RAF Rotor Blades

C. Beaty;n1127758 said:
JC, during my early days of gyrocoptering, my partner and I wanted to find the limit of pitch setting for rotor blades.

We set our Bensen type metal blades at the upper limit of their pitch adjustment and with the two of us hand spinning the blades, managed to get them started.

Top speed was ~20 mph with the stick against the forward stop. More power; climb; less power; descend, all at 20 mph. It felt like riding a screw controlled by the throttle.

We didn’t have an accurate rotor tachometer; simply a bicycle generator running a voltmeter.

Other gyro flyers tried the same thing with other metal blades available at the time with similar results.

I wouldn't recommend trying this with rotor blades that had a negative pitching moment coefficient.



Like slow-flying a FW STOL plane with deployed flaps and the nose very high... I understand that both lift and drag increase a lot, but lift generation is still enough. If I'm not wrong, that would mean that the rotor turned more slowly, and that the fuel consumption was very high, thus lowering endurance & range...
 
Chuck,
The high Cmo of Bensen's profile and its low torsional stiffness mades possible a large angular flapping before reaching the excessive stall of the retreating blade: Thus in flight, the stop of the stick was reached before the divergence of a1.
But with a moment coefficient just zero, the divergence appears before the stop of the stick. In this case the blades hit their stops in flight
 
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Chuck thanks for sharing that story with us,it sounds like you were right on the edge,but that's what test pilots do, they go right to the edge.!!
 
JC, the Bensen rotor blades had 0.032” upper skins and 0.050” lower skins; the other metal blades of that era, Stanzee and Rotordyne. had 0.032” aluminum skins wrapped around an aluminum spar.

The stick was against the forward stop because the Bensens of that time were mostly CLT and had no effective horizontal stabilizer, resulting in an extreme nose high attitude of the airframe at ~20 MPH.
 
C. Beaty;n1127745 said:
The designers of the RAF-2000 gyro, like most retail gyro designers, used the ”by guess and by gosh” method and wouldn’t have the foggiest notion of how rotor RPM related to air density. They knew from experience that rotor RPM increased with altitude.

My guess is that they increased pitch with altitude because they were concerned about the strength of blade/hub attachment and wanted to restrict rotor RPM.

So most gyroplane designers of commercial gyroplanes have no clue of effect of density altitude on true air speed? Because rotor RPM is nothing more aerodynamically on a blade section than speed. Really Chuck? May be you should restrict this comment to a subset or particular manufacturer(s).
 
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C. Beaty;n1127784 said:
JC, the Bensen rotor blades had 0.032” upper skins and 0.050” lower skins; the other metal blades of that era, Stanzee and Rotordyne. had 0.032” aluminum skins wrapped around an aluminum spar.
The stick was against the forward stop because the Bensens of that time were mostly CLT and had no effective horizontal stabilizer, resulting in an extreme nose high attitude of the airframe at ~20 MPH.

Chuck,
Are you trying to tell me that the Bensen's blades can not be deformed enough by the Cm0, to explain the stick on its front stop? This would only be because of the flapping angle a1?
Yet, my simulations shows that the steady flapping angle does not exceed 3 degrees, even with Mu = 0.35
And this is confirmed by the measures shown in the NACA report 475. While you say that the control reaches 8 ° less than the tip plane (Stick on the front stop).
My opinion is that the supplement is due to the deformation of the blades. What is missing?
Thank you for your clarifications
Sans titre1.png
 
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I expect you’re correct, JC. I know that Bensen wood rotor blades had considerable positive moment coefficient that caused the stick to reach the forward stop at ~60 MPH.

But I still have difficulties understanding how Bensen’s metal blades could have had a strong nose up moment; being derived from an 8H12 airfoil along with sufficient torsional flexibility to produce so much flapping.
 
I am having some difficulty is trying to figure out how a twisted blade would affect the driven/driving areas of the rotor. It seems to me that a greater relative angle of attack near the hub would increase the area of the stalled region. This would seem to me to make retreating blade stall induced vibrations of the rotor to increase. I may have this backwards in my head though. It would also help if I could find at what incidence untwisted rotors such as on the Mto or AR-1 are set at. Also is the twist of the RAF rotors constant and linear from root to tip, or is more twisted just at the root. Hope this all makes sense.

RogerS
 
Roger,
If the twisted blades are set to get the same rpm as with non-twisted blades, then the angle of attack at 0.75 R is almost unchanged, and the
twisting only changed the distribution of lift along the blade span. Thus, a negative twist moves the thrust vector of each blade a little closer to the center.
The lift component closer to the center gives less cone, which reduces the vibration due to the cone
The drag component closer to the center gives less drag vibration
The widening of the stalled area on the retreating blade is almost without effect due to low airspeed at this place
The lower angle of attack on tip decreases the ratio L/D in this high airspeed area and increases a bit the general drag of rotor.
 
This helps. I see that if the rrpm is unchanged the overall angle of attack is the same. The summation of the whole blade is the same. I wasn't visualizing it as a negative twist. So if the RAF is set with the tip at 1.5 degrees, this is less than a straight blade would be at the tip? It would still help if I knew what angle the straight blades were normally set at. I understand what you are saying about the coning. So the twist moves the driving section closer the center and at the same time the lift vector is angled forward more, increasing driving force, but over a shorter section of the blade? Thanks for the education.

RogerS
 
Roger,
A usual pitch setting for autorotation with flat blades is about 3.5 degrees relatively to the zero lift direction of the profile used)

When RAF says 5 ° at the root and 1,5 ° at the root, it is;
1) - at rest (in flight there can exist a dynamic torsion due to the load)
2) - on the bottom of their profile (it exist a difference relatively the direction of 0 lift)

To obtain the same rrpm as with flat blades assuming a zero deformation due to the load, it will take 5.9 ° at the root (ie 5 ° with respect to the bottom) and 2.4 ° at the tip (ie 1.5 ° with respect to the bottom), as makes RAF
 
I found this Javafoil analyses of an RAF rotorblade on a CD labeled 2003.
The pitching moment is close enough to zero to be ignored.
Javafoil and other computer airfoil analyses programs accurately determine pitching moments but lift/drag ratios are iffy. The improvement shown for the Clark Y nose may not be as great as Javafoil indicates.

RAE blade.JPG
 
Thank you both. I've got a better grasp. Years ago I spent a lot of time looking at javafoil while trying to design a set of stall regulated blades for a 10kw grid tied wind turbine. You guys would have been a big help and would have made short work of the task.

RogerS.
 
Mike G;n1127727 said:
Jean Claude’s spreadsheet allows the input of rotor twist at different distances along the blade and suggests that to agree with the Rrpm measured in flight the blade needs to be twisted in the opposite direction to what RAF say it is and what Eddie has confirmed it is.
However Chuck has just told us that his analysis of the RAF aerofoil suggests that the aerofoil has a zero pitch moment, the plot thickens.

The plot clears when I favor the zero pitch moment indicated by Chuck, rather than the too slow Rrpm measured in flight by Eddie. Analog tachometers are often poorly calibrated.
 
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One of the questions I asked in poste#13 was

I’m still not clear about the angles being talked about. Does the RAF manual refer to the pitch angle of the blade relative to the chord axis, the zero-lift axis or simply the flat undersurface of the blade? Since it would be practically impossible for the average owner to measure the pitch relative to the zero-lift axis or the chord axis I suppose that it was relative to the flat undersurface which would be the most simple, does anybody know?

JC is telling us that RAF probably gave the pitch angles relative to the flat underside of the blade. And that gives us a result that can make Chuck's observation about zero twisting and the calculations agree, but with the assumption that the rotor tacho might be under reading. Comments RAF owners???

I also asked:

Does anyone have more information regarding the development of these blades and angles that could help us understand better their performance?

Come on RAF drivers we are trying to understand your gyro rotor performance for you; you could at least join in and help by giving us some more data to play with.

Mike G
 
When I purchased The Predator she had RAF blades.

The flight rotor rpm was around 320 solo (1,100 pounds) and around 340 near gross.

My 30 foot eight and a half inch chord Sport Copter blades spin a little faster at the same weights with the same rotor tachometer (340 to 360 rotor rpm).

I never measured the twist in the RAF blades and I had the older hub bar.

I replaced the RAF blades because of cracks in the training edge that were growing.

The Predator first flew in 1999. I don't know what version of RAF blades they were.
 
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Vance

Thanks, I don't know anything about Sportcopter blades, do you have any data/drawings? The things I'm interested in are:
diameter
chord
profile
pitch (relative to which reference, chord line, zero lift axis or other)
twist
blade weight
blade CofG spanwise
hub bar/teeter block & blade bolts weight
hub bar length
pre cone angle
undersling

If you don't want to publish the data here, please PM me.

Mike
 
The total knowledge I have of the Sport Copter blades are in my last post.

I recall them being heavy but I don’t remember the number.

They are very flat bottomed and not anything like the RAF blades in profile.

I purchased them set up so I don’t know the pitch.

Sorry I can’t be more help Mike.
.
 
Were raf blades originally designed as a helicopter blade and then used on a gyro? The reason I ask is that it has more pitch at the root than the tip, it should be the other way for a gyro as the airflow is up through the rotor instead of down like a helicopter.
 
Here are the ordinates and Javafoil plot of a SC rotor I found on a CD dated 11/10/04.
The ordinates were extracted from a sample blade section by mounting on a milling machine table with digital XY readout.
Hopefully, the dangerous negative pitching moment has been corrected in more up to date rotors.
Disregard the 0012 label on the plot; I had evidently forgotten to correct it when running the SC blade.

SC blade.JPG
 
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