Rotorblades for the Swift

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,743
Location
Florida
Gyros using the Bensen rotor system without collective pitch are far more tolerant of rotor blade profile errors than are helicopters or even jump takeoff gyros where collective pitch is a necessity.

A rotor blade with a negative (nosedown) pitching moment introduces what is in effect, nosedown cyclic pitch as a result of periodic blade twist. The advancing blade twists more nosedown than the retreating blade, tilting the rotor tip plane axis in a nosedown direction and in the case of a Bensen type rootrhead, moves the thrust vector rearward toward the gimbal pitch pivot. This requires more aft stick movement and increases stick pressure, requiring either more trim spring tension of a reduction of offset between spindle bolt and pitch pivot of the rotorhead.

Helicopters are far less tolerant of rotor blade profile errors because collective pitch loads can rapidly become unmanageable. Here’s an E-mail exchange I had with John Uptigrove (Mosquito Helicopter originator) about DW 9” rotorblades for his latest project, the 2-place Swift shortly before his fatal wire strike accident. The main holdup of the Swift was the lack of a suitable 9” rotorblade.

From: John Uptigrove
To: Ernest Boyette
Cc: [email protected], dwight
Sent: Tuesday, January 17, 2017 10:49:56 AM
Subject: RE: New blades

Ernie/Chuck,

I set the blades up with the adapter I had made so that the feather axis is at 27% chord. Now when I have the blades aligned and remove the pitch link they drop nose down to an estimated 10-15 degrees nose down. That is just due to the cg of the blade being well forward of the feather axis I picked. But when I run them up and try to fly the collective force is so high that I can barely see the skids trying to move a little and I have as much force on the lever as I want to put without fear of breaking something. I don’t think its due to centrifugal flattening nearly as much as it due to a strong negative pitching moment from the airfoil. I am going to have to make a trim tab to try to bring the cp back to the feather axis. I will let you know how much tab it takes to get that done. I think the pitching moment might affect their use on a gyro as well.

John

From: CHARLES [mailto:[email protected]]
Sent: Tuesday, January 17, 2017 1:33 PM
To: John Uptigrove <[email protected]>
Cc: Ernest Boyette <[email protected]>; dwight <[email protected]>
Subject: Re: New blades

John, these rotor blades are Boeing VR12 airfoil section, widely used on modern helicopters

The attachment is a plot of pitching moment Vs angle of attack and is nearly negligible. That's not to say that the trailing edge closure couldn't be at the wrong angle and contribute a nose down moment. If so, it can be easily corrected.

(pitchng moment plot)

The aerodynamic center of the VR12 is at ~26% of chord which is where the chordwise CG should be located. I haven't personally checked a sample section but Ernie told me it was quite close.

The CG of a complete blade can be checked by removing all attachments, laying it on a flat surface and balancing on a round rod set at +45º, drawing a line along the rod axis and then rebalancing with the rod at -45º. The CG then being at the intersection of the two lines.

John, you may not remember but years ago, before everyone was wired for Internet, you had built a helicopter with rotorhead based on the original Rotorway layout and couldn't understand why everything flopped over to one side. You phoned me and we were able to determine that you hadn't linked the feathering bearings together so they only responded to collective pitch. I think that was in the early '90s.

Anyhow, if there's anything I can do to help with this current problem, just give me a call or E-mail me. My phone # is:813-

Regards, CB

From: John Uptigrove
To: CHARLES
Cc: Ernest Boyette
Sent: Thursday, January 26, 2017 3:19:55 PM
Subject: RE: New blades

Chuck,

Good to hear from you! Not sure how I missed your email earlier. Yes, I think maybe the flat trailing edge is giving it the heavy nose down aerodynamic characteristics. I’m just finishing up some trim tabs to adjust the aerodynamic center. I’ll fly in the next couple of days.

I definitely remember that conversation about the rotor head as it still makes me wonder a bit how the earlier heavier version I made stayed in centered even though it had nothing to keep it there. It must have just been the gyroscopic stability of the heavier head was stronger than the offsetting force from the blade coning. The smaller, prettier head I came up with was lighter and so didn’t have enough gyroscopic stability and flopped to the side. Best of all was your answer when I asked why the earlier one worked. “Black Magic!” Haha! I’ll never forget that!

Hope you’re doing well. Still flying and designing?

John

From: CHARLES [mailto:[email protected]]
Sent: Friday, January 27, 2017 1:30 PM
To: John Uptigrove <[email protected]>
Subject: Re: New blads


John, the pitching moment of an airfoil is quite sensitive to the trailing edge angle. It is possible that the angle of the trailing edge closure of Ernie’s new blades is wrong.

If the tabs you add reduce collective pressure, you might make up a tool for bending the TE closure angle by brazing a couple of pieces of angle iron to the jaws of vise grip pliers so as to bend a ~4 inch length of the TE at a time.

It’s 60ºF at 3 PM as I write this and I’m about to freeze; how do you guys manage?

Regards, CB


John Uptigrove<[email protected]>

To CHARLES Copy Ernest Boyette, dwight

Chuck,
So after my experience bending the trailing edge of the 7” blades causing the upper surface to distort somewhat I decided to go with adding trim tabs. But after installing and bucking about 8 rivets in the first part of the length of aluminum to the upper surface I could see it wasn’t going to be pretty as it was bowing up between the rivets (spaced 2” apart) likely due to bucking the rivets. So I tried bending again just to see on a 6” section of the inboard end of the blade and found it didn’t distort the upper surface at all. No idea why the difference. So I took a shot in the dark and bent from 6” inboard of the tip to 36” inboard of the tip up approx 9 deg. Filled the rivet holes in the one blade with epoxy and tried it. Flew perfectly with about 3 lb of lift on the collective. Blades are smooth as glass. Jims would run up smooth and then start to shake when close to lift off likely due to the chordwise cg being offset in one of the blades and not matching the lift center. Seemed like they took less power as well but difficult to judge with the small amount of hovering I did with the other blades.

Just need to do some heat shielding under the exhaust and hopefully I can start getting some hover time in.

John
 

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,743
Location
Florida
Would "Leading" the blades slightly have the same effect on a heavy collective?
My first thought was that it could, Cam but on second thought, "leading" the blade could make it unstable; pull in a little collective and it tries to go to full collective.
 
Last edited:
Top