Aerodynamics - Stall Recovery [Archive]

Dave Jackson

16th Mar 2002, 00:01

A plane can recover from a stall by pitching the nose down, increasing the forward air speed and restoring lift on the airfoil.. .. .Assuming that a helicopter was manufactured which had;<ul type="square"> Symmetry, due to twin (side-by-side or intermeshing) rotors.</li> Advancing Blade Concept.</li> Absolutely rigid rotors.</li> Adjustable horizontal tail (elevator).. .</li>[/list]If this helicopter was to experience totally stalled rotors, why could it not do the same as the plane? Pitch the nose down, increasing the forward air speed and restore the lift on the airfoils? <img border="0" title="" alt="[Confused]" src="confused.gif" /> . .Replies? to <img src="http://www.unicopter.com/pissonit.gif" alt="" />Idea.. . . . [ 15 March 2002, 20:06: Message edited by: Dave Jackson ]

helmet fire

16th Mar 2002, 07:07

Is this a trick question?. .. .Forward speed generally IS the cause of blade stall (ie RBS), thus nosing over and increasing speed exacerbates the situation. . .. .For the overpitched blade in the hover (IE either stalling throughout the revolution or exceeding engine power due drag build up), I should imagine that gaining forward airspeed will help on the advancing side at least. But how would you have the control authority to achieve the nose down?. .. .Either way, the lowering of collective pitch remains the simple solution (and it works for both problems).. .. .I am not altogether sure how ridgidity affects these issues, but on face value I would think it was as marginal as my spelling prowess.. .. . <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />

IHL

16th Mar 2002, 07:25

Dave : You have way to much time on your hands.

Dave Jackson

16th Mar 2002, 07:55

helmet fire,. .. .Trick question??? That's not a very nice question? <img border="0" title="" alt="[Smile]" src="smile.gif" /> . .. .The idea, if there ever was one, is that the helicopter is hovering, or close to it. The pilot is soloing in the craft, after having completed the mandatory 2-1/4 hours of dual training. . .. .Blade stall due to low RPM is the second highest cause of fatal accidents. Should the rotor speed inadvertently fall below (100% - ??) RPM in a conventional helicopter, the blades will not be able to withstand the upward flow of air (too too high an angle of attack) and they will fold up. . .. .In this hypothetical helicopter, the blades are too too strong to fold up. Therefor, similar to a plane during a stall, the nose of the helicopter drops until the angle of attack on the advancing blades is small. Then power, gradual collective and aft cyclic are applied to the two rigid counter-rotating rotors. The helicopter slowly pulls out of the dive.. .. .Control authority is regained because the center of gravity, is ahead of the center of the stalled rotors. The elevator on the horizontal stabilizer is connected to the cyclic. I.e. the helicopter acts like a plane; ~ a plane which has rotating wings (CW and CCW).. .___________ . .. .There are a few minor problems to work out; like having a large enough blade (wing) area and the advancing tips maybe wanting to go sonic. But, these are just inconvenient details.. .. ._____________. .. .IHL,. .. .Too true; but something may come from all of this. <a href="http://www.unicopter.com/unicopter.html" target="_blank">UniCopter</a>. . . . [ 16 March 2002, 06:47: Message edited by: Dave Jackson ]

Nick Lappos

16th Mar 2002, 13:30

What if we assume a rigid tail rotor with a Notar back-up, on a Tuesday, in a solar system with a red sun (for Superman fans!) and blue Kryptonite as the main power source?. .. . <img border="0" title="" alt="[Wink]" src="wink.gif" /> <img border="0" title="" alt="[Wink]" src="wink.gif" /> <img border="0" title="" alt="[Roll Eyes]" src="rolleyes.gif" />

ShyTorque

16th Mar 2002, 18:18

If the aircraft is in the hover with a 2 and a quarter hour? solo student at the controls, it is likely to be in ground effect, approximately 2/3 (single) rotor span.. .. .Allowing the nose to fall automatically in this configuration would have a very unpleasant effect on the student's underpants and on the aircraft, even if it had a nosewheel (which is for steering with, not landing on).. .. .(Although the UK Royal Helicopter's pilots might have found an automatic nose-down pitching moment in the hover useful just recently......hehehe)

[email protected]

17th Mar 2002, 01:51

Dave, are you sure you mean that blade stall is the cause of lots of accidents or do you mean overpitching, which is not the same thing at all. If the rotor has slowed down either the engine has failed or not enough power is being produced by the engine - probably because the pilot has not opened the throttle enough or the governor is at max chat.. .Your hypothetical helicopter could only have totally stalled rotors in a perfect still air hover with a high collective pitch setting and exceedingly low Nr.Even then the differnces is rotational speeds along the length of the blade would mean that some portion of it is not stalled.. . The nose pitch up/pitch down associated with collective increase/decrease occurs due to flapping to equality as the retreating and advancing sides will not have the same airspeed except in a perfect still air hover.

Dave Jackson

17th Mar 2002, 02:32

Nick. .. .Unfortunately, there's not enough room for both a tail rotor and Notar, since the FAA has insisted that a drogue chute, or 5 or 6, be incorporated into the tail. Other than that, everything else will be included. . .. .Las Vegas still give this helicopter better odds than the tilt rotor. Just joking. . .. ._____________. .. .Shy Torque. .. .No, No, No! This helicopter is so easy to fly that the pilot's license is simply an endorsement on his driver's license. The helicopter is not in ground effect. Actually, it's at 35,000 ft, since this is the height required to come out of the dive.. .. .______________. .. [email protected]. .. .Oh, Oh. Getting serious.. .. .The following was clipped and then pasted on a web page entitled Governor / Robinson R-22. I cannot recall if this statement was attributed to helicopters in general, or is specific to the R-22.. .. .>"A review of NTSB accident briefs clearly demonstrates that wire strikes are the primary cause of fatal accidents, followed by blade stall due to low RPM and continued flight into IMC."<. .. .This hypothetical helicopter has absolutely positively really rigid rotors. Therefore 'flapping to equality' is not a consideration. It also has twin counter-rotating rotors that incorporate the Advancing Blade Concept. This will reduce the 'advancing / retreating blade' concern.. .. .Everything works so well in 'virtual' reality. <img border="0" title="" alt="[Smile]" src="smile.gif" />

ShyTorque

17th Mar 2002, 03:29

Dave,. .. .Ah, yes I see now.. .. .Just don't allow this machine to fly at 34,999 ft or less.. .. .It will be best not to have a pilot in it at all since he would need oxygen above 14,000 ft. <img border="0" title="" alt="[Wink]" src="wink.gif" /> . .. .In my opinion, any pilot who allows the Nr to reduce to the stall following a wire strike and subsequently continues into IMC deserves all that is coming to him. <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />. . . . [ 16 March 2002, 23:34: Message edited by: ShyTorque ]

helmet fire

17th Mar 2002, 03:29

Dave,. .Perhaps the nose down pitching could be continued to the point where the aircraft is falling completely inverted, thus reducing the induced flow for the same AoA and making the aircraft more efficient. Though it might be somewhat unnerving for Mr 2 and a half hours and his pax!!. . <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />

Dave Jackson

17th Mar 2002, 03:47

helmet fire. .. .Excellent idea. Since the craft has rigid rotors and no coning angle, we'll allow it to have negative pitch, as well.. .. .Recovery from the dive will be by applying forward stick. After achieving level flight, a quick 180-degree roll will complete the recovery.. . <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> . .. .Is there any money to be made by flying an aerobatic helicopter at air shows?. . . . [ 16 March 2002, 23:49: Message edited by: Dave Jackson ]

heedm

17th Mar 2002, 04:02

Dave, I think a better way of picturing stall recovery on a fixed wing is to imagine the pitch down as a way of decreasing angle of attack. If you're inverted in a power off loop and pull too hard, you release back pressure on the stick to break the wing from a stall. You point your nose more skyward and your power is off so it's not an airspeed increase that breaks the stall.. .. .Now consider your helicopter. Pushing the nose down decrease the angle of attack only on the advancing blades. The best you could hope for results in a tremendous dissymmettry on the forces at the rotor mast.. .. .Of course, the entire blade shouldn't be stalled. With the tangential velocities increasing with the radius and keeping twist to a minimum, only a portion of the blade should stall initially. If the blade can flap then the stalled portion would result in a loss of lift that allows the blade to flap downwards. This downwards flapping increases the angle of attack and breaks the stall. Haven't experimented with this myself I can only speculate that you would get an increased Nper vibration. A very rigid rotor wouldn't allow this insipient condition to be noticed.. .. .One thing still confuses me. Under what conditions are you planning on needing a recovery technique? With a governed rotor you should only encounter this problem with too high a DA or too heavy a helicopter. If you're making this helicopter simple to fly then the simplest solution is to overpower it for the published DA/AUW limits. The excess of power is a much easier way of preventing low Nr stalled blades than introducing a piloting technique to recover from it.. .. .Oh yeah, there already is a recovery technique. Increase power and lower collective.

Dave Jackson

17th Mar 2002, 08:22

Hi heedm. .. .Many moon ago I took some aerobatic flying instructions. The strongest memory is that of knowing where the bag was. <img border="0" title="" alt="[Smile]" src="smile.gif" /> . .. ._________________. .. .Lets assume that this 'special' helicopter is being piloted by Fred. Fred has just received his driver's license endorsement and is enjoying his first solo flight. The engine is fine but the governor goes to west. In fact, it goes west, east, north and south, because it just disintegrated. Fred is enjoying the scenery and doesn't notice that the rotor has dropped to 26 RPM.. .. .A moment later, he slowly passes a skydiver, who is in free fall. Fred is not worried. He knows that the skydiver is only traveling at about 130 mph and he knows that his Really Rigid Rotors will not folded up. After doing a few rolls and tucks with the skydiver, he feels that its time to pack in the fun.. .. .Fred lets the helicopter drop nose down, to decrease the angle of attack on the advancing blades, and then he brings up the RRPM. Next, he slowly raises the collective to apply lift on the advancing blades, of the two counterrotating rotors. He's not too concerned about the retreating blades because his 'special' helicopter came equipped with ABC. For a few moment he procrastinate whether to pull out of the dive using aft cyclic or, for a change, use the elevator. He decides to use the cyclic, since his hand is already on the stick. . .. .While returning home he begins thinking that this might get boring after a while, because it's so simple, but for now it's fun. Maybe next time he'll start off with a vortex ring state.

heedm

17th Mar 2002, 10:02

Dave, almost sounds like you'll be recommending all the driver's licence holders with a Synchrolite endorsement to take a 40 hour safety course on helicopters. <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> . .. .It's not enough to give these guys book knowledge. I think everyone reads that you have to steer into a skid to control it, but most people end up screwing that up the first time they hit ice.

ShyTorque

17th Mar 2002, 12:47

I still think Fred will need ABS equipped underpants!. .. .Seriously, though, I don't care what he does as long as the heli doesn't have front and rear foglights to be abused.. .. .Will Volvo make this un-crashable aircraft?

RW-1

18th Mar 2002, 23:14

I can only go by what I know.. .. . <blockquote>quote:<hr /> If this helicopter was to experience totally stalled rotors, why could it not do the same as the plane? <hr /></blockquote>Because our rotor pitch is not really related to the nose attitude.. .. .Getting the nose down in the plane is what we accomplish by lowering collective, you have lowered the angle of attack on the airfoil.. .. .Problem is with our rotor, when it stalls we really drop, and the relative wind angle goes to nearly vertical in seconds, which that happening, one can lower collective all we want, but likely would never get the rotor AOA back under AOA crit.. .. .This doesn't happen in the fixed wing airplane, and the nose down can get you well under the crit AOA.. .. .Now if we added negative pitch to the rotor system, like in my model heli, I wonder what you would have there.. .. .The other reason (to me) is that with a stalled rotor, you would lose cyclic authority which you would need if the nose lowering were going to have an effect at all.. .. .I hope I made partial sense.. . . . [ 18 March 2002, 19:40: Message edited by: RW-1 ]

Dave Jackson

19th Mar 2002, 11:42

RW-1. .. .You may well be correct. My thinking is all theory; but it might be workable.. .. .>"Problem is with our rotor, when it stalls we really drop, and the relative wind angle goes to nearly vertical in seconds"<. .. .The 6-blades of the * absolutely rigid* rotors will provide vertical drag and so will the horizontal stabilizer. If the helicopter's center of gravity is forward of the rotors' center of vertical drag, then the helicopter will start pitching nose down, just like a plane. At some point the rudder and elevator will start acquiring authority. When the helicopter is pointing and moving straight down the velocity will still be within operational limits. The blades will now have reacquired the ability to provide thrust. The helicopter is now flying, albeit flying vertically downward. The application of gradual collective and aft cyclic should now be able to pull the helicopter out of this steep dive.. .. .Ah, theory is wonderful.

RW-1

20th Mar 2002, 20:31

It would be interesting to see. <img border="0" title="" alt="[Smile]" src="smile.gif" /> . .. . <blockquote>quote:<hr /> The 6-blades of the * absolutely rigid* rotors will provide vertical drag <hr /></blockquote>I agree, but if the heli losing lift begins that vertical descent, could they handle the loads? Or would they bend back and snap prior to having the horizontal stab authority being reached for the nose down moment?. .. .My other thought is that if we are talking actual blade stall at that point, you lose RPM due to drag going up, leading to reduced rotational forces allowing them to bend up more. Then I go back up to the above para.. . . . [ 20 March 2002, 16:34: Message edited by: RW-1 ]

CyclicRick

21st Mar 2002, 00:40

I think you are all going off on a tangent. Why not just fit leading edge slats which deploy automatically at high pitch angles curing the stall problem and also negating the need for lots of RPM! Simple, then most good ideas are.. .Mind you the free fall para chap could grab hold of the tail and deploy his chute which will certainly give you more time to read the manual and pop the floats.

RW-1

21st Mar 2002, 01:42

Only going by the rotor stall to vertical or near vertical descent and associated relative wind change on the rotor airfoil: . .. .Your slats would still be of no use, even if the implementation of such a leading edge device and it's associated weight could be built into the rotor airfoil.. .. .What pitch angle would you set them for? for the purpose of our discussion (only) you may assume a stalling angle of, hmmm 10 degrees AOA.. .. .I'm not knocking your suggestion, but the topic is that the rotor HAS stalled, the rotor would still stall even with your leading edge device on it at some AOA, and then we are back to the scenario.. .. .When the rotor blade actually stalls, the change in relative wind in seconds goes from below 10 degrees to something in the neighborhood of 70-80, if not 90 degrees. . .. .The one thing we rotorheads are certain of, is that besides maintaining rotor RPM, to not let that blade stall, for thru the limitations of the blade pitch mechanics, there would in that situation be no way (yet, we are talking Dave's theory) to get that AOA back under 10 degrees and providing lift again.. .. .I've always thought we could use some good negative pitch in the system for an event, problems I see in doing so is that once you have stalled the rotor though, that you get into the relative wind change above, and then with the slowdown ifrom drag and Nr loss, you end up with blowback whacking the boom, and then the rotor no longer matters. . .. .Plus (to me) even if you could get the blades pitched down say to a 45-50 deg negative AOA, the loads on the rotor and rest of the aircraft, would be dropping like a rock already if not into free fall, can anyone take this end of the scenario and possible issues both positive and negative to a successful recovery using negative pitch?

Lu Zuckerman

21st Mar 2002, 02:36

It seems that everybody posting on this thread expounds upon their theories relative to how to overcome the stalling of the blades in Dave’s theoretical rotor system. How about discussing a blade that will not stall no matter how much pitch is added (within the limits of the control system). Such a blade design exists.

Dave Jackson

21st Mar 2002, 03:15

RW-1. .. .>"... but if the heli losing lift begins that vertical descent, could they [the blades] handle the loads? Or would they bend back and snap ..."< . .. .<a href="http://avia.russian.ee/vertigo/hiller_x-2-235-r.html" target="_blank">An interesting picture.</a>. .. .The 'intent' is to produce a beam similar to <a href="http://www.unicopter.com/1054.html" target="_blank">DESIGN: UniCopter ~ Rotor - Blade - NACA 00xx - Spar</a> out of pultruded carbon within the next six months. Hopefully, this will give a better understand of the strength, deflection and feasibility of the Really Rigid Rotor.. .. .Of course, the feathering bearings, hub, static mast and frame must also be much stronger.. .. ._______. .. .>"... possible issues both positive and negative to a successful recovery using negative pitch?"<. .. .Your idea about negative pitch may work, at least theoretically. You mentioned that "...the relative wind angle goes to nearly vertical in seconds ...". Like you said, this may require that the pitch go all the way down to -45 to -90-degrees, to initiate recovery. The rotor and blades will still have to be very strong, for all the previously discussed reasons. . .. .____________________. .. .CycleRick. .. .I would agree with RW-1's comments. . .Many more like that and RW might be promoted to RW+1. <img border="0" title="" alt="[Smile]" src="smile.gif" /> . .____________________. .. .Lu. .. .It will be interesting to understand more about this blade. Can you elaborate or suggest how and where to look.

Lu Zuckerman

21st Mar 2002, 05:24

To: Dave Jackson. .. .The blade in question was unique in its construction and in its’ aerodynamics. I may get the number of spanwise elements wrong but I believe there were five spanwise elements, each of which was a hollow extrusion. The elements were made in such away that they could be linked together by interlocking the elements together in a manner similar to tongue and groove lumber forming a constant aerodynamic profile.. .. .With the elements interlocked a bonding liquid was pumped into the interlocking spanwise areas and the negative twist was incorporated and the twist locked in by a fixture. The blade was then placed in an autoclave and the bonding material was cured making the blade into a monolithic structure capable of reacting all of the aerodynamic and mechanical loads normally found on a helicopter rotor system.. .. .Being hollow high velocity air could be pumped in via flexible couplings with the air being introduced through the rotor mast. Air was ejected from the tip providing the propulsive force to turn the rotor at speed. The upper surface of the outboard portion of the blade had a series of rectangular holes, which allowed air to flow out at high velocity. The high velocity air would adhere to the upper surface due to the Coanda Effect providing a large amount of lift with the result of the blade not stalling out due to high pitch or due to retreating blade stall. The idea was patented and it was planned to put the helicopter into production but raising the necessary funding proved to be the downfall of the program. The blade structural design concept was offered to the American Military and the major helicopter companies but it was rejected due to the not invented here attitude. The military rejected the idea for other reasons relating to their support of the designs they were already flying. . .. .The helicopter although smaller than many of the larger helicopters flown by the military could out lift any of them.. .. .The design of this helicopter never got to the point of cutting metal however a smaller prototype had been flown on many occasions proving the concept.. .. .All of the above took place at Boeing Field in Seattle.

RW-1

21st Mar 2002, 19:46

An interesting proposition for a blade Lu.. .. .But even with the coanda effect, which I can see eliminating stall at higher pitch angles than typically found in todays systems, and retreating blade stall, how much more of a cushion would it provide?. .. .Dave,. .. .On the negative, I only brought that up for my model heli's fly with negative for inverted flight, the model having much more power thrust to weight thamn most full scale operations. In a model set to auto I typically have anywhere from -3-4 degrees set in there, likely more to maintain model Nr than anything else. some in the model commumnity feel you need negative to "get it down from alt" normally, however I have disproved that in demonstration.. .. .I was thinking the nearest thing that might fly that could demonstrate what we might be looking at would be an X plane derivitive. Having a spinning rotor that would lock down at some point and those airfoils then providing conventional lift in forward flight. If we placed that aircraft in the hover and then brought down Nr (or increased pitch allowing it to stall, I wonder if it could handle such loads, already having to have to handle the loads of having the blades locked down.. .. .Ooh, is there $$$ involved in that promotion? <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />. . . . [ 21 March 2002, 20:54: Message edited by: RW-1 ]

Lu Zuckerman

21st Mar 2002, 21:01

To:RW-1. .. .It flies just like a normal helicopter but with the increased lift I believe it could hover and land with much less pitch than a normal powered rotor system at any given weight.

RW-1

22nd Mar 2002, 00:59

Ahh .... .. .I can agree with that, and that you likely then would not be prone to stall within the limits of the control system itself, (can I take that to mean you wouldn't be able to, appropriately powered, droop it in Nr to a point where we get blade stall and the conditions we're discussing?). .. .I believe what I mean in asking above is that we could design a blade that would not be stalled in the limits of the control system, but how does said design deal with power failure/ or (gasp!) pilot innatention to detail and allowing Nr to droop, for I would suppose that even that system at some point with low RPM would stall (we're talking higher AOA, not a non-stalling airfoil anyways right?) Or would this system only be succeptable to that type of situation, now haviong a higher atalling AOA for normal operations?. .. .And I do have to say I like the new signature. <img border="0" title="" alt="[Eek!]" src="eek.gif" />. . . . [ 21 March 2002, 21:04: Message edited by: RW-1 ]

Lu Zuckerman

22nd Mar 2002, 04:33

To RW-1. .. .Although the upper surface blowing adds to the lift it is also necessary to add pitch to go into a hover or to climb straight up. If the blades are at low mechanical pitch they are generating lift but not enough to raise the helicopter off the ground. Degree for degree of collective pitch when compared to a normal helicopter the upper surface blowing will generate a greater lift.. .. .In order for the upper surface blowing and the tip jet blowing to cease which would result in a stall condition it would take a complete power failure (both engines) and then the helicopter would autorotate just like a powered helicopter.

Dave Jackson

22nd Mar 2002, 04:45

Lu,. .. .Thanks. I found 29 patents that involved Coanda effect and rotor blades. They are interesting ideas, but if Coanda cannot provide enough force to offset rotor torque it probably will not give enough force for lift. . .. .Time will tell.. .______________. .. .RW-1. .. .Here's some more interesting ideas: . .<a href="http://www.vtol.org/uavpaper/NavyUAV.htm" target="_blank">VTOL UAVs Come of Age</a>. .. .The Bell Eagle Eye would make for a nice personal vehicle. <img border="0" title="" alt="[Cool]" src="cool.gif" />

heedm

22nd Mar 2002, 05:21

Lu, was there any mention of the power penalty required to pump high velocity air through all the blades?. .. .Granted, that penalty gives you a safety benefit, but if it considerably reduces your envelope, then it may not be desirable overall.

Lu Zuckerman

22nd Mar 2002, 08:01

To: Dave Jackson. .. .Quote:. .. .“Thanks. I found 29 patents that involved Coanda effect and rotor blades. They are interesting ideas, but if Coanda cannot provide enough force to offset rotor torque it probably will not give enough force for lift”.. .. .Response:. .. .I do not know what torque you are referring to. The upper surface blowing does not provide much propulsive force to rotate the blades. The tip discharge jet nozzle performs that work.. .. .To: heedm. .. .Quote:. .. .“Lu, was there any mention of the power penalty required to pump high velocity air through all the blades?. .. .Granted, that penalty gives you a safety benefit, but if it considerably reduces your envelope, then it may not be desirable overall”.. .. .Response:. .. .I am not knowledgeable relative to the engineering aspects and how much power is required to provide the propulsive force. What I do know is that the helicopter design had two gas turbine engines coupled to shafts that drove turbine compressors, which provide the compressed air. The jet efflux is passed down twin booms and provides some forward thrust and the efflux impinges on control vanes for directional control. The compressor discharge passes through air ejected intercoolers which minimizes the temperature of the air passing down the blades.

Dave Jackson

22nd Mar 2002, 10:46

Lu,. .. .Sorry. I miss read your posting and thought that Coanda was the sole source of forces.

RW-1

22nd Mar 2002, 19:32

<blockquote>quote:<hr /> The Bell Eagle Eye would make for a nice personal vehicle. <hr /></blockquote>Oh yes! It would!. .. .But I'm for my own solo-trek even though it would be a chute ride if I have a power failure <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> . .. .Two feet into the air and going ... hehe ....