over again. And, 2) If the cyclic is moved from the center of travel forward and aft from the center of travel how can they have different pitch settings for forward movement and for aft movement.

The rigging procedures are very vague and this includes the rigging procedure for the tail rotor. The rigging procedures for the R-22 are totally contrary to the procedures used to rig any other helicopter. The R-22 rigging procedure can introduce excessive pitch into the blades. If the blade angle readings are taken at the 25% span and the readings are 8.5 to 9.0 then the reading at the root would be around 14.25 degrees and this is with no collective. The collective reading with the blades in the neutral position is 11-12-degrees. With maximum cyclic input the pitch reading at the root could exceed 25-degrees which is quite high for most helicopters. This angle will in most cases increase with adjustments for autorotation.

My hat is off to the mechanics that maintain these helicopters and to the pilots that trust that their mechanics have a full and total understanding of the procedures.

:(

Blade pitch stall marginTwo things spring to mind
 

Two things spring to mind. The reason the cyclic stick movement is limited by a ring of metal at its base, is that the cyclic stick tube is both of a larger diameter and thicker wall than the push rods, so the base of the cyclic stick would seem an obvious place to control excess movement without loading the pushrods to excess. On the adjustment of fore/aft, I think Mr Robinson could not get equal movement each side of centre due to the bellcrank setup on the mixer. Or may be he thought "Better give it a bit more aft cyclic, they will need that when they are about to put two grooves in Terra Firma. Either way, when you try designing one youself (control linkage) its a real ******. That Mr Robinson is a very clever chap. (sorry to deviate from the thread, the wine has done me head in)

Pitch or is it bitch?
 

To: bugdevheli
Still just another number

What you say may be true but the cyclic control limit plate is in conflict with the design specifications of the FAA for helicopters. The other thing may have to do with bell crank throws but it is the method used to adjust the basic pitch of the blades. This is done by adjusting the push rods connected to the swashplate where on any other helicopter this would be done by adjusting the pitch links. When the push rods are adjusted for forward cyclic the range is established by the cyclic limit plate and the push rods adjusted for the blade angle. When checking for aft cyclic range the same push rods are adjusted to get the blade angle. This will change the angle set for forward cyclic and the mechanic is left chasing his tail just like a dog.

Read the R-22 maintenance manual and see if you can make sense of the main and tail rotor rigging procedures.

:(

Some more information from Prouty's book, which might be of interest.

Longitudinal

The avg. 'aft' value of 11 helicopters is +13.45º.
The avg. 'forward' value of 11 helicopters is -8.04º.
The average longitudinal cyclic range of all 18 helicopters is 23.54º.
The greatest is 34º.
The Robinson R22 is -9.0 to +11.0 (20º)

Lateral
The avg. 'left or right' value of 11 helicopters is +/- 7.43º.
The average lateral cyclic range of all helicopters is 15.07º.
The greatest is 20º.
The Robinson R22 is -9.5 to +6.0 (15.5º)


Of the 18 helicopter listed; 6 have equal forward and aft cyclic, and 12 have greater aft cyclic than forward cyclic.

Stall Margin
 

I may have mistated my question and maybe its more complex than I thought. What I should have asked is: what is the average blade angle of attack in a hover with respect to the air?
Do helicopters operate normally with the rotor close to stall angle of attack?

Johnson's Helicopter Theory mentions the term stall margin several times but gives no hint as to what is a good stall margin.

Most airfoils stall in the range of 12 to 16 degrees angle of attack. I am wondering if the R22 flies with blades closer to stall than other designs. That would explain why the R22 rotor slows rapidly after engine stoppage.

The only way to tell what the stall margin of the rotor is uses the Coefficient of thrust to solidity ratio. This factors in the tip speed (a very important contributer).

Coefficient of thrust divided by solidity. Ct is thrust divided by density, disk area and tip speed squared. Solidity is the total blade area divided by the disk area.

R-22 values (assume thrust is equal to weight):

Disk area 496 sq feet
radius 12.58 feet
Tip speed 676 feet per second
chord 0.6 feet
Max gross weight 1370 pounds

For a gross weight R-22, Ct/sigma is .084, which is very normal for a typical helo (a Black Hawk is about .08). The rotor begins to stall at about .17 and is fully stalled at about .20.

Conclusion, the R22 rotor is very normally loaded, and not near stall in normal conditions.

The referenced R22 paper warns that the conclusion about stall based on a computer analysis that is somewhat flakey (as I think it is) especially when such programs are not backed by flight test data:

http://www.ntsb.gov/publictn/1996/SIR9603.pdf

R22 Data
 

Nick,
Robinson R22 specs:
Gross weight 1370lb
Rotor radius 12.58ft
Blade chord 7.2ft
RPM 530
Airfoil 63-015

The rotor rpm could be 510 to 530, (I am not sure) the other data should be correct.

Slowrotor,

The hovering rotor is more efficient at higher angles of attack. But, as the coefficient of lift increases, so does the coefficient of drag. This implies that, at the initial loss of power, the rotor's inertia will be 'consumed' faster.

You might find the US patent 6,007,298 of interest. It involves using high pitch settings to optimize hover efficiency. It should be noted that the application is for unmanned helicopters.

slowrotor,
forgive me if I misread your second post, but it appears you may want a more simple explanation. Your question seems to suppose that the R22 blades stall rapidly after an engine failure because they operate with a reduced stall margin. The answer is: no.

As all the formulas and experts above have pointed out, there is nothing unusual in the R22s blade loading, nor pitch setting in the hover.

As you are aware, pitch setting is only one of many factors involved in blade stall - the others being speed of the airflow over the airfoil, angle of the airflow hitting the airfoil, G loading, the usual suspects such as density, and many more that Nick et al could rattle off. As such, the pitch setting (such as the 12 to 15 degrees you mention) can be inconsequential without considering all the other factors too.

But the one that concerns your proposition about the blades slowing rapidly and stalling is not really related to the "stall margin" as much as it is to do with the rapid slow down. The blade slows because there is no thrust (engine has failed) to overcome drag. No different to any other helicopter. What is different about the R22 is blade inertia. Blade inertia is very low, so drag is able to rapidly slow the blade (and hence the blade stalls more quickly), where as with an aircraft like the Huey, it's blade has large inertia and will slow down far less quickly than the R22. This is somewhat oversimplified and it ignores pitch angle at the time of engine failure, etc, etc, but you get the picture.

Couple the low inertia blade (ie relatively rapid stall) with the generally low pilot experience levels attracted to the type and is it any real suprise the stats highlight the issue? But that does not mean there is anything wrong with the design - it is more an expected outcome from the factors involved.

Hope that helped.

retreating blade stall
 

helmet fire
Thanks for your explanation of the rapid loss of rotor rpm because of low rotor inertia.

I am still trying to understand what the cause of the rotorhead separation described in NTSB report actually was. The report does'nt give any explanation. The 1996 report says that early (1980-81) separations were thought to be caused by low rotor rpm and that problem was fixed with an engine governor and raising the low rpm warning from 90 to 97 percent rpm I think it was.
But the main purpose of the report was the fact that a rotor failed at normal rpm and that fact concerned the NTSB and concerns me.
In that case it seems likely to me that the blade may have stalled because of a gust. A stalled airfoil tends to dive and could hit the tailboom under the right conditions possibly. I dont know, this is all new to me, most of my experience is fixed wing and fixed wings do not often fly normally at high angle of attack close to stall.I do fly gliders close to stall climbing in a thermal normally but that is never near the ground.

It appears that helicopters do operate at close to stall angle of attack in normal flight (at least that is what the Robinson manual says).

So do helicopter pilots need to be concerned with retreating blade stall and possible blade divergence into the the tailboom in turbulence or is the blade designed to handle retreating blade stall?
Also could a low inertia blade be more likely to diverge when hit by a strong gust?

Slowrotor:
I said earlier not to hang too much on that old NTSB report, but I've been loth to go into all the nauseous detail about why it's suspect. I see I'll have to fill in a few details.
At the time of the report, the NTSB was run by a man called James Hall, a political appointee of Clinton's (he'd raised a lot of money for Al Gore's Vice-Presidential campaign and got his 120,000 a year post as a reward). Hall had no aviation knowledge whatever. He was a lawyer.
Hall started out by attacking the ATR and the US commuter airlines, and he tried to ground the Boeing 737. He was on TV a lot, making a name for himself. And he went after Robinson in a big way.
The Trial Lawyers Association was at the time demanding that the NTSB give them the same access to accident investigations as the manufacturers, and Hall shifted NTSB policy in that direction. Robinson was then facing several lawsuits. One California congressman, a lawyer and a personal friend of the father of a pilot who died in a Robinson crash, went to Hall to see if he could help him overcome Frank Robinson's intransigence. (Robinson carries no litigation insurance and fights every case to the wire).
The NTSB went back over 14 years of accident data and put together the list that you have, that purports to show that Robinson helicopters would be flying along minding their own business when suddenly the rotor would inexplicably dive through the tailcone.
Every one of the accidents on that list was caused by low rotor rpm stalls, low-g manoeuvres or other known causes. The first one on the list, the pilot had turned the fuel off. In some cases they had eye-witness reports that the rotor had slowed up; in others it was obvious from the damage that the rotor hadn't hit the tailcone until after it struck the ground. There was no mystery about any of them.
A lot of field investigators at the NTSB were very unhappy with that report. The field investigators are the people who collect information, establish all the observed facts about an accident, make a wreckage distribution diagram, document the pilot's credentials and so forth. But the Board determines probable cause. The Board is made up of people like Hall, with little or no aviation knowledge. Politics and litigation intrudes where it should not. The Board was behind this report.
The reason that Robinsons were over-represented in the accident figures in the early days (they're very much under-represented today) was bad instruction. The R22 multiplied the number of people learning to fly by a huge factor. But in the USA, there weren't enough experienced instructors to handle the business.
Under the FAA rules, a fixed-wing instructor who had 50 hours on helicopters was automatically a helicopter instructor - an insane situation which was at the root of students' inability to handle many emergency situations. Fixed-wing instructors were in fact starting helicopter schools out of the boots of their cars, a situation which Robinson petitioned the FAA six times to amend before they finally put a stop to it. Here in the UK, where there was no 50-hour rule, we never had the kind of problems the US had.
The accident rates were largely brought down by Robinson's own actions, including the rescinding of the 50-hour rule, the Safety Courses, Robinson's own qualifications for insurance and other measures. The only hangover from those days is the SFAR, a face-saving measure for the Board which wasn't needed then and isn't now.
There are no more 'unexplained' accidents with the Robinson than with any other helicopter. Retreating blade stall is not something an R22 pilot need be too concerned about. The blade will not suddenly dive through the tailcone in turbulence. Keep the RRPM where they should be, react properly in case of emergency, and don't listen to ghost stories.

The technical aspects of the Robinson R-22 rotor have been much discussed on this forum. To the disgust of some, while to the entertaining, or education of others.

There are very few ways of evaluating the safety of the Robinson. One is to statistically determine if the craft has a poorer or better safety record then comparable helicopters. Another is to re-evaluate the craft and its rotor, using today's level of engineering capability. It is doubtful whether either means could arrive at a definitive answer.

I strongly believe in the future advantages of (very) high rotor rigidity. Therefor, there is a personal dislike of 'loosey-goosey' rotors and blades.

The trend has been toward greater rotor rigidity. This is born out, in part, by the evolution from teetering rotors, to articulated rotors, to hingeless rotors. Robinson has taken the opposite approach. It has increased flexibility, by taking a teetering rotor and incorporating coning hinges.

It is questionable whether the advantages of these coning hinges outweigh their disadvantages.

A distortion of the facts.
 

To: t'aint natural
Over 500 posts. I really must consider clicking here so I can order a Personal Title.

If you really believe what you wrote in your post above I would strongly suggest that you contact the below listed NTSB Investigator:

Mr. Ron Price
National Transportation Safety Board
490 l’Enfant Plaza S.W.
Washington, DC
20594

Ron was one of the senior investigators on the various Robinson Loss of control accidents.

Send him a copy of your post and see what he has to say about your "facts".

:*

Before I buy a R22
 

t'aint natural,
Thanks for your insights. I did review much of the past postings at this site about the R22. If the controversy of the R22 is getting old then I am sorry I used the R22 as an example in my question.
Still hoping to get some input as to what blade angle of attack is common for helicopters in general.

By the way, my first ride in a R22 was with Frank Robinson back in 1980, I have nothing bad to say about what Frank has done to help the little guy fly helos. But I want all the facts before I buy.

slowrotor

slowrotor,

I think that you are trying to apply fixed wing theory to rotary wing flight. There is no typical "blade angle of attack" like there might be for fixed wing forward flight. Pitch angle and angle of attack are different in rotary wing (also true for fixed wing - think "alpha"). Angle of attack varies with manouevre, wieght, airflow, blade relative airflow, etc, etc. Use all the groovy formulae given to you earlier, and you will see that you can vary so many of the input values that to talk of a general, or typical value is relatively meaningless in this context.

I repeat my earlier post that there are many different factors affecting blade stall, and angle of attack is but one. To isloate this one factor, or to draw some sort of inference about a particular design based on a single factor in a complex scenario is to miss the whole picture.


I note now that these questions seem to stem from the old R22 report which attempts to indicate that there is some sort of blade stall issue isloated to the R22. You will note that these accident statistics are not replicated in Australia in any way - and they operate perhaps the most demanding of all R22 roles - cattle mustering. This role involves maximum exposure to the possibilities of blade stall: continual ops at max all up weights, extremely high DAs, high manouevre loadings, etc, etc - but there no "blade stall issues" in the accident statistics. No rocket scientist required to figure it out. They are a very statistically reliable aircraft in these neck of the woods.

By the way: a stalled blade does not suddenly fly down and chop through the tailboom. You can research that one on this forum too.

I humbly suggest a commercial pilot level of aerodynamics of RW flight theory may be a good way for you to start if you are at all concerned by the report.