Double Hang Test Accuracy vs. Rotor Coning Angle

Brian Jackson

Platinum Member
An academic question to the more informed...

When performing a double hang test to pinpoint VCG, I understand the blades must be on because they are a significant portion of the AUW. However, I imagine there is a measurable change in COM of the rotor when its sagging vs. coned in flight. Rough mental guess; maybe 1/3 of total tip vertical deflection (I can do the math later). But whatever the ratio, it will change by some amount. We'll call it 2" for now just for demonstration. We'll call the rotor 50 lbs. and AUW 500 lbs to keep the math easy. So in this example adding an inch to rotor height moves the VCG of the ship+pilot 1/10". Thusly, our 2" change in rotor COM would affect a 2/10" change in the ships VCG.

Granted it's not huge, but I'm curious if this coning angle difference is generally taken into account. I don't recall reading about this in the archives. Thanks.
 

Vance

Gyroplane CFI
No Title

If you had asked me before I installed the larger prop and the IO-320 on The Predator I would have told you two inches are not going to make a significant difference.

Based on that experience changing the CG to thrust line an inch made a noticeable difference in the way The Predator flies.

My tanks are slightly below the propeller thrust line (eight inches to the center of the round tanks) and I can feel the difference in the way she flies from full (22 Gallons) to seven gallons. She has maximum takeoff weight of 1,400 pounds.

I feel you are confused on a higher level Brian; and that is a good thing.
 

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gyrojake

Gyro Rehab Candidate
Things ain't always what they seem they should be.
I set most of my machines up with a slightly high thrust line.
Centerline trust and low thrust lines fly very similar.
I consider 3 to 4 inches inches above, high
I consider 0 to 4 inches below, low.
In a climb, a high thrust line machine wants back stick to maintain climb
A low thrust line and a center thrust machine in a climb likes forward stick to maintain a climb.
I feel adding back stick in a climb normal and safe.
High thrust line has a nose up attitude with loss of power
This happens because your power source above thrust line is trying to pitch your air frame down to the ground
Loss of power on a low trust line has a radical nose dive and more pitching during fight.
That is because your power source is pitching your air frame up
Thrust line is chosen by experience
CLT is chosen from ignorance and the common thought this is better and safer.
Within the limits of safety all formats are good.
Exceed these limits and well, ya better be a good pilot
I feel a bit high is safest and naturally manageable.
And yes the blade droop makes a minor difference.
 

Doug Riley

Platinum Member
The blades don't have to be on the machine for the (usual) first hang test, from the teeter bolt. But, yes, they do have to be on the machine (ideally trussed up to coning angle) for the second test, where the hang point is some distance away from the teeter bolt.

It's also possible to calculate the effect of moving weights around -- so you could use arithmetic to correct for the effect of coning instead of trussing up the blades. Example: 500-lb. gross. Blades NOT trussed up for the double hang. Blades' CG is a foot lower in droop than coned. Blades weight 50 lb. (10% of gross). SO, when the blades cone, 10% of the gyro's mass moves up a foot. That means that the gyro's center of mass (CG) moves up a distance of 10% of the amount the blades moved -- or 1.2 inches up.

IMHO, the nose of the aircraft should drop gently when the engine quits. Otherwise, you execute a mid-air flare, losing airspeed just when you need it. (Bensens used to do this.) The nose-drop thing can be over-done, though. More than a couple inches LTL is more than necessary. Radical LTL may be dangerous. Radical HTL definitely is dangerous.

CLT is like cooking pasta -- your underlying ingredient has no flavor, so you can add flavors to suit your taste. CLT adds no nose-up or nose-down bias with power changes..

A horizontal stabilizer, immersed in the prop slipstream. cam be adjusted to mimic either higher or lower thrustline. Example: On a CLT machine, a H-stab with a bit of negative incidence (leading edge lower than trailing edge, relative to the direction of the propwash) mimics slightly low thrustline, and is the normal setup in FW planes. This setup in FW lingo is called "longitudinal dihedral."
 

Brian Jackson

Platinum Member
Thank you gentlemen. I now have an improved understanding of the interrelationships. Being completely OCD about this build, I’m curious how one would truss the blades without affecting the measurement. Is this truss perhaps a light weight pole fastened to the top of teeter block with the blades winch-strapped to it half way out until loaded state coning angle achieved? I’m anticipating needing to know this angle first, which would require a preliminary hang test at proper blade support points (without damaging the blades) and measure the string line to the teeter bolt. The thought being that this measurement could be recreated with strap tension when trussing the rotor for the double hang test. Wouldn’t it be easier just to send it into space and spin it?
 

Doug Riley

Platinum Member
No Title

Brian, to be honest, I just did the test with the blades drooping. I figured that lifting the blades to cone angle would only make the CM position more favorable to stability (higher CM). Close enough is good enough with horseshoes and hand grenades.

The craft I tested was my early-model Air Command. It tested out significantly HTL -- about 6 inches. Trussing the blades wasn't going to make THAT go away. I reported my result as "5-6 inches HTL," thus including enough of a fudge factor to capture the droop effect. See awful, pixelly 1990 photo, attached.

Incidentally, I cobbled up this test method based on a book of grade-school science-fair experiments.

If you must "truss up," I'd first try a rope attached somewhere outboard of each blade's mid-span. Use Dacron, not nylon, which is too stretchy. Cone up and tie the line to each blade BEFORE you lift them into place. Use your best self-tightening knot, matey. Don't ruin the trailing edges. Mounting them on the teeter towers will be a pain without the droop to keep the rotor assembly upright.
 

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Vance

Gyroplane CFI
I know you would like your gyroplane to fly well out of the gate and you are doing what you can to make that happen.

If it turns out it doesn't fly like you want just make some adjustments on the seat height to get her to fly like you want.

After building such a big project this will seem like a minor task.

Many years ago I found I could predict the success of a racing project by how full my mistake bin was.

More metal in the mistake bin indicated a higher chance of achieving the project's goals.

If there was no metal in the mistake bin it indicated the project was aiming low and would not help me learn and grow.
 

Doug Riley

Platinum Member
Vance, that is a much more uplifting way of looking at it than mine -- I look in the discard bin and think of the time and money wasted. Except that maybe it wasn't ENTIRELY wasted. The learning curve is an uphill road with frequent tolls to pay....
 

Brian Jackson

Platinum Member
Another thing that occurred to me was a forum thread a while back about using scales under the tires to derive COM by calculation. Something was bugging me about this besides the iffy results. On the ground with pilot and full fuel the landing gear is deflected upward, at least for ships with suspension systems. So hanging with blades coned is the only way I see to be truly accurate.

Doug, on a side note, I’m building a cruciform tail with rudder to similar proportions of a tall tail but larger HS area for a Bee style ship. HS mounts lower than thrust line, about 1/3 the distance from thrust line to tailboom. It’s been advised to use 2 or 3 degrees of negative incidence on the HS. Is that rule still true in this configuration with a CLT ship?
 

Doug Riley

Platinum Member
Brian, the 2-3 deg. neg. incidence is about right for a plans-built 'Bee. You need little-to-no incidence for a truly CLT craft. A touch won't hurt; too much will produce an exaggerated "Dominator effect," in which the nose rises too much when you power up. This is the safe direction for the nose to go (down-with-power is bad), but in excess it causes you to slow down unless you add forward stick together with your power-up.

The H-stab on a cruciform tail probably provides the best torque compensation if it's right in the center of the propwash. OTOH it gets the fastest propwash air about 2/3 of a prop radius out from the center. In either location, the HS will lose a noticeable amount of effectiveness with power down, or off. This is thanks to the slower and more confused air that the HS receives when there's little/no propwash.
 

PW_Plack

Active Member
Being completely OCD about this build, I’m curious how one would truss the blades without affecting the measurement.
Brian, not to feed the OCD, but any rigging to truss the blades is going to affect the result, too! (Well, OK, maybe a snack for the OCD...)

You're proposing resolution which greatly exceeds the precision. This is like measuring the diameter and circumference of a wheel with a yardstick and calculating pi out to 20 decimal places. Remember - The hang test goal is generally expressed as a range of nose-down angle with the stick centered. Will you find a way to center the stick within 0.1 inch? Will you feel you need new cheek plates or ballast if you gain 10 pounds?

I've seen hang tests done with a weight substituted for the blades. It's really a PITA to handle and protect the blades, trussed or not, during a hang test. A rigid steel bar cut to the weight of your rotor, with a hole drilled to allow bolting it at the teeter point, is much easier, and doesn't change its balance if it tilts slightly one way or the other during the test.

I even saw one scrapped hub bar adapted for dumbbell weights to allow adding/subtracting weight during hang tests.

Try a hybrid approach. Do a hang test with a rigid dummy weight matching your rotor centered on the teeter bolt. If the result lands near the center of the range listed as acceptable, be happy! Then, the math to fine-tune for coning will be simple and accurate.

Having followed your journey for several years now, it makes me happy for you to see your list of things-to-worry-about this evolved!

Paul
 

Brian Jackson

Platinum Member
Thank you Paul. This sounds like a reasonable approach. And yes the years have evolved the to-do list. Still much to do on the build but planning ahead. I appreciate you and the others here on RWF helping me get this far. Question though... am I correct in assuming the proxy weight's COG be located at teeter height to mimic that of the coned rotor?
 

Doug Riley

Platinum Member
Brian: Yes, the purpose of the undersling is to place the COM of the coned rotor at the teeter bolt. So putting the COM of the dummy weight there simulates reality.

As Paul warns, beware of wasting effort on that 20th decimal place!
 
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