I am looking for a way to find the correct blade angle on an autogyro I am building. There is never enough wind to get a good feel for how the blades will spool up and the kind of lift they will produce. I have thought about using a convertible, but that has proven to be difficult. The blades are my own design and have a symmetrical airfoil. Are there any standards to start with? Does -2 to -4 degrees sound right?

Thanks for the input

"Julch" (Can't we please use our real names?!)

>>>...have a symmetrical airfoil. Are there any standards to start with?...<<<

I will refer you and this "string" to our rotor blade expert, Bill F.

In the mean time, I would suggest a flat-bottom airfoil, very light construction and if you want to experience true autorotation, begin your attempts accelerating into autorotation using flat blade pitch.

Dear "J..."

I'll try to uphold Steve's confidence! <G>

First off, in order for an autogyro rotor to "autorotate" the blades of the rotor must be producing lift with a minimum of profile drag. So symmetrical airfoils can be made to work, but there are, as Steve says, better alterrnatives.

The Clark-Y is a good airfoil. In fact a regular Clark-Y profile that is made a bit thinner than the "correct" pattern is probably a very good choice. I believe the regular Clark-Y has a max thickness of something like 11.72%. It has a zero lift angle of attack of -3.51 degrees.

The wind tunnel tests that Les Garber and I did a couple of years ago revealed that the SG6042 produced superior lifting performance. And flight tests seem to bear this out. This airfoil has a very slight undercamber toward the trailing edge. (The Clark-Y is flat from about 10% back from the leading edge all the way to the trailing edge.) The SG6042 has a zero lift angle of attack of -4.92 degrees and is 10% thick.

Using a Clark-Y profile that is 10% thick should give very good performance and the blade is easier to make since it doesn't have the slight undercamber.

Eric Sanders' Compufoil program will give you the profiles of these airfoils and you can manipulate the thickness too.

The wind tunnel information is summarized in an RC Modeler article published in the August 2001 issue pp 70-76.

Hope this helps.
Bill

Dear "J...."

In my previous post, I neglected to include that a symmetrical airfoil needs to have a positive angle of incidence or, if it is a wing,
a positive angle of attack. Otherwise it doesn't produce lift.

So for a symmetrical airfoil rotor blade to get into autorotation, it must have a positive angle of incidence relative to the plane of rotation.

However, a blade with positive incidence is very very very difficult to get up to speed so that it is producing enough lift to get into autorotation.

That is why the Clark-Y and the SG6042 are so well suited to autogyro rotors, They produce lift even at negative angles of incidence!

In fact a well adjusted rotor (that is one that has been fiddled with enough to start and yet go into autoroation) will easily get into autorotation by simply walking into a 5mph wind holding the rotor at about a 45 degree angle to the wind.

It is worth spending a lot of time, getting the angle of incidence of the rotor blades just right. In fact a rotor that won't spin up into autorotation can be "fixed" by adding as little as one layer of masking tape under the trailing edge of the blades. Conversely, a rotor that starts too easily, probably is simply windmilling and not really going into autorotation. Here the fix is to add a layer of masking tape at a time under the leading edge of the blades.

By the way, three blades are better than two and 4 blades are better than 3.

Bill

Can some one tell me Y 3 BLADS are beter than 2 ?
I flew on Ken Brock gyro 2bladed of course.

Dear "Oldf...2"

Three blades generated more lift than two and four somewhat more than three in our wind tunnel tests. See RCM August 2001.

These are measured observations. Possible explanation might be that more blades give a larger "solidity factor."

Bill

I'll add something here in reference to multibladed systems vs. 2-blade, teetering systems. (Keep in mind I am discussing only findings derived by lots of experimentation on MODEL autogyros.) Also, the following is NOT comparing net lift, but ease of control; roll-pitch stability:

A multibladed rotor system, using individually flapping blades and not too light a disk loading, is noticeably more stable throughout the model flight envelope. Particularly on gusty days, the multibladed system is much less "bothered" by bumps and breezes. Without pilot input it seems to compensate for these irritations. With normal pilot input, the gyro just flies so much smoother.

My one reservation is on VERY LIGHT disc loaded, multibladed designs: fast rotor rpm ALSO promotes stability. A multibladed system in full autorotation turns slower than a 2-bladed system of the same dimension and weight. Since very lightly disc loaded model rotor systems rotate more slowly, you may not notice the stability improvement. Load the system a little, though and you will.

THANK YOU, I KNEW THERE HAD TO BE A REASON FOR THREE
BLADES.



TWO BLADE SYSTEMS ARE EASIER TO BUILD FOR
HOME BUILT GET ON & FLY GYRO'S.

Hi Steve...

In viewing the video from this September's autogyro fly-in in Martinsville, it appears that the multi-bladed gyros do indeed fly more smoothly. The first day provided significant gusts aloft and the multi-blade gyros appear to handle those conditions very smoothly.

However, Leon Wolf's Whistler really bored through the air at great speed and didn't seem much affected either. It may be that the Whistler's wings help to provide stability.

Certainly, a teetering two blade rotor would seem to be less smooth simply because the two blades (increased coupled mass) have to move together to accomodate bumps, etc. I wonder how well a two-blade, flapping rotor would work?

Bill

Bill,

>>>...I wonder how well a two-blade, flapping rotor would...<<<

Over half my testing has been with 2-bladed, individually flapped, elastomeric heads. Using the same exact model with flights alternating heads (only minutes apart) using the exact same blade lengths, etc., the 3 and 4-bladed systems seemed so much more forgiving. (Granted, they each were 1 and 2-blades heavier...? Doubt if 2 and 4 oz. made much difference, though.)

Keep in mind the Whistler you spoke of is really a windmill (lots of negative blade pitch) and flies a lot more on the wing than the rotor during cruise. You take those wings off, add lateral tilt for control, reduce negative blade pitch so it can slow down like a gyro and I'll bet a dollar to a donut the multibladed head would fly much smoother than a 2-bladed, individual flapping head.

Isn't a big part of the "cure" for the K-Kopter a multibladed head?

Based in Emery Wayman's modified K-Kopter, you are absolutely correct!!

However, my performance at Martinsville in Sept, might suggest otherwise!! <G>

Bill

IT looks like I started something here.
I AM HAPPY TO LEARN OF THE EXTRA STABILITY
THAT THREE OR MORE BLADE'S PRODUCE. :-)

The fewer amount of blades, the more effecient it is. Maybe not stable wise but more effeicient.

My two cents

I fly the very 'light' models you speak of ( disc loadings of around 3.5). I have extensively tried both two and three bladed hubs using the same type of hub construction ( wood polypropylene sandwich) and identical blades. It became very clear early on that three blades gave more lift and increased stability than the two blade hub.
Rpm on the two blade hub was noticeably higher but was more difficult to initially spin up.
I will in future only use three blade hubs as I found the easier build of a two blade hub is more trouible than its worth getting it to perform right.

These are just my own findings so hardly represent a conclusive result.

Sean

Hi ya'll,
2 blades versus three thoughts and on going investigation.
2 blades have a great advantage when simplicity is concerned.
1/3 less effort to produce blades and the blade hanger can be
simplistic.
Bill F's tunnel tests got me interested in lower aspect ratio.
Thought being if the area of 2 equaled the area of 3 then perhaps lift would be similiar?
Then in the chord sizes blades have there are aerodynam
ic reasons why wider chords could be more efficent.
So having little else to do I am investigating.
So far ground and "car tunnel" tests have been very encouraging, some even astounding.
Currently I am about to commence flight tests and will report those findings.
I have reports of the car tunnel tests. Could Email them if you would desire, OK?
Will see ya'll at Hernando next time? "Autogyros over Spring Hill" Word is it is already organized!
Good luck!

Hal [email protected]