# Weight and Balance

#### Mike Leimetter

##### Mike Leimetter
Apologize if this has already been discussed somewhere but a search really didn’t come up with much. When doing weight and balance calculations on a gyro, is the hang test the definitive answer? My handbook gives me the min and max angles, but that’s with 5 gals fuel, pilot, and all equipment usually carried. If it falls in that range you’re good? How do you tell what max pilot/passenger weight is? Keep adding weight/fuel till it’s out of CG? What about the alternative old school approach of weighing all 3 gear, measuring the arm etc? I’ve seen that as a listed method also. Or do you do both? Sorry such a newbie question but I’m almost at that point of needing to get it done.

Mike: The two methods (the "old school" one is typical for fixed wing) measure the same thing, and so are equivalent. Because horizontal CG location has a different significance for a gyro than for a fixed wing, the hang test more directly tells you what you need to know for a gyro. If you use the fixed-wing CG method, you'll have to do some trigonometry. Here's what's different about gyro CG's:

First of all -- in flight, the gyro frame is free to hang from the rotor. It's going to hang at whatever angle will put the whole craft into equilibrium (= no tendency for the frame to rotate). The rotor pulls up-and-back at about a ten-degree angle aft of vertical. Therefore, we set our hang angle so that the centerline of the rotor's pull runs down from the teeter bolt through the CG. IOW, the line connecting the teeter bolt and the CG should lean aft at ten degrees, relative to the gyro's horizontal datum line.

Unlike the fixed wing case, the gyro's horizontal CG location does not affect the aircraft's stability (other things do, but that's a separate subject). Instead, the hang test explores whether the frame will ride at a comfortably level stance at cruise, with the wheels about in their on-the-ground orientation, and the controls in the middle of their travel.

If your gyro is "out of hang," it'll ride either nose-down or nose-up, and the stick will be near one end of its travel. If you're way nose-heavy, you're apt to land nosewheel-first, a very squirrelly situation indeed.

All of this assumes that the prop thrustline either runs right through the CG, or, if it's higher (so-called HTL), that a horizontal stabilizer counteracts the prop's tendency to push the nose down. Without an H-stab doing this work, the hang specs need to be altered from the usual ten degrees to something less, or the gyro will ride nose-down. For the sake of stability, it's far better to counteract any nose-down tendency caused by the prop, using an H-stab -- and leave the hang spec at ten-ish degrees.

Thank you Doug for the great understandable response. 119 views and only one answer might lead me to believe many out there don’t properly understand gyro weight and balance, or understand how to communicate it effectively. I hope others read this and learn as well.

Thank you Doug for the great understandable response. 119 views and only one answer might lead me to believe many out there don’t properly understand gyro weight and balance, or understand how to communicate it effectively. I hope others read this and learn as well.

It isn't that. W&B in gyroplanes is the same as W&B in airplanes. The only difference and why they do hang test which no modern gyroplane kits require, because its been done for you by designer, is to make sure you won't be at some ridiculous angle of dangle which can create some ergnomic as well as control travel issues. At extremes it can create aerodynamic issues as well I suppose. In all modern designs and kits the designer tells you that your seat weight (front seat specially in a tandem) can range for say 120 to 260 pounds. That is the range they already know and have tested that will allow acceptable control travel and correct angle of dangle with their rotorhead and their tail setup. You then do not need to figure that. In these gyroplanes there is no room for you anyway to adjust the angle of dangle. Everything is welded and fixed already.

Seems like you happen to be building a scratch built old school design where you do need to do that and there you do need to understand more because designer is allowing you to adjust cheek plate assembly I am guessing to change this angle. That won't be the case if you were dealing with any new gyroplane. So a lot of end users won't be able to help you with what they did not have to deal with.

the line connecting the teeter bolt and the CG should lean aft at ten degrees, relative to the gyro's horizontal datum line.

Unlike the fixed wing case, the gyro's horizontal CG location does not affect the aircraft's stability (other things do, but that's a separate subject). Instead, the hang test explores whether the frame will ride at a comfortably level stance at cruise, with the wheels about in their on-the-ground orientation, and the controls in the middle of their travel.

If your gyro is "out of hang," it'll ride either nose-down or nose-up, and the stick will be near one end of its travel. If you're way nose-heavy, you're apt to land nosewheel-first, a very squirrelly situation indeed.

All of this assumes that the prop thrustline either runs right through the CG, or, if it's higher (so-called HTL), that a horizontal stabilizer counteracts the prop's tendency to push the nose down. Without an H-stab doing this work, the hang specs need to be altered from the usual ten degrees to something less, or the gyro will ride nose-down. For the sake of stability, it's far better to counteract any nose-down tendency caused by the prop, using an H-stab -- and leave the hang spec at ten-ish degrees.
Doug,
I'm curious to what is included in your "(other things do, but that's a separate subject)."

I feel the horizontal CG location is indeed very critical to the gyro's dynamic stability.

Agree with all you mention about the "10ish degrees" line from the teeter bolt to the CG.
But we need to go a step further.
I specify 11.5° to 12° hang angle on my machines. This assures that the CG is just ahead of the 10°ish degree line you mention.

All objects will rotate around their CG.

With this configuration, a sudden increase in lift will be pulling up through this 10° line that is behind the CG. This will cause the aircraft to rotate forward, removing pitch from the blades, returning it to equilibrium.

If a sudden reduction in lift is experienced, just the opposite. the nose of the aircraft will rise, adding pitch to the blade again returning it to equilibrium.
(Dynamically stable)

With a lesser hang angle, a sudden increase in lift will again be pulling up through this 10° line, but which is now ahead of the CG. This will cause the aircraft to rotate back, adding pitch to the blades, increasing the load even more!
(Dynamically Unstable)

A thrust vector directly through the CG is still stable, but it puts all corrective measures in the pilots hands.

An offset rotor head and trim spring is doing the same thing. (If the pilot allows it to do so).

Denis

Apologize if this has already been discussed somewhere but a search really didn’t come up with much. When doing weight and balance calculations on a gyro, is the hang test the definitive answer? My handbook gives me the min and max angles, but that’s with 5 gals fuel, pilot, and all equipment usually carried. If it falls in that range you’re good? How do you tell what max pilot/passenger weight is? Keep adding weight/fuel till it’s out of CG? What about the alternative old school approach of weighing all 3 gear, measuring the arm etc? I’ve seen that as a listed method also. Or do you do both? Sorry such a newbie question but I’m almost at that point of needing to get it done.
I'm pretty sure the reason there are so few comments is the fact that figuring out the balance of the gyro is involved enough to be quite a write-up. Many people know how to do it, yet don't know how to write it. Have you done a hang test yet? I believe finding out where your machine sits CG wise is a good start. A double hang test or "hang & wheel balance" would pin point the CG. Then you would know if you need to adjust your mast angle, rotor to mast side plates, and/or weight distribution.

Bobby, you need to get back into gyros. Miss you on Saturdays. You can mess with my Chinese engine gyro.

Bobby, you need to get back into gyros. Miss you on Saturdays. You can mess with my Chinese engine gyro.
I'm flying every chance I get! Recently completed 200hrs inspection/maint. Just dread the drive thru Houston.

Bobby. Glad you are still thinking gyros.

Doug,
I'm curious to what is included in your "(other things do, but that's a separate subject)."

I feel the horizontal CG location is indeed very critical to the gyro's dynamic stability.

Agree with all you mention about the "10ish degrees" line from the teeter bolt to the CG.
But we need to go a step further.
I specify 11.5° to 12° hang angle on my machines. This assures that the CG is just ahead of the 10°ish degree line you mention.

All objects will rotate around their CG.

With this configuration, a sudden increase in lift will be pulling up through this 10° line that is behind the CG. This will cause the aircraft to rotate forward, removing pitch from the blades, returning it to equilibrium.

If a sudden reduction in lift is experienced, just the opposite. the nose of the aircraft will rise, adding pitch to the blade again returning it to equilibrium.
(Dynamically stable)

With a lesser hang angle, a sudden increase in lift will again be pulling up through this 10° line, but which is now ahead of the CG. This will cause the aircraft to rotate back, adding pitch to the blades, increasing the load even more!
(Dynamically Unstable)

A thrust vector directly through the CG is still stable, but it puts all corrective measures in the pilots hands.

An offset rotor head and trim spring is doing the same thing. (If the pilot allows it to do so).

Denis
Great points!
I am probably going to catch a ration of \$#!# for this, but I am interested to see what other designers have to say about this redneck technique!

When I designed my first radio control gyro which was a tractor configuration, I figured out the horizontal CG by aligning the blades for and aft.
The balancing the entire aircraft from a point on the front blade that bisected the disk at 30% of the disk area.
The rotor head was not direct control, it was a rudder elevator aircraft.
When I switched to direct control, I locked the head with a couple of shims.
After balancing at 30%, I would add a bit of nose weight to bring it to 27% for stability on the first flight, then reduce the weight until it flew the way I wanted it too.
I use the same process on fixed wings, except I tend to keep moving the CG back until it gets unstable, then move it slightly forward of that point for maximum efficiency.
Obviously you can not do this to full sized aircraft, the rotor is not strong enough...
The other interesting thing was that when balanced this way, it would hang as expected when suspended from the center of the rotor head.
Here is a diagram (Not a tractor) to explain the balance point.
I basically treated the rotor as a round wing and balanced it accordingly.....

Hi, Denis:

The older Bensen spec assumed a rotor disk angle of more like 12-13 deg. Rotor disk angle is a measure of the rotor's lift-to-drag ratio (i.e. its efficiency). The draggy ol' Bensen blades did in fact fly at 12-13 deg., so Igor's specs called for an ideal hang angle of 3 deg. aft of vertical measured on the mast. The mast was already raked at 9 deg., so the total nose-down angle would be 9+3 = 12.

Blades have gotten better, so we can dial down the assumed rotor thrustline angle from 12-13 to 10-ish.

You're right that having the CG ahead of the rotor thrustline is a stable setup, in a gyro just as much as in a FW plane. This setup provides static longitudinal stability just as you describe, but...

The CG will not end up ahead of the rotor thrustline in flight just because you set the hang angle higher! Why? Because the airframe is going to find its own equilibrium as it "dangles." That is, regardless of the hang setup, the CG will land right on the rotor thrustline unless another force is there to displace it. The frame will simply dangle at whatever angle zeroes out all the forces and moments on and about the CG. So...

If we want the CG to stay ahead of the rotor thrustline, we need to introduce another force to push it there. What's our source-of-force? It can be any number of things, but the two most common in gyros are (1) low prop thrustline and (2) a downloaded H-stab.

Abid, you're right of course that a well-engineered gyro design will have had all this work already done. The aircraft will come with max and min weights for fuel and people -- stay within these limits and the gyro will fly properly. But...

Certain early, stabless high-thrustline gyros were laid out using Bensen specs -- which, in fact, were quite wrong for those designs. So, much depends on the quality of the engineering that went into the design. That's always the case with "experimental" aircraft. Know your designer!

Just a note. Rotor thrust line right through horizontal CG probably will feel neutral in certain stability scripts to the pilot. We probably want to avoid that. Stabilizer being effective will allow the right orientation and hang in expected and allowed attitudes to make it t behave on its own as positively stable without much input from the pilot reducing pilot workload.
And yes low thrustline could also be used but generally in new designs H stab at the back does the job. It is drag so you want to do just what you need and not too much more.

Roger that.

What complicates this whole business is the Bensen-style gimbal head, with excess offset and a trim spring to balance things out. The bottom end of the spring is normally fixed to the frame. Pitch rotations thus produce changes in stick pressure that can be either helpful or not.

Example of an unhelpful reaction: A gyro with uncompensated high thrustline will nose down when power is increased. This slacks off the trim spring (IOW its bottom end rises), producing a stick-forward change in control pressure. If the pilot maintains a light grip (as per standard technique), the stick WILL move forward, dropping the nose even more. This is classic static instability with respect to power setting.

Next, add a H-stab that JUST counteracts the high-thrustline nose-dip. Now, when power is increased, the nose won't dip and the rotor head will not contribute to power instability.

Now go a step further. Add a H-stab that has some download, and is immersed in the propwash. Now when the power is increased, the nose will rise (because the H-stab experiences increased airspeed inside the wash, thereby increasing download). The trim spring will now pull harder on the torque bar, creating an aft-stick pressure. The (likely) aft movement of the stick will add to the nose-up rotation.

Is the third scenario good? Depends. It's possible to overdo it, to the point where the gyro actually slows down when you add power. While this is much safer than diving when power is added (first scenario), we'd ideally like the gyro to hold its airspeed, and simply begin climbing at that speed when throttled up. Life is simpler that way.

The takeaway is that airframe stability DOES matter in a rotorcraft with a Bensen-style trim-spring head.

I think you’re right Abid I know that there are some VERY smart folks here on the forum, which is why I posed my question. I knew some of the smart ones would compliment in eventually. Doug explained it very well so I’ll do the hang test (maybe a double hang test) just for the learning experience. Thank you Doug, Denis and AirCommandPilot for chiming in as well as Abid.

Roger that.

What complicates this whole business is the Bensen-style gimbal head, with excess offset and a trim spring to balance things out. The bottom end of the spring is normally fixed to the frame. Pitch rotations thus produce changes in stick pressure that can be either helpful or not.

Example of an unhelpful reaction: A gyro with uncompensated high thrustline will nose down when power is increased. This slacks off the trim spring (IOW its bottom end rises), producing a stick-forward change in control pressure. If the pilot maintains a light grip (as per standard technique), the stick WILL move forward, dropping the nose even more. This is classic static instability with respect to power setting.

Next, add a H-stab that JUST counteracts the high-thrustline nose-dip. Now, when power is increased, the nose won't dip and the rotor head will not contribute to power instability.

Now go a step further. Add a H-stab that has some download, and is immersed in the propwash. Now when the power is increased, the nose will rise (because the H-stab experiences increased airspeed inside the wash, thereby increasing download). The trim spring will now pull harder on the torque bar, creating an aft-stick pressure. The (likely) aft movement of the stick will add to the nose-up rotation.

Is the third scenario good? Depends. It's possible to overdo it, to the point where the gyro actually slows down when you add power. While this is much safer than diving when power is added (first scenario), we'd ideally like the gyro to hold its airspeed, and simply begin climbing at that speed when throttled up. Life is simpler that way.

The takeaway is that airframe stability DOES matter in a rotorcraft with a Bensen-style trim-spring head.

AR-1 was/is the third scenario. That is what it will do. Compared to AutoGyro, ELA and Magni tandems that still do nose dip and speed up with power application. People think AR-1 flies the same as AutoGyro etc. I don’t say much to it except to make a mental note that they are not very perceptive pilots and wouldn’t without training make good test pilots.
AR-1 will slow down about 6 knots when you go from low cruise power to full throttle in a 915. You have to retrim. I have reduced the negative angle on the H stab in the last two years to curtail this effect without flipping over to the other side. Also from doing that reduces drag.

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I’m glad to have found this article and thanks for the great dialogue. I have an Air Command pre elite, done the recommended upgrades and added a “big-n-ugly” H-stab and my flight instructor test flew it for me just prior to my solo and during the testing the engine froze up then life happened and I’ve been grounded for a few years. Just now trying to get going again and when I correct the engine situation I will pay attention to this info.
I have the all flying rudder and mounted the H-stab about 7” above the keel putting it in the slip stream about 5 1/2” to 6” above the propeller tip with 0.0* download. My instructor never mentioned anything about adverse reaction but all he did was many t.o. and landing and around the pattern. Of course all this after some refresher training to get back in the groove. I hope this isn’t out of context to this thread if so please forgive I’m new at these forum writings have read many of them. Keep up the good work comments?

Not the best practice to do an all flying tail/rudder for many reasons but you aren’t going to go that fast in an AirCommand

I'm flying every chance I get! Recently completed 200hrs inspection/maint. Just dread the drive thru Houston.
I Think they were talking about me. I sold my machine and haven't been out there in quite a while.

Abid, sounds like you have your design dialed in. Nice!

In my limited time in a Magni-configuration gyro, I did notice that initial nose-dip upon power-up. The effect is small and apparently harmless; just irritating to a stability nerd. What may be the case with those gyros is that the H-stab as installed has no negative incidence, and the nose must dip a little before the H-stab starts to "bite."

I found that a -3 deg. incidence on the Gyrobee (with a small engine, 2-3 inches HTL and a 6 sq. ft. immersed H-stab) produced Scenario #2. OTOH, my 912S tandem Dominator, with substantial LTL plus an immersed H-stab, was over-compensated for power changes -- powering up from cruise power to full power required quite a large trim change to avoid slowing down. Scenario #3 on steroids. More examples:

The earliest model lowrider Air Command would fly very nose-low at high airspeeds. I noticed this both in flying mine and in flight videos of a friend flying his. This is an example of a gyro that was laid out using Bensen's hang specs, but its uncompensated HTL actually demanded a different hang spec. Once you add the factory H-stab to the lowrider Air Command, though, it becomes a different aircraft. The "low-nose syndrome" largely disappears. (My dinky 447 lowrider Air Command would do over 80 IAS using a high-pitched prop, but was twitchy and scary at that speed pre-H-stab. It became quite solid after the H-stab upgrade.)

IIR, the hang spec for an original RAF-2000 was around 4-5 degrees nose-down (recall that Bensen's was -12). This is an example of departing from the classic hang spec in order to enable a very HTL gyro to fly level. An RAF-2000 with a seriously-compensating H-stab would need to revert to something like the Bensen spec. Otherwise, it would fly way nose-UP.

Abid, sounds like you have your design dialed in. Nice!

In my limited time in a Magni-configuration gyro, I did notice that initial nose-dip upon power-up. The effect is small and apparently harmless; just irritating to a stability nerd. What may be the case with those gyros is that the H-stab as installed has no negative incidence, and the nose must dip a little before the H-stab starts to "bite."

I found that a -3 deg. incidence on the Gyrobee (with a small engine, 2-3 inches HTL and a 6 sq. ft. immersed H-stab) produced Scenario #2. OTOH, my 912S tandem Dominator, with substantial LTL plus an immersed H-stab, was over-compensated for power changes -- powering up from cruise power to full power required quite a large trim change to avoid slowing down. Scenario #3 on steroids. More examples:

The earliest model lowrider Air Command would fly very nose-low at high airspeeds. I noticed this both in flying mine and in flight videos of a friend flying his. This is an example of a gyro that was laid out using Bensen's hang specs, but its uncompensated HTL actually demanded a different hang spec. Once you add the factory H-stab to the lowrider Air Command, though, it becomes a different aircraft. The "low-nose syndrome" largely disappears. (My dinky 447 lowrider Air Command would do over 80 IAS using a high-pitched prop, but was twitchy and scary at that speed pre-H-stab. It became quite solid after the H-stab upgrade.)

IIR, the hang spec for an original RAF-2000 was around 4-5 degrees nose-down (recall that Bensen's was -12). This is an example of departing from the classic hang spec in order to enable a very HTL gyro to fly level. An RAF-2000 with a seriously-compensating H-stab would need to revert to something like the Bensen spec. Otherwise, it would fly way nose-UP.

On AR-1 after a lot of careful test flight and in flight angle measurements I tweaked the mast angle slightly on new frames and ended up with 1.5 degree negative incidence on the symmetrical H-stab center chord line as a good number that seems to work well across the speed range. The tail in AR-1 is about 12 inches farther back than say MTO Sport. MTO Sport H-stab I think has negative 3 to 4 degree incidence.

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