Aerodynamics - Stall Recovery

[Lu Zuckerman]

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]

RW-1. .. .&gt;"... but if the heli losing lift begins that vertical descent, could they [the blades] handle the loads? Or would they bend back and snap ..."&lt; . .. .<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.. .. ._______. .. .&gt;"... possible issues both positive and negative to a successful recovery using negative pitch?"&lt;. .. .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]

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]

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" />. . . . <small>[ 21 March 2002, 20:54: Message edited by: RW-1 ]</small>

[Lu Zuckerman]

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]

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" />. . . . <small>[ 21 March 2002, 21:04: Message edited by: RW-1 ]</small>

[Lu Zuckerman]

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]

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]

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]

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]

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

[RW-1]

</font><blockquote><font size="1" face="Verdana, Arial, Helvetica">quote:</font><hr /><font size="2" face="Verdana, Arial, Helvetica"> The Bell Eagle Eye would make for a nice personal vehicle. </font><hr /></blockquote><font size="2" face="Verdana, Arial, Helvetica">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 ....