Rotor blades tips?

[C. Beaty]

Ernie has evaluated various tip configurations on Dragonwings; teardrop shape, droop tips and vane tips to turn the vortex the other way. Of all configurations, the squared off tip worked as well or better than any.

In actual measurements I made a number of years ago by plotting rotor disc angle of various rotors at constant airspeed, DWs had the lowest drag of the rotors available at the time; Skywheels, Rotordyne, Rotorhawk and Bensen. DWs had a lift/drag ratio of ~8:1 at 50 mph on a 500 lb. machine, second best were Skywheels at ~ 7:1.

Results were published in a Sunstate newsletter at the time. Someone must have a copy.

DWs on a Mini-500 provided a substantial performance increase over the stock blades. If the rotor rpm was allowed to drop into the yellow in a 3-ft. hover, the stock blades could not be powered out and a landing was inevitable. With DWs, simply roll on more power and fly away.

There is no scientific literature on square vs. rounded tips and it is reasonable to expect the aerodynamicists at Hughes knew what they were doing when they squared the tips on the Hughes OH-6/500.

There is considerable scientific literature on delaying the formation of shock waves on helicopter rotor tips, swept tips, ogee tips and arrowhead looking things. That’s not an issue with ordinary gyros.

Teardrop shaped tips do look nice on rotors but with respect to performance, it’s simply lipstick on a hog.

 

[magilla]

I agree with Chuck B on this, after reading about the Sikorsky swept tips on the UH-60, and the BURP Blades on the Wessex Lynx.

However - one OBVIOUS benefit for removable tips is this: If you end up getting blade rash from a blade striking the runway during prerotation / taxi run, it would be FAR FAR less expensive to remove and replace a plastic tip than to replace/fix the whole blade.

UH-60 blade tips run 14K each. Blades are 125K. You do the math on how much you are saved if you make contact. We had a rotor hit a branch that fell from a tree on a tactical LZ. Four blade tips damaged.... easy to replace. Would have been a 500k fluke accident and a Class C Army Accident instead of the 64k Class D or E (don't remember which) that it ended up being. Removable blade tips = replaceable blade tips.

Also, if you have the capability of doing balancing, the location of the screws/bolts to attach the tips is the PERFECT place to make fairly large weight adjustments to balance blades. A very tiny weight here, as in grams, would equate to several ounces of weight at the roots.

Just my 2c.

 

[WaspAir]

... and the BURP Blades on the Wessex Lynx.

Burp is what you do after those Hooters meals discussed earlier in this thread. BERP was the Westland trick tip (the E is for Experimental, I think). I wonder if there is a Hooters near the Westland test facility.

. . .one OBVIOUS benefit for removable tips is this: If you end up getting blade rash from a blade striking the runway during prerotation / taxi run, it would be FAR FAR less expensive to remove and replace a plastic tip than to replace/fix the whole blade.

Which leads to the lawyer joke of the day:

What's the difference between a lawyer and a vulture?


Removable wingtips.

 

[SnoBird]

...sounded to me like he was agreeing with you and adding yet another loss that you hadn't. Some volume of air would move through the blade and it would do what it does and it isn't clear to me that it's negligable...Which didn't seem to match your response to SnoBird who was agreeing with you and adding another contributing effect that gets addressed by end caps, blunt or curvy.

Thanks Larry for the supportive comments, you nailed my sentiments exactly. Although I don't think Dennis intended to be derogatory towards me, I'm guessin' he was just funnin' with his remark (i.e. I took no offense by it).

Another potentially interesting pursuit might be to place an aerodynamically contoured tip that has strategically located "aspirator" holes. The holes could bleed the slightly compressed air being pumped through the rotor, and if designed carefully, might energize or otherwise enhance the behavior of the boundary layer at the tip in a way that actually might decrease tip drag slightly (compared to a standard, capped, squared off tip). I suspect if many hours and many research $$$ were dumped into the project, you might just about offset the losses caused by pumping the air through the blade. :lol:

 

[DennisFetters]

Although I don't think Dennis intended to be derogatory towards me, I'm guessin' he was just funnin' with his remark (i.e. I took no offense by it).

Not at all! I was just joking around, and sorry if anyone gained anything from it than that.

Although, It saddens me to see so many people misunderstanding the significance in rotor-tips. But, that is why some peoples products turn out to be better than others. Time tells all.

 

[Arnie Madsen]

Just curious... Does anybody out there know how many Hooters Buffalo Wings it takes to activate a standard bypass valve at sea level?

I dont know the precise answer Dennis , but I do know you sure made us laugh on your first forum post. Welcome.
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Does anyone agree that special blade tips can usually only benefit a rotor with high tip speeds and high aircraft forward speed such as modern helicopters designed for speed and gyros like Carter experimental MU project ?

If our typical gyro's are not in this league is that why square tips work fine ?

magilla I never thought about tips being used as protection against tip damage. Good idea.

Chuck or anybody There are modern small and light weight remote air pressure sensors available. I think some trucking outfits use them. If a person really wanted to test air-pumping rates of an open rotor could a couple of these sensors (properly) mounted inside the tips be used ? I am thinking if air is being pumped , the (internal) blade air pressure would be higher (at the tip).

I also recall some designers drilling small holes in sealed tips to avoid "compressability". Are they referring to internal air pressure or have I mis-understood something ? (I understand external compressability issues , but these holes drilled in the tip left me scratching my head)

Thanks.
Arnie
Bell 47

 

[C. Beaty]

Arnie, the centrifugal pressure buildup inside a rotor blade is the same as the ram or stagnation pressure that would be measured at the tips with a pitot tube.

Pressure = ½*p*V² p= air density, 0.0023 at sea level.

With 400 fps tip speed, stagnation pressure would be 184 lb/ft² or 1.28 psi. This would be enough to distort an airfoil if not relieved.

Standard practice is to vent the tips with small bleed holes and to nearly seal the root end. Then any distortion taking place as the result of centrifuging occurs at the root end where it’s not critical.

Vent holes at blade tips also allow condensation to be expelled.

I’ve attempted the calculation of power loss from centrifugal pumping but there are too many unknowns such as the discharge coefficient of an open tip and internal friction for more than a ball park number. I’ve measured ~2 hp on a tail rotor running at about 500 fps tip speed of less than ½ the cross section area of a typical rotor blade.

 

[RotoPlane]

Chuck or anybody There are modern small and light weight remote air pressure sensors available. I think some trucking outfits use them. If a person really wanted to test air-pumping rates of an open rotor could a couple of these sensors (properly) mounted inside the tips be used ? I am thinking if air is being pumped , the (internal) blade air pressure would be higher (at the tip).

I am sure you are right Arnie, the air will be compressed more at the tip and doing so will pull a vacuum at the root.

I am designing blades now for my gyro, based on an idea that the blades can be a giant centrifugal compressor. It seems plausible that if air is blown into the root, this air will be accelerated down the blade by centrifugal force and exit out a rear-facing tip nozzle. This accelerated air will try to pull more air from the root, by lowering the air pressure there, thus allowing the blower to give more air flow than it would statically. I figure it will take ~17 hp to run the blower and spin my 22' rotor to 350 rrpm, at 0 deg pitch. The blower runs continuously and so will help maintain the rotor rrpm during flight and lessen the disk AoA and drag.

The remote air pressure sensors you mentioned is a good idea. They would give the pressure at the tip and the root with 0 rrpm (rotor brake on) and at 350 rrpm. That would tell how much air the blades are pumping.

 

[SnoBird]

Although, It saddens me to see so many people misunderstanding the significance in rotor-tips. But, that is why some peoples products turn out to be better than others. Time tells all.

Did you ever test your blades with and without the end caps? If so what was the test configuration and what were your results? Reason I ask is because if the root was taking in air and pumping it out of the tip, the air flowing through the open end will effectively serve as a form of aerodynamic tip by itself. In other words, the nature of the airflow over the tip will not be the same as an open tip that does not have air flowing through it (i.e. with the root capped). Although I haven't verified this experimentally, the airflow coming out of the tip would probably serve to streamline the tip as it moved through the air, thereby reducing the drag somewhat at the tip. However, the rotor would require more power to continually centrifuge the entrained air out of the tips. The only place the autogyro rotor can get that power is to extract it from the air. This would result in a net higher angle of attack for the rotor and would effectively equate to increased induced drag for the rotor system (meaning the engine would have to produce more power to compensate). So although you'd likely see some type of decrease in tip drag (by pumping air though the tip), you'd probably lose at least that much energy as a result of an induced rotor drag increase. So by capping the ends, you may have increased the performance of the rotor not so much because of the specific shape or by reducing drag at the tips, but because pumping losses were halted by the caps presence. It's even possible that when air is flowing through the blades, the open ends were more streamlined than the capped ends (although that's pure speculation without running tests).

As an aside for anyone who might be interested, in theoretical aerodynamics just about any physical shape or form can be mathematically modelled using "sinks", "sources", "vortex sheets/filaments", "doublets" etc. You can add them and subtract them and mathematically manipulate them in a way that simulates real physical shapes (like airfoils, for instance). The air flowing out of the tip of a hollow rotorblade is a real life source (albeit imperfect considering real life variables and the structurally segmented airfoil profile). If you mathematically modeled it and combined it with the oncoming freestream (which varies continuously on a rotorcraft blade in forward flight), you'd see it form a sort of streamlined fairing shape at the tip of the blade. The only catch is that the shape would be squishy and would shift-shape during every revolution depending on how fast the rotor was spinning, the airspeed and where it was in its azimuth in forward flight (because the magnitude and direction of the oncoming freestream varies tremendously at the tip under these circumstances).

 

[Master Roda]

Oh, I want to hear more.