Sport Rotors Airfoil(s)

The need for a trim spring and its necessary tension depends upon the geometry of the rotorhead in addition to rotor cyclic flapping angle.

Any rotor, whatever its characteristics, can be made to fly without a trim spring by adjusting offset between pitch pivot and spindle bolt.

The stabilizing effect of a Bensen offset gimbal rotorhead is diminished as offset is reduced.
 

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How did you infer that Chris?

Surely not from the discussion I was having with Juergen about a statement in the book “Helicopter Theory” by Wayne Johnson?

The SC sample section I have is quite tail heavy. A tail heavy rotorblade produces a very light stick. The requirement for trim spring tension reflects cyclic flapping angle, ie., the amount of rotor "blowback."

When you judge tail-heaviness or nose-heaviness in a blade, do you use the 25% rule, or is the center-of-pressure more applicable.

I am asking your opinion because the thickest part of the SC blade seems to be nearer 35% of chord; not necessarily COP, but generally.

Another thing caught my eye from your image of the RAF blade v. 8-H-12, and that is the aft part of the top surface. The RAF has straightened this out, and it appears SC does the same, while the 8-H-12 is rounded up a little. Some would suggest this would initiate air delamination and turbulence, I suppose it would be a matter of degree; but it would seem this would create an opposite effect from TE reflex; and would tend to move the COL aft in normal flight while allowing some relaxation as things slowed down. I wonder if this is the "inflexed" Jon wrote of.
 
If pitching moment coefficient is zero, the aerodynamic center and center of pressure are one and the same. For an 8H12, the aerodynamic center is located at 26% of chord.

Modern airfoil data does not normally include center of pressure information because it is not a fixed point but varies with airspeed if cambered. If needed, center of pressure can easily be calculated from aerodynamic center and pitching moment coefficient.

An example is this plot of the NACA 4412, a common FW airfoil but unsuitable for rotorblades unless equipped with a huge trailing edge reflex.

The product of area, dynamic pressure and pitching moment coefficient gives the nosedown pitching moment in ft-lbs. Knowing the aerodynamic center location enables calculation of the point at which it would balance; the center of pressure.
 

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Chuck..

Chuck..

Thank you for the response about trim springs, so if I understand you right ANY gyro can operate without trim springs, but the sportcopter rotors are MORE in need of them than other blade designs? If so then why are they designed (the machines) without them?
I am sort of like the race car driver who is talented at driving but has no idea what makes them run. In other words I can fly but don't understand the math behind what makes them go (kinda like gyroron put it).
After all is said and done, I have been flying a lightning for 2 .5 years now and have never been surprised or dissapointed in its handling characteristics. It is a fine machine and does exactly what I ask of it.
I am curious as you have such a better understanding of airfoils than I do what kind are you using on your gyro? who made them and what is the benefit of their shape or materials or construction over what I am using, if one?
Are you using a two seater heavy gyro or something more compareable to my lightning?
I hope you are able to explain it to me in laymans terms, I am educated but not in mathmatics.
Thanks in advance for your time.
Ben S
 
Thank you for the response about trim springs, so if I understand you right ANY gyro can operate without trim springs, but the sportcopter rotors are MORE in need of them than other blade designs? If so then why are they designed (the machines) without them?
I am sort of like the race car driver who is talented at driving but has no idea what makes them run. In other words I can fly but don't understand the math behind what makes them go (kinda like gyroron put it).
After all is said and done, I have been flying a lightning for 2 .5 years now and have never been surprised or dissapointed in its handling characteristics. It is a fine machine and does exactly what I ask of it.
I am curious as you have such a better understanding of airfoils than I do what kind are you using on your gyro? who made them and what is the benefit of their shape or materials or construction over what I am using, if one?
Are you using a two seater heavy gyro or something more compareable to my lightning?
I hope you are able to explain it to me in laymans terms, I am educated but not in mathmatics.
Thanks in advance for your time.
Ben S

This is why I posed the question of this "inflexed" area in the RAF blade when compared to the 8-H-12. The SC blades also seem to have it, and SC has removed some of their trailing edge reflex as well. So while the 8-H-12 aerodynamic center may be around 25% of chord, the RAF and SC blades may be at 30% or more.

I would think that this flattened area would tend to move the center of lift rearward as rotor speed increased.

This is just speculation on my part.

I can really appreciate the math and theory that many hang their hat on; as it provides a general understanding of airfoil performance; but in reality, there is not an aerodynamicist who would hang his hat on theory, no matter how good the math and understanding. Airfoil programs create simulated approximations based upon human understanding; and they are invaluable for this. However, the real world is where the wing hits the air, and nearly every publication on airfoils and aerodynamics appeals to real world testing as the final reality.

I wonder if someone would take the time to plot the RAF and SC blade coordinates for the different chord widths so they could be run in a computer simulation. Shouldn't be too difficult to set up.
 
If pitching moment coefficient is zero, the aerodynamic center and center of pressure are one and the same. For an 8H12, the aerodynamic center is located at 26% of chord.

Modern airfoil data does not normally include center of pressure information because it is not a fixed point but varies with airspeed if cambered. If needed, center of pressure can easily be calculated from aerodynamic center and pitching moment coefficient.

An example is this plot of the NACA 4412, a common FW airfoil but unsuitable for rotorblades unless equipped with a huge trailing edge reflex.

The product of area, dynamic pressure and pitching moment coefficient gives the nosedown pitching moment in ft-lbs. Knowing the aerodynamic center location enables calculation of the point at which it would balance; the center of pressure.

Please forgive my improper terminology Chuck.

Thanks for the input.

Terry
 
Ben, if you’ll look at the sketch of the rotorhead I posted earlier, the pitch pivot is set forward of the rotorhead spindle. The weight of the machine hanging from the pitch pivot tries to tilt the rotorhead nosedown but it’s balanced by the trim spring.

Upon encountering an upward gust, the increase of rotor thrust overpowers the trim spring and tends to tilt the rotorhead nose down as it should if the machine is to head into the relative wind. An upward gust, combined with forward speed shifts the angle of the relative wind from head on to one rising at an angle. That’s how all stable aircraft behave.

An aircraft that pitches nose up is unstable, magnifying the effect of the gust.

There are some gyros that would be nearly impossible to fly except for a Bensen style offset gimbal rotorhead.

I have flown a number of gyros with reduced rotorhead offset so that the trim spring is eliminated but all were less stable than Bensen’s stock layout. A gyro using a Bensen standard rotorhead, zero pitching moment coefficient blades and with the right trim spring rate will fly indefinitely hands off.

I’ve flown Bensen wood blades, Bensen metal blades, Hughes helicopter blades, homebrew metal blades that were built from a 3/8 inch thick spar, homebrew metal blades of 6” chord and symmetrical airfoil, homebrew symmetrical blades that had the leading edge radius formed in a press brake with trailing edges pulled together and riveted, Rotordyne blades, Stanzee blades, Rotorhawk blades, SkyWheel blades, homebrew Boeing-Vertol VR-7 rotorblades of 7 inch chord and 6 inch VR-7 rotorblades on a 3-blade rotor and last but not least DW rotorblades. The VR-7s were built around a laminated birch spar with vertical grain balsa aft section and fiberglass skins.

The 3/8-inch thick blades were the first gyro blades to use structural bonding without wraparound skins. That was in 1972.

My favorites were Hughes helicopter blades with their 5 pound tip weights; roll out of a tight turn, flare and play helicopter for a few seconds. DWs are the lowest drag blades and are properly balanced about the aerodynamic center with a zero pitching moment airfoil.
 
Thank you Chuck...

Thank you Chuck...

I think I get what your saying, My Sportcopter does not "nose up" when I thermal it on the gusts coming off the hill near my runway. As for the types of blades you have flown, that is a lot more types than me, maybe someday I will fly other machines with different blades on them but after looking at your pics of those ones with foam in them I sure am glad Jon builds his the way he does. They are tough as nails.....I KNOW, I put them to the test! (see earlier post)
It looked from your list that you haven't yet tried a set of Sportcopter blades on your machine, perhaps you should? You might just love them, they are VERY well made and finished.
Thanks again for the info.
Ben S
 
Ben, if you’d be so kind as to send me a short section of your crashed SC blades, I’d like to run it on a computer airfoil program. I’ll pay the postage in you can find a piece that isn’t distorted.

I only need a piece 2-3 inches long and we’ll know pitching moment coefficient if any. Computer “wind tunnels” aren’t the greatest thing for determining behavior as the airfoil approaches stall but are dead accurate for pitching moment and angle of zero lift.
 
Chuck...

Chuck...

Sorry but I had told my out of work teenage son that if he took them to the recycle plant for me he could get some money for the aluminum.
He didn't tell me how much he got for them, but I suspect it was a good amount two rotors and a hub bar aren't light!
Ben S
 
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