Dominator thoughts on CLT/drop keels

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When Chuck and Ernie conceptualized the Dominator, and given the wild variety of engines weight and power, available today, what engine was the design originally planned for? What determined the offset of the drop keels upper fuselage to that of the lower fuselage member? This can be assessed by the double hang test after the build is completed but a tough time to find out major mistakes were made in engine choices and or drop keel distance variations. If you might refer me to an ancient thread which can answer this or know from personal experience.
 

Gyro28866

David McCutchen
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Just my opinion,
but I think the 2 keels are in parallel planes. The amount of offset is determined by the diameter of the Propeller you want to swing, keeping the thrust line passing through the vertical CG. The amount of offset in the 2 keels will determine the length of the Main and Nose wheel landing gear. My Tandem Dominator is pretty tall, but I swing a 74" three blade Prop behind a 100 hp Hirth.
Based upon stories I have heard. I think it started with a Bensen frame keel being cut behind the Mast and a Rotax engine being lowered and getting the Prop Thrust line back down to the vertical CG. That forced them to create the drop keel to be able to have a rudder. It started with the standard Bensen Rudder, which morphed into a tall tail which latter morphed into the Horizontal Stabilizer being placed into the thrustline, which basically cancels P factor.
The original concept was an attempt to correct for the High thrustline Rotax powered "Bunt-O-matic" modified Bensen which was responsible for many deaths. The Dominator is one of the best, if not the best design ever developed.
But my opinion is obviously prejudiced!
 
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Just my opinion,
but I think the 2 keels are in parallel planes. The amount of offset is determined by the diameter of the Propeller you want to swing, keeping the thrust line passing through the vertical CG. The amount of offset in the 2 keels will determine the length of the Main and Nose wheel landing gear. My Tandem Dominator is pretty tall, but I swing a 74" three blade Prop behind a 100 hp Hirth.
Based upon stories I have heard. I think it started with a Bensen frame keel being cut behind the Mast and a Rotax engine being lowered and getting the Prop Thrust line back down to the vertical CG. That forced them to create the drop keel to be able to have a rudder. It started with the standard Bensen Rudder, which morphed into a tall tail which latter morphed into the Horizontal Stabilizer being placed into the thrustline, which basically cancels P factor.
The original concept was an attempt to correct for the High thrustline Rotax powered "Bunt-O-matic" modified Bensen which was responsible for many deaths. The Dominator is one of the best, if not the best design ever developed.
But my opinion is obviously prejudiced!
Goodness, a 74” 3 blade. That is a big one.

As per the offset and prop/engine size, it looks like the drop keel improved the handling enough with a variety of power plants and most left well enough alone without attempting to tweak to perfection.

Once in the air, the parasitic drag, which varies with airspeed, would probably offset any perfection a builder might have achieved in CG/thrust line alignments. I expect the horizontal tail now stabilizes the fuselage at higher speeds which might offset some of the drag induced by lower influences such as landing gear.

At the point of the application of power, I expect the case has little to do with drag, at least not initially, but rather inertia. The distance the CG is below the push of the line of thrust will introduce the pitching motion which is caught by the experienced pilot by slight back pressure or just ignored and trimmed to to the new airspeed. Both corrections become second nature if you know the characteristics of your machine.

I believe we can agree that modifying an airframe to move the CG closer to the line of thrust plus the tall vertical tail and a sufficiently sized horizontal stabilizer does wonders to the handling of the fuselage where as it wants to follow the lead of the rotor.

Humans are fallible and we need a machine which will help correct our mistakes or inattention, not exacerbate them. These design qualities can improve safety.

Thank you for the reply.

Bill
 

Doug Riley

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Rowdy: I'm pretty sure that the drop-keel designers initially eyeballed the raising of the seat, to locate the pilot's belly button on the centerline of the prop (or similar rule-of-thumb).

Carl Schneider went one step farther and made his front keel (with seat attached) movable up and down the mast. He then test-flew various seat heights 'til he arrived at the handling qualities he liked. Both Carl and Ernie Boyette ended up with low-thrustline (LTL) configurations. In these config's, adding power raises the nose automatically -- in some cases so much that the gyro slows down with added power unless you add forward stick pressure. Whether this is a good thing, or merely not too bad a thing, is open to debate.

But you don't need to resort to eyeballing, or guessing, or a lot of trial-and-error. The procedure for laying out the major masses so that the CG lands on the prop thrustline is the same one as used to solve weight-and-balance problems in FW planes. This procedure is set out in any FW flight-training manual.

The only difference between the fixed-wingers and us is that we do the analysis along the aircraft's vertical axis, while the FWers do it along the horizontal axis. The more complete your accounting of the various masses (rotor with head, fuel, pilot, engine, landing gear, pod, etc.), and their individual CG locations, the better your paper planning will work out in real life.

Hint: a seated human in most cases does, in fact, have a CG located around bellybutton level. I risked looking silly by testing this notion while sitting in a plastic bucket seat on the floor and rocking back and forth.

BTW, it's inaccurate to blame the scores of PIO accidents in Bensen-style gyros on the higher thrustlines dictated by redrive engines. The Bensen bloodbath long predated the arrival of the redrive Rotax and similar setups. The peak of Bensen fatalities was, IIR, in 1967. Bensen's refusal to use an effective H-stab, slight HTL even with a 4-foot prop, and low-damping (light + high RRPM) rotor had much to do with this tragic record.
 
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Rowdy: I'm pretty sure that the drop-keel designers initially eyeballed the raising of the seat, to locate the pilot's belly button on the centerline of the prop (or similar rule-of-thumb).

Carl Schneider went one step farther and made his front keel (with seat attached) movable up and down the mast. He then test-flew various seat heights 'til he arrived at the handling qualities he liked. Both Carl and Ernie Boyette ended up with low-thrustline (LTL) configurations. In these config's, adding power raises the nose automatically -- in some cases so much that the gyro slows down with added power unless you add forward stick pressure. Whether this is a good thing, or merely not too bad a thing, is open to debate.

But you don't need to resort to eyeballing, or guessing, or a lot of trial-and-error. The procedure for laying out the major masses so that the CG lands on the prop thrustline is the same one as used to solve weight-and-balance problems in FW planes. This procedure is set out in any FW flight-training manual.

The only difference between the fixed-wingers and us is that we do the analysis along the aircraft's vertical axis, while the FWers do it along the horizontal axis. The more complete your accounting of the various masses (rotor with head, fuel, pilot, engine, landing gear, pod, etc.), and their individual CG locations, the better your paper planning will work out in real life.

Hint: a seated human in most cases does, in fact, have a CG located around bellybutton level. I risked looking silly by testing this notion this while sitting in a plastic bucket seat on the floor and rocking back and forth.

BTW, it's inaccurate to blame the scores of PIO accidents in Bensen-style gyros on the higher thrustlines dictated by redrive engines. The Bensen bloodbath long predated the arrival of the redrive Rotax and similar setups. The peak of Bensen fatalities was, IIR, in 1967. Bensen's refusal to use an effective H-stab, slight HTL even with a 4-foot prop, and low-damping (light + high RRPM) rotor had much to do with this tragic record.
 
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Thank you for the practical useful information. I will look at bellybuttons very differently now. On a more serious note, the datum point on a single engine fixed wing is typically the tip of the propeller spinner. To calculate what we are trying to do, we are to use the thrust line of the propeller.

Many aircraft engines have a CG indicated the Engine Operation Manual. Should a vertical line be drawn at the point to intersect the horizontal line of thrust? Could this be used as a more defined datum point to begin the calculations? Many designers as I believe Bensen did, tilted the engine to help the thrust line intersect the CG point. Is this just nit picking? How’s that for a Southern term?

Dr Bruce Charnov’s book, From Autogiro to Gyroplane, does a remarkable job documenting the struggle In overcoming the technical difficulties early on. But it is hard to get many references to the more modern successes which has lead to the boom we are experiencing today. So many breakthroughs were made in people’s personal workshops. Such information is so scattered and illusive. This is what makes this forum so important and golden.

Than you all,

Bill
 

Jazzenjohn

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I see it a bit differently. The keel itself can be high or low but its really the seat height determining the pilot position that makes the difference. You might want a low front keel with more room under the seat, especially necessary if you're using a pump handle joystick for instance, it also means you will have a smaller/shorter front fork. If you want to use a walking beam stick, a higher front keel would use a shorter stick and some of the traditional walking beam control sticks like the brock stick will fit without modification. It generally requires a longer front fork.
 

Vance

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I like your thoughtful approach to gyroplane design Bill.

The datum line on a fixed wing aircraft at the spinner is picked so a pilot doesn’t have to struggle with negative and positive numbers for calculating the horizontal center of gravity.

The center of the spinner will also be on the thrust line but may be well above or below the center of gravity.

To calculate the vertical center of gravity you can use something as simple as the ground for a datum line and it is basically the same process as calculating the horizontal center of gravity.

During the design phase you can weigh things (mass), determine where the center of gravity for each mass is and see how far above or below the propeller thrust line that center of mass is (moment arm).

To calculate the horizontal center of gravity you pick a datum line (I use the tip of the pitot tube on The Predator) to calculate the center of gravity.

Once she is built you can do a double hang test to see where your center of gravity is and make adjustments to get things the way you want.

On The Predator making a one inch change in the height of the center of gravity in relation to the thrust line made a large change in the way she flies.
 

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I see it a bit differently. The keel itself can be high or low but its really the seat height determining the pilot position that makes the difference. You might want a low front keel with more room under the seat, especially necessary if you're using a pump handle joystick for instance, it also means you will have a smaller/shorter front fork. If you want to use a walking beam stick, a higher front keel would use a shorter stick and some of the traditional walking beam control sticks like the brock stick will fit without modification. It generally requires a longer front fork.
Totally agree and I agree again with your thoughts on a more sturdy, at least in appearanc, nose gear configuration. Sport Copter, Snow BIrd and Avromania ( I’m sure I misspelled this) with its slanted keel does a great job of raising the seat and pilot without sacrificing strength.
Great points.
 
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I like your thoughtful approach to gyroplane design Bill.

The datum line on a fixed wing aircraft at the spinner is picked so a pilot doesn’t have to struggle with negative and positive numbers for calculating the horizontal center of gravity.

The center of the spinner will also be on the thrust line but may be well above or below the center of gravity.

To calculate the vertical center of gravity you can use something as simple as the ground for a datum line and it is basically the same process as calculating the horizontal center of gravity.

During the design phase you can weigh things (mass), determine where the center of gravity for each mass is and see how far above or below the propeller thrust line that center of mass is (moment arm).

To calculate the horizontal center of gravity you pick a datum line (I use the tip of the pitot tube on The Predator) to calculate the center of gravity.

Once she is built you can do a double hang test to see where your center of gravity is and make adjustments to get things the way you want.

On The Predator making a one inch change in the height of the center of gravity in relation to the thrust line made a large change in the way she flies.
Lycoming list the center of gravity for the 0320 D2A as to the rear of the first two cylinders aft of the prop flange. The weight wet with all accessories including, surprisingly enough, the engine mount is 278 pounds. 160 hp at 2700 rpm. She is an old engine and I don’t expect that out of her. The center of gravity and the crank position co exist. I was impressed the engine was that well balanced so to speak.

I love the engine and have logged many hours behind one not in front of one. But and there is always a but, that engine requires EGT,CHT and other instruments at least those as I feel necessary and comfortable with and panel real estate is a problem.

Here is one…..How many instruments does the FAA require on an experimental aircraft? Look it up. Zero! I almost choked when I read the regs.

The nose gear is going to take a lot of thought. With a moment arm as far out as a tandem machine affords, weight and strength is an issue. I was told by a very experienced Gyroplane pilot that Gyroplanes are married to asphalt. I do so want to create a rugged enough front gear as I do not have to worry on grass runways. If the nose wheel folds she will be a yard dart and this poor boys flying days are over.

Your whole machine looks sturdy including the bracing on your nose gear.

Thanks for the advice. I may or may not make the drive from Texas to the desert at the end of the month but it is tempting.

Bill
 
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