Aerodynamics - Stall Recovery
 

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;</font><ul type="square">[*]<font size="2" face="Verdana, Arial, Helvetica"> Symmetry, due to twin (side-by-side or intermeshing) rotors.</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica"> Advancing Blade Concept.</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica"> Absolutely rigid rotors.</font></li>[*]<font size="2" face="Verdana, Arial, Helvetica"> Adjustable horizontal tail (elevator).. .</font></li>[/list]<font size="2" face="Verdana, Arial, Helvetica">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.. . . . <small>[ 15 March 2002, 20:06: Message edited by: Dave Jackson ]</small>

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" />

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

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>. . . . <small>[ 16 March 2002, 06:47: Message edited by: Dave Jackson ]</small>

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" />

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)

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.

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.. .. .&gt;"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."&lt;. .. .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" />

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" />. . . . <small>[ 16 March 2002, 23:34: Message edited by: ShyTorque ]</small>

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" />

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?. . . . <small>[ 16 March 2002, 23:49: Message edited by: Dave Jackson ]</small>

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.

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.

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.

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?

I can only go by what I know.. .. . </font><blockquote><font size="1" face="Verdana, Arial, Helvetica">quote:</font><hr /><font size="2" face="Verdana, Arial, Helvetica"> If this helicopter was to experience totally stalled rotors, why could it not do the same as the plane? </font><hr /></blockquote><font size="2" face="Verdana, Arial, Helvetica">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.. . . . <small>[ 18 March 2002, 19:40: Message edited by: RW-1 ]</small>

RW-1. .. .You may well be correct. My thinking is all theory; but it might be workable.. .. .&gt;"Problem is with our rotor, when it stalls we really drop, and the relative wind angle goes to nearly vertical in seconds"&lt;. .. .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.

It would be interesting to see. <img border="0" title="" alt="[Smile]" src="smile.gif" /> . .. . </font><blockquote><font size="1" face="Verdana, Arial, Helvetica">quote:</font><hr /><font size="2" face="Verdana, Arial, Helvetica"> The 6-blades of the * absolutely rigid* rotors will provide vertical drag </font><hr /></blockquote><font size="2" face="Verdana, Arial, Helvetica">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.. . . . <small>[ 20 March 2002, 16:34: Message edited by: RW-1 ]</small>

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.

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?