Retreating Blade Stall No 2

[SASless]

They (H-19's to me) were notorious for that kind of behavior!

Along with a horrible shimmy in the forward landing gear (bungee cord centering device)....that killed any acceleration when the shimmy started....which on a running takeoff could be interesting!

[JohnDixson]

Was That Blade Stall?

bastOn,

That was indeed classic S-55 stall.

Nice looking ship in the picture.

Thanks,
John Dixson

[Pofman]

SASless. The UK H19s(Whirlwind) were fitted with nose wheel lock pins(either in or out) as the majority were for ship use. Initially we did EOLs with the pins out and as you say there was a lot of wheel shimmy. So eventually we flew with the nose wheels locked unless ground taxiing. The EOLs were now much nicer and became really nice in the gas turbine(Mk 9 & 10) when the RRPM were raised to 220 rpm. Almost as easy as the Wessex!

[AnFI]

Flapping ....

the confusion arises IMO, as previously clearly stated, as a result of the reference system used to describe flapping.

Most people (UK CAA for example) would use the definition of Flapping being the Angular displacement of the longditudinal axis of a blade referenced to the HUB AXIS (or hub plane) - this has some merit since this is the Flapping which a flapping hinge actually has to accommodate. however for forward flight the Tip Path Plane can be inclined forward more than the Hub Plane - leading to the odd consequence that Flapping, defined in that way, would be UP on the retreating side (!) - NOT WHAT MOST PEOPLE WOULD EXPECT. It is fairly 'meaningless' for understanding helicopter flight except in as far as it does show clearly how a blade's flapping hinge will physically move. It would result in a different value for Flapping if the CoG were changed in a nil wind hover for instance - it is not what people mean when they use the term flapping. Simply (by this definition) it is the difference between Tip Path Plane and Hub Plane (accommodated by Flapping Hinges, Teetering Hinges or blade (root) flexing depending on the Rotor Head).



More commonly / usefully Flapping can be described with the reference being the Tip Path Plane - if a blade is rotating in the TPP and encounters a change in cyclic pitch or cyclic airspeed change (advancing/retreating sides) then the blade would Flap (at a rate) into a new plane of rotation - this is the definition most commonly used for describing the Flapping to Equality phenomenon - the TPP would change resulting in FlapBack. In this case it is clear that to maintain a constant accelerative attitude the cyclic stick (Control Plane) will have to be progressively displaced forward (in general) sufficiently to effect a Pitch (and AoA) dissymmetry sufficient to negate the need for any Flapping - as a result no Flapping occurs.

There is another plane to which 'Flapping' could be referenced - this is the Control Plane - it was most favoured in the Dark Ages before the breakthrough into the understanding of helicopters which was marked by Cierva allowing his blades to freely flap - this marked the beginning of the viable helicopter. This plane is a little arbitrary - if the Blades are Referenced to this Plane then then Pitch does not vary around the cycle at all but no part of the disk physically rotates in this plane ( the merit to using this plane is that it can be the plane of the swash plate - which is marginally interesting ) although largely useless as a Reference in a modern helicopter it does have the interesting feature that Referenced to that (arbitrary) plane the Pitch does not vary cyclically but instead the blades ONLY FLAP. Does anyone use the Control Plane as a reference for Flapping? - An Analysis of Flapping With Respect to the Control Plane would be fairly obscure and unhelpful. It is also noteworthy that a (truly) rigid rotor with NO FLAPPING HINGES and NO BENDING would still be said to 'Flap' relative to the Control Axis

For Crab to begin to be right he would have to be using this Reference Plane (Control Plane), and he should he have said so (since it is NOT the Default Plane of Reference) , he should also have agreed when I pointed out that the likely cause of the (his) confusion was the Reference Plane he was using. NOT only that but he would also have had to be consistent in the use of that plane - which he was not - frequently using descriptions relevant to other reference planes.

So in short an argument could be made for Crab's correctness on this issue - but it would be a pretty weak argument IMHO - and he clearly was not talking about Flapping referenced to the Control Plane - (was he SASless?)

The friendliest 'judgement' would surely be that both were correct - and to say:
Referenced to TPP; the Pitch varies Cyclically (as required to make the appropriate AoA such that 'lift' is constant around the disk) and the Blades Do Not Flap - except to change the plane of rotation.
or
Referenced to the Control Axis; Pitch is constant around the disk and the blades Flapping is Huge (and extremely difficult to analyse any element of this definition of Flapping attributed to (apparent) Cyclic Speed Variation!)

sorry a bit rushed.

If anyone wants to read someone else saying it 'in print' then this reference says it (approximately) quite nicely (reading from 4.3.3 on) click the link: Art of the Helicopter - John Watkinson - Google Books


please vote (SASless) - should Crab continue to do my spell-checking?

[[email protected]]

An FI - strange that you have gone away from Leishman and started paraphrasing a different source about reference planes after I pm'd you this

AnFI, please feel free to make me look bad but I don't think you will succeed since your explanations are muddled - I am not an aerodynamicist and I suspect neither are you - various publications define flapping with respect to different axes and the most efficient according to Gessow and Myers (who taught Leishman) is to use the control axis and not the tip path plane. I have pored over more than enough textbooks to believe the way I argue the case for flapping is, if not mathematically precise, an acceptably understandable explanation of something that can be both calculated and measured in the real world. Do you not think it strange that all the teaching for the military, the CAA, the FAA and many other established authorities talk about flapping in the same terms as I do? If you really know better then write a book or explain it clearly (unlike your staying visual posts) in words that are clearly understood. As for your experience - you have never stated it publicly so feel free to pm me with it and your name so I can verify it. I won't care if you turn out to have more hours than me or are God's gift to aviation - you write a lot of unintelligible posts on PPRuNe which just confuse the less experienced guys on here - that is a shame - if you really are so experienced you should be passing that on properly so the GA PPLs can understand it.

I await someone who really knows what they are talking about (Shawn Coyle or Nick Lappos) to comment on your assertions All you are actually doing is plagiarising someone else's work and taking certain statements out of context - I suspect the whole text on flapping in that book might give a different perspective - what is his take on RBS for example?

[army_av8r]

ok, ok, ok... so today I was flying left seat in an OH-58D. Crossing through the Rocky Mountains. DA was around 13000. VNE was charted around 87. My right seat pilot was flying at the time, and was trying to catch up to the flight. During an acceleration the aircraft began to "shudder" and a mushing feeling was felt. it was followed by the nose trying to pitch up, and his first reaction was to apply forward cyclic due to the nose pitching up... as the forward cyclic was applied the vibrations continued to increase and the aircraft rolled RIGHT... I have read this whole thread a few times previous to today, and I also know for a fact that it rolled right. the pitching up was at max 5 degrees becuz he applied forward cyclic. The roll was 15 to 20 degrees... RIGHT. I have the event on video from a gopro. I will also go on to say that none of this pitching or rolling was violent, and the vibes were not anything severe. we have been trying to figure out all day what is causing the right roll, now that we have seen it first hand... could it be due to improper control inputs( forward cyclic) which increases blade angle at approximately 9o'clock? im assuming the text books only cover the situation from the perspective of the pilot doing the right thing and pulling aft cyclic to slow down/ doing nothing at all? im intrigued, to say the least. I am a 2000 hour CFII in RW, and also have a good bit of time FW. I know what I saw and felt. Now I NEED to know why.

[[email protected]]

AnFI - from your own source document

An observer turning with the tip path plane would see the feathering action changing the pitch of a given blade sinusoidally about the collective pitch setting at one cycle per revolution but as he is turning in the tip path plane he would not see any flapping (except for harmonics).

then

An observer turning with the control axis in forward flight would note that the pitch of the blades was constant, but that the blades flapped up at the advancing side of the nose of the machine and down at the retreating side.

and finally

There is an exact equivalence between the degree of flapping seen by an observer on the control axis and the degree of feathering seen by an observer on the tip path axis. Figure 4.3(c) (not available in the preview) shows that when the control and tip path axes differ by θ degrees, if the blades reach a peak feathering angle of θ degrees with respect to the tip path axis, they will flap to a peak deflection of θ degrees with respect to the control axis.

The reason for the difference between the tip path axis and the control axis should now be clear. In the hover, there would be no permanent difference as the blades follow the cyclic stick. However, in translational flight, the blades encounter highly asymmetrical conditions that pull the tip path axis away from the control axis.

He goes on to say that confusion arises when the axes are mixed which is where I think you are having a problem accepting flapping PS I think you have made up 'hub axis' and think you mean 'shaft axis' - read your source again!

[fijdor]

Thanks av8r I don't feel all alone anymore, also this happened in a Bell product as well, 2 bladed system.

JD

[AnFI]

I am not plagarising anyone - since unlike you I understood this before - I think I said something in a very early post on this thread along the lines of "Crab's misunderstanding most likely comes from his confusion about which reference axis (or plane) he is using - but that's another story" - I prompted you to start reading the text books - NO you are wrong about LEISHMAN he DOES STATE IT very CLEARLY at the ref I gave - the only reason to mention these publications and authors is to show that it's not just me who understands the helicopter in this way - since you wrongly charged me with that. I hope you can honorably say that as a result you now understand something fundamental to helicopters which you previously had not. You can call the Hub Axis the "Shaft Axis" if it makes you feel better - but it's no more correct than the "Drive Axis" "Input Axis" "Head Axis" etc and it doesn't matter too much if you want to refer to the Plane either - call it the 'driving plane' and 'driven plane' if you like. One thing is quite clear - you did not understand it before and you almost do now - admit it (for once)

[army_av8r]

Well, after watching the video about 1000 times... I have decided that the right roll was only experienced when a forward cyclic input was made to oppose the pitching up. When it first vibrated it only pitched up. No roll at all. it makes sense that if the disc only stalls at the 90 point, you would only feel nose up, and past 90 the relative blade airspeed begins to climb. Forward cyclic would increase that blade angle and aggravate the stalled condition and force a longer stall in terms of blade rotational position. Add your 90 degree phase lag and guess what, right roll. But I have been wrong before. Lol

[JohnDixson]

OH-58

Army, your notes refer to a Vne of 87 for that condition, but your posts do not say whether that Vne is a stall boundary or exists for one of the other typical speed limiting factors. I must add that for a counterclockwise rotor, a right rolling tendency attributed to stall is difficult to envision.

Here's a suggestion: get on the phone to Bell and get connected to their test pilot office, then talk to the guy who did the 58D structural demonstration for the Army model and explain your flight experience. He can get some assistance from his flight test engineering pals and most likely get to an explanation for this event. When you get it, post it here as it is most interesting.

Thanks,
John Dixson

[army_av8r]

I completely agree that a right roll made no sense. However, I also have no way of proving that what I experienced was indeed RBS. it could have been just a mountain rotor that we hit at the same time a the pitching up. Who knows? I also don't know when our vne is due to rbs and when its due to certification requirements. I will research and post what I find

[[email protected]]

AnFI - you keep making yourself look like a petulant child - I did understand this before and you have not led me to make any fundamental change in how I believe a helicopter works. The fact that you linked to an article which I used to refute your assertions was just fun

I had read the text books a long time (about 21 years ago when I did my A2) before you started blathering about how rotors don't flap.

I hear rumours that you are an experienced instructor - well, based on what you have written on pprune on this and other subjects, you must have some very confused students.

[SASless]

fijdor,

OH-58D.....built by Bell but four bladed head....but turns the same direction of course.

All this should be a good question for the Bell Test Pilots.....as the Conventional Wisdom is in RBS....the helicopter will pitch up and roll in the direction of rotation of the rotor system.

We now have two reports of a Bell Helicopter rolling the opposite direction than CW says should happen.

Sounds like a couple of flights the Factory could arrive at some solution to the puzzle.

Does the application of forward cyclic during the onset of RBS cause a roll in the opposite direction where with no application of forward cyclic....the aircraft would perform as expected?

Would a Sikorsky design be any different in this regard than the Bell design?

[[email protected]]

In order to make the nose go down (blades low at the 12 o'clock) the swashplate must be tilted down towards the 3 o'clock position to allow for phase lag. That sudden acceleration due to forward cyclic flapping the blades down on the right hand side of the disc might be a factor in this right roll - I don't know, but it may be worthy of consideration - acceleration cross-couples are not unusual in helicopters.

[JohnDixson]

Stall Characteristics at SA

SAS, I'm not sure how to address your " different than " with regards to SA vs Bell stall characteristics. Have a lot of experience with the former and zero with the latter.

Over the years, the various new rotors at SA have been subjected to flight into stall, as you can imagine. It is an interesting subject and far too long a story to type in here. The increasing number of rotor blades from the 3-bladed S-55 has softened the roll rate for the cases where the rotor is driven into stall, straight ahead, one " G ". In those instances, one notices an increase in N/rev vibration and a measurable tendency for the lateral stick position at the trim points to migrate toward the right. That agrees with the consistent results we have seen in dynamic cases as well, where the rolling tendency is always left.

I might mention one very different but common rotor behavioral effect, ( that quite frankly does not explain the story as related by Army ), and that is the gyroscopic effects of control inputs. Hence for a clockwise rotor, a pure positive ( nose up ) pitch up cyclic input will result in the rotor/aircraft pitching up, and then, rolling right, with the roll rate a function of the pitch rate. Easiest to observe with all of the electronic stability enhancing equipment turned off.

Allow me to amplify the comment regarding N/rev vibration encountered with stall. If the rotor is flown into the edge of stall, the signature will be N/rev, but if driven further into deep stall, to include stall flutter situations, then the signature will be more complex, with additional blade response harmonics present, which can, and often do, set up beat frequency vibrations sensed in the cockpit as something near 1/rev.

Thanks,
John Dixson

[AnFI]

Crab Check:

In order to make the nose go down (blades low at the 12 o'clock) the swashplate must be tilted down towards the 3 o'clock position to allow for phase lag.

Is that type specific?

[[email protected]]

AnFI - hardly - opposite direction rotors and aircraft without swashplates will be different. Additionally, since phase lag is compensated for by different arrangements of jack position and advance angle (how far the pitch change arm is ahead of the feathering axis of the blades) on different helicopters, the exact tilt of the swashplate will depend on those factors.

John -you are describing an acceleration cross couple that is very evident on the Lynx - a hard pull up needs left cyclic to oppose a marked tendency to roll right. Similarly on other aircraft (Wessex, Sea King) a hard right turn (steep turn or quickstop for example) needs left cyclic to prevent further right roll as the cyclic is brought aft to increase the G.

[SASless]

That agrees with the consistent results we have seen in dynamic cases as well, where the rolling tendency is always left.

Brother Dixson says his experience testing Sikorsky aircraft.....RBS causes the aircraft to roll in the same direction as the blades rotate. American....left, French/Russian....right.

That is my experience as well.....less the Chinook which being Tandem rotor was a different kettle of fish as two heads complicate the reactions....depending upon whether both or just a single head was actually stalling.

[fijdor]

"Brother Dixson says his experience testing Sikorsky aircraft.....RBS causes the aircraft to roll in the same direction as the blades rotate. American....left, French/Russian....right.

That is my experience as well.....less the Chinook which being Tandem rotor was a different kettle of fish as two heads complicate the reactions....depending upon whether both or just a single head was actually stalling."



Were those tests conducted (in real life) to a high DA? or to the maximum operating altitude of the aircraft?

Or is it whatever we got in between 0 to 10,000 ft is what you will get (in theory) between 10,000 ft to 20,000 ft ?

Just asking.

JD