View Full Version : Blade Pitch
05-04-2005, 09:35 PM
I am experimenting with some different blade configurations on my honeybee and wondering if anybody has some practical experience they might want to throw my way. I'm using 25 foot bensen blades and I've tried both extremes. First I pitched them all the way up. I have a prerotator, so they weren't too bad to get spinning (although it certainly wasn't easy and I started to flap them, but shoved the stick forward in time to avoid any damage) but they were still kind of a pain to get spun up and REAL draggy once they were. Then I tried taking all the pitch out, well that didn't work at all, the rotor RPM was way up, and they were barely creating enough lift to get off the runway. Then...it got dark and I ran out of time. So, If there is some magic pitch that works really well, or if there is something I'm missing please let me know!
05-04-2005, 09:51 PM
Correct pitch would be sumwhere between the two settings you'v already tryed. :D
05-05-2005, 05:48 AM
Bensen blades had scribe marks on the bottom of the blocks where you make the pitch adjustment. Align the marks, and away you go.
I believe it gives you 2º of pitch.
05-05-2005, 06:32 AM
Diameter and pitch can traded off against each other to some extent. The object of the game is to get tip speed down without stalling too much of the inboard portion of the blade (especially the retreating blade).
With 22-foot Bensens, we found back in the 70's that a pitch setting about one scribe mark's width higher than the factory marks gave better performance. This amounted to increasing the pitch from 1.5 deg. (the factory pitch) to a little over 2 deg.
On a larger rotor diameter, however, especially on a light gyro, the factory marks ought to give you enough pitch. I use only 1 deg. on 24.5-foot Rotordynes on my 'Bee.
In the end, however, blades with non-flush rivets are so inefficient that they are not very practical on low-powered gyros such as the typical Gyrobee -- no amount of pitch/diameter adjustment can get rid of the drag of those rivets. The breakthrough that made 40 hp gyros practical was the smooth rotor blade with an accurate airfoil section, a la Skywheels, Dragon Wings and some others.
05-05-2005, 06:47 AM
Well, I'll have to use a protractor to try that 1 degree setting since my hub bar doesn't have factory scribe marks (it was home made many moons ago by gyro enthusiast Tom Milton). As far as the inherent draggyness of the blades go, I found this on Ralph's bee site:
The light disc loading has other consequences that are not so obvious, one of these being a low rotational speed for the rotor system. A gyro rotor doesn't do any more work than necessary to generate the lift needed to keep the craft in the air. Because of its light disc loading, the Gyrobee's rotor turns at only 305 rpm in flight, compared to 400 rpm or more for the typical single-seat machine. This has two positive effects. The first is low parasitic drag. Parasitic drag increase with the square of rotor speed, so much less of the power of the engine is needed to overcome parasitic blade drag in the Gyrobee and that's more of the available power to fly the aircraft. The second effect of the low rotational speed of the rotor system is that the Gyrobee is quite insensitive to the blades you use. For example, riveted blades (like the Brocks) are often notably less efficient on most gyros compared with laminar flow designs. Since the blades are rotating more slowly on the Gyrobee, these differences are much less pronounced.
I will try 1 degree and experiment some more, thanks for the input!
05-05-2005, 07:12 AM
Read our good friend Ralph's words carefully. It's true that lower RRPM makes the rivet problem (and the related "lousy airfoil" problem) somewhat less severe. It's very much still there, however. Exposed-rivet blades are noticeably less efficient, even on a Gyrobee. The Bensen airfoil, with a flat bottom, a lap joint on bottom and lots of camber way forward, suffers an additional efficiency penalty.
If you're sticking with those blades regardless, definitely get some accurate pitch marks on your hub. I'd try to have marks for 1.0 deg. and 1.5 deg. and work within that range for starters. 0.5 deg. is about .009" on a one-inch radius out from the pivot point, though, so it's hard to see. Bring reading glasses if you're over 40.
05-05-2005, 12:04 PM
I want to stick with the bensen blades for 2 reasons. Firstly, they are very light and since they are so light I don't think my wunderlich prerotator is throwing things off too much. Secondly, I am getting a good deal on them ;) I will experiment with different pitch settings and see what happens.
05-05-2005, 02:25 PM
I talked to the guy who makes them at Bensen and he says that the correct pitch is 1.2 degrees and he sets them by putting the teeter bolt into a v block and checks the blade angle at the tip with a Starrett guage and then scribes the line. The scribed line is not so easy to set accurately in my limited experience. I've made some aluminum bars that clamp onto the pitch blocks on both the hub bar and the blade and stick out about 16". My idea is to set them at the scribed lines at the end of the bars where 1.2 degrees is an easier to discern .335". (Actually I scribed 1, 1.5, and 2 degrees) I'll let you know how this works out if you want. Herman Yates at Bensen days said that 1.5 was what he found to be the best setting.
05-06-2005, 07:55 AM
I was under the impression that bensen blades were no longer made, mine were made in the 80's. If the guy who made them is still around though, that's good because I always like starting out with factory specs and then making adjustments from there.
05-06-2005, 08:43 AM
This is paraphrased from Ralph Taggart's Gyrobee site;
"Use a pair of rods clamped on either side of the pitch blocks and pointing forward relative to the blade.
If the rods measure 57.3 inches from the center of the hub bar to the far end of the rod, 1 inch of separation is equal to one degree of pitch."
This sounds like a simple and very accurate way to set the pitch.
05-06-2005, 09:40 AM
I never saw that on his site... Good info. Mine are pretty much the same thing but shorter. I read somewhere that the stock Butterfly without prerotator uses Bensen blades but I don't know that as fact. The Butterfly site doesn't say. The pictured blades seem to be DW's.
05-06-2005, 09:48 AM
The stock butterfly does use Benson blades, otherwise known as Brock blades. Not nearly as efficient as the Dragon Wings, but much easier to start by hand.
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