RFD rotor head and Dragon Wings spring tension question

mvadney

gyroboy
Joined
Aug 8, 2004
Messages
44
Location
Arlington
Aircraft
Sportcopter Vortex
Total Flight Time
300
Hello everyone.

I don't post very often, but I have a question about trim spring setup for the typical RFD rotor head and DragonWing combination. With my Vortex SportCopter I don't need a trim spring, but with my newest gyro project that uses a RFD rotor head I have to use a set of trim springs to make things right.

Can anyone that has this combination, usually Dominators, what springs are you using.

Also, what does a Dominator usually have for a good keel hang check number? 8 degrees with half fuel and centered controls or what?

Thanks for your input,
MikeHangCheck20190721.jpg
 

Smack

Re-member?
Joined
Nov 3, 2013
Messages
543
Location
Georgetown
Aircraft
Kitfox IV / F1 Rocket / Magni M-16 / Beech 18
Total Flight Time
500+
Is that a trike control bar ?
What engine is that? HKS?
Brian
 

j bird

Gold Supporter
Joined
Nov 11, 2005
Messages
1,697
Location
Cave Junction,OR.
Aircraft
Dominator/Airworthiness Certificate 9/06/12
Total Flight Time
26.5 duel,RAF,Sparrow-Hawk,Cavalon,Calidus.
Ernie told me 8* to 12* degrees, closer to 12* better, as you burn of fuel your keel will be flying at less and less degrees.
 

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,656
Location
Florida
The RFD rotorhead uses Bensen standard 1 inch offset between rotor axis and pitch pivot. This will require a light trim spring such as a screen door spring for trim at normal cruise speed when using zero pitching moment rotor blades; ie, blades that don’t try to twist either noseup or nosedown when moving through the air at several hundred mph.

My speculation is that Bensen arrived at an offset of 1 inch when he started out using his wood rotor blades that had a positive (noseup) pitching moment that provided “hands off” flight without the need for a trim spring. After going to metal rotor blades with zero pitching moment, a trim spring was required but the feedback of a component of rotor thrust into the controls was stabilizing.

An upward gust generates a nosedown force in the control system, tending to keep the gyro headed into the relative wind.

When I started out flying Hughes helicopter blades in the 1970s, I decided the use of a trim spring was a nuisance during ground handling so built a rotorhead with offset reduced to 5/8 inch. Hands off at 50 mph and no annoying trim spring. However, my friend Pete Johnson, a very savvy guy from Swainsboro, GA used to come to SRC flyins and fly my gyro and informed me that I had sacrificed stability. Pete had experimented extensively with various combinations of offsets and trim springs and came to the conclusion that a 1 inch offset and low rate trim spring provided the best stability. I realized Pete was right and went back to a 1 inch offset and trim spring.offset.JPG
 
Last edited:

mvadney

gyroboy
Joined
Aug 8, 2004
Messages
44
Location
Arlington
Aircraft
Sportcopter Vortex
Total Flight Time
300
Is that a trike control bar ?
What engine is that? HKS?
Brian
Hello Brian,
Yes, it is like a trike control bar. I know the arguments against it, but I really wanted to give it a try. Your hands are no where near where they would be when using a stick so I am hoping that won't be a big problem. Practice and more practice to go along with that too.
The engine is an HKS 700T (80hp). I have a problem with it in that it puffing some blue smoke out the turbo that I have yet to resolve.
Thanks,
Mike
 

mvadney

gyroboy
Joined
Aug 8, 2004
Messages
44
Location
Arlington
Aircraft
Sportcopter Vortex
Total Flight Time
300
Ernie told me 8* to 12* degrees, closer to 12* better, as you burn of fuel your keel will be flying at less and less degrees.
Thanks J-bird. My keel was showing between 6 something and 8 something with fuel empty and fuel full. I guess that I will be pulling the head back a bit now.
 

mvadney

gyroboy
Joined
Aug 8, 2004
Messages
44
Location
Arlington
Aircraft
Sportcopter Vortex
Total Flight Time
300
The RFD rotorhead uses Bensen standard 1 inch offset between rotor axis and pitch pivot. This will require a light trim spring such as a screen door spring for trim at normal cruise speed when using zero pitching moment rotor blades; ie, blades that don’t try to twist either noseup or nosedown when moving through the air at several hundred mph.

My speculation is that Bensen arrived at an offset of 1 inch when he started out using his wood rotor blades that had a positive (noseup) pitching moment that provided “hands off” flight without the need for a trim spring. After going to metal rotor blades with zero pitching moment, a trim spring was required but the feedback of a component of rotor thrust into the controls was stabilizing.

An upward gust generates a nosedown force in the control system, tending to keep the gyro headed into the relative wind.

When I started out flying Hughes helicopter blades in the 1970s, I decided the use of a trim spring was a nuisance during ground handling so built a rotorhead with offset reduced to 5/8 inch. Hands off at 50 mph and no annoying trim spring. However, my friend Pete Johnson, a very savvy guy from Swainsboro, GA used to come to SRC flyins and fly my gyro and informed me that I had sacrificed stability. Pete had experimented extensively with various combinations of offsets and trim springs and came to the conclusion that a 1 inch offset and low rate trim spring provided the best stability. I realized Pete was right and went back to a 1 inch offset and trim spring.View attachment 1144998
Hello Chuck.
As always your answers are nice aero-lessons. I appreciate that. I have seen very light trim springs used, but when working the control bar fore and aft while performing this hang check I was surprised by how much force it took to move the controls hence the question. My guess is that it all lightens up when actually flying.

Can you also compare this style head to the Sport Copter design please?

Sincerely,
Mike
 

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,656
Location
Florida
Hanging from a rope, there is no cyclic flapping so the full 1” offset is producing a nosedown moment. In flight, cyclic flapping moves the rotor thrust vector toward the pitch pivot, reducing offset and control moment.

Rotors with a noseup pitching moment have increased flapping, moving the rotor thrust vector even more toward the pitch pivot, reducing control force. A negative pitching moment rotor blade causes the advancing blade to twist nose down more than the retreating blade, suppressing cyclic flapping and causing high stick force unless offset is reduced.

Correctly designed rotor blades, both helicopter and gyro, will have a zero pitching moment coefficient. In other words, if the airfoil is cambered; that is if the mean line, the line midway between upper and lower surfaces looks like a Venetian blind slat, a trailing edge reflex, equivalent to an up elevator, is necessary to cancel the nosedown pitching moment.

The only two post WWII gyroplanes to receive FAA certification, the A&S 18A and the McCulloch J-2 have symmetrical airfoils but symmetrical airfoils have abrupt stalls that makes hand starting difficult. Bensen began with symmetrical airfoils but switched over to cambered blades because of the hand starting difficulty.
 

ultracruiser41

Gold Member
Joined
Nov 2, 2003
Messages
2,555
Location
NC
Aircraft
Cavalon,Snobird+, Ryan PT 22,Eagle 150B, Titan T51, Thatcher CX4, Radial Rocket Gyro, HummelBird
Total Flight Time
Too many to count
When hanging the control forces will alway be heavier....you are trying to move all the weight of your machine.......in the air.....the rotors are supporting the weight thus easier control inputs
 

mvadney

gyroboy
Joined
Aug 8, 2004
Messages
44
Location
Arlington
Aircraft
Sportcopter Vortex
Total Flight Time
300
Another question: What happens if the downward keel angle is too small? i.e 5 degrees or even flat? What would be the result?

Hey Chuck. If the trim spring is something as light as you say then why does Ernie warn about not flying with DragonWings unless you have a spring in place? Wouldn't you be able to compensate manually?

Thank you all,
Mike
 

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,656
Location
Florida
The stick pressure isn’t very high when flying DWs without a trim spring and with 1” offset. I suppose Ernie’s concerned that a student pilot might be caught unawares.

Of course without a trim spring, if something did go wrong such as pilot incapacitation or loss of control continuity, the gyro would smack into the ground nose first at high speed.
 
Last edited:

mvadney

gyroboy
Joined
Aug 8, 2004
Messages
44
Location
Arlington
Aircraft
Sportcopter Vortex
Total Flight Time
300
Thanks Chuck.
Could I trouble you for your opinion on the low keel angle effect? My Sport Copter Vortex is something like 13 degrees as I recall. What happens if the keel on my new gyro was like 3 degrees. Does it only effect the centering of controls or what? Thank you again. Oh, and my new gyro is around 7.5 average btw.
 

C. Beaty

Gold Supporter
Joined
Apr 16, 2004
Messages
9,656
Location
Florida
Keel and mast angles are quite arbitrary; generally, the mast angle should be aligned with the rotor thrust vector to minimize bending loads and keel angle, inasmuch as it affects landing gear angle should permit the proper angle of landing gear.
Bensen picked a mast angle of -9º relative to the keel to minimize mast bending load and to result in a level keel at normal cruise speeds.
 
Last edited:

mvadney

gyroboy
Joined
Aug 8, 2004
Messages
44
Location
Arlington
Aircraft
Sportcopter Vortex
Total Flight Time
300
Would anyone happen to know what the preferred keel angle would be for Dominator?
 

j bird

Gold Supporter
Joined
Nov 11, 2005
Messages
1,697
Location
Cave Junction,OR.
Aircraft
Dominator/Airworthiness Certificate 9/06/12
Total Flight Time
26.5 duel,RAF,Sparrow-Hawk,Cavalon,Calidus.
Come on, 8* to 12*, Call it 10*, right in the middle, I set my Dominator to 12*
 

j bird

Gold Supporter
Joined
Nov 11, 2005
Messages
1,697
Location
Cave Junction,OR.
Aircraft
Dominator/Airworthiness Certificate 9/06/12
Total Flight Time
26.5 duel,RAF,Sparrow-Hawk,Cavalon,Calidus.
Or you could call Ernie and ask him, 813-634-3370
 
Top