Hang test / RTV.

[birdy]

A while back someone asked why do a hang test.
I'v been look'n through the threads but can't find it.

I said sumth'n simple like," the hang test is to gauge the extent of the nessasary inbuilt correction to the effects of thrust."

Someone else [Douge I think] corrected me by say'n it was to be sure that the COM is set ahead of the RTV, or something like that.

The question,
If the machine WAS set to the correct hang angle,THEN the rotor blades were changed to a set that cruised at a different disc AOA,wouldn't the hang angle need to be changed to compensate for the 'moved' RTV.
IE; a set of blades that flew flatter would need ???????

[chuter]

My understanding is that the hang test is done to ensure enought travel in the control system in the pitch axis.

Where the RTV ends up is a function of thrustline/CG relationship and aerodynamic forces.

Experts, is that right?

[birdy]

Hmmm.............., Doug , Chuck, Udi ????

[mceagle]

Not Doug, Chuck or Udi but I will have a go.
The hang test angle should be slightly more than the rotor thrust vector angle, placing the C of G slightly ahead of the rotor thrust vector for improved stability.
The off shoot of this is that the joystick will operate close to the central position. Technically, the hang test should be changed when changing from very efficient rotors to very draggy rotors.
The hang test is only an average of what is considered to be a good starting point for safe test flying. It may need to be further adjusted for optimum comfort and performance, depending on many other factors.

[chuter]

Tim,

I'm trying to think through your post; it seems to me that if you setup a HTL gyro (without HS) like you said, no matter how it hangs it would fly with the CG behind the RTV, resulting in negative static stability. :confused:

[darrellwittke]

The hang test is to ensure you have enough control travel in the rotor head (ie. to center the stick and rotor head together), from what I remember. The rotor is going to fly at what its airfoil demands it fly at through the air, the gyro frame is going to dangle at whatever aerodynamics and gravity decide it must dangle at, and the hang test is a convenient means to correllate the two to the control range, methinks.

[Udi]

The hang test does not have a direct effect on the location of the CG vs. RTV in flight. Neglecting pitching moments such as engine thrust line and aerodynamic forces (HS, drag, etc.), the CG would always hang right in line with the RTV. When the gyro hangs nose-low in hang test -- say you move the battery to the nose -- the gyro would also hang more nose-down in flight. But the location of the CG vs. the RTV would not change! Move the battery to the tail, and the gyro would fly nose high, but still the CG would be in line with the RTV. In order to affect the location of the CG vs. RTV you must apply additional pitching moments that would move the CG forward or back of the RTV.

A HTL gyro has the engine pushing the nose down, thus moving the CG behind the RTV. Airframe drag usually does the same. We normally use a down-loaded HS to lift the nose in flight and move the CG ahead of the RTV. We may also use a down-loaded stab to counteract the nose down moments of the engine and drag.

The main purpose of the hang test is to ensure that the cyclic is centered in flight. If the nose is hanging too low, you may run out of back stick for maneuvering or landing. If the nose in hanging too high, you may not have enough forward stick to bring the nose down during takeoff or at high speed flight. A light nose is safer than a heavy nose.

The second purpose of the hang test it ensure that the HS would fly level, or with a slightly negative pitch during normal cruise speed.

Udi

[PW_Plack]

...and in any event, it's not going to hang any differently based on rotor drag, or even with the rotor removed.

[Udi]

It would, however, hang differently in flight with a more (or less) draggy rotor, Paul.

Udi

[chuter]

It would, however, hang differently in flight with a more (or less) draggy rotor, Paul.

Udi,

See if I'm getting this: a rotor with less drag flies at a given airspeed with the disk being flatter, so the RTV is angled more rearward (rearward under the rotor). This would mean that the HTL gyro would fly more nose down with a less draggy rotor?

[Udi]

That's right, Michael. And if that HTL gyro has a stab, the stab would fly at a more negative AOA, producing more down force, and moving the CG more forward than it was before in relation to the RTV. This gyro would be more statically stable (or less unstable, as the case may be).

Udi

[chuter]

Thanks Udi,

So if you've got a HTL gyro without a HS, a more efficient rotor makes it less stable; but if your HTL gyro has an HS, a more efficient rotor makes it more stable! Cool. :cool:

[Udi]

Michael - flying with a more nose down attitude, with no stab, doesn't necessarily make the gyro more unstable. If the drag-induced nose down pitching moment hasn't changed, and the engine nose down pitching moment hasn't changed, the distance of the CG from the RTV would remain the same even if the RTV itself has moved back a few inches. The nose down stance of the airframe dose not, all by itself, change the static stability of the gyro.

Part of the stability equation is the rotor itself. Some rotors are more stable than others. Rotor stability may be the most complicated and least mentioned subject in gyroplane aerodynamics. For me, at least, it is the most difficult subject. People often forget to think about that when they change their rotor systems.

Udi

[chuter]

I think it's clearing up; HTL, no stab, more efficient rotor flies flatter, allowing the RTV to move rearward. This just rotates the whole package more nose-down, but doesn't change CG/RTV relationship, therefore, the stability factor hasn't changed.

[ggfossen]

Please pardon a bit of old ignorance, but you all are confusing me. I'm new, here, but I'm curious. To put things in perspective. I soloed a T-craft in 1963, and ended my flying career with a total of 20 hours logged. Now I'm old with a big new shop, and some extra time, so I've been lurking in the shadows.

Problem is, your abbreviations are confusing me. I understand Angle of Attack, and Center of Gravity, but what, exactly is HTL, HS, & RTV.

Also, wouldn't the angle of thrust, or the way/angle of the engine mount affect the position of the machine in flight?

Gary

[Dean_Dolph]

....Now I'm old with a big new shop, and some extra time............Nah, you ain't old, you are posting here aren't you? :) ....Problem is, your abbreviations are confusing me. I understand Angle of Attack, and Center of Gravity, but what, exactly is HTL, HS, & RTV.

Also, wouldn't the angle of thrust, or the way/angle of the engine mount affect the position of the machine in flight?

Gary I know what you are saying about the acronyms! I created an acronym list to support the glossary the was created here but no one seemed interested in archiving it and then again it might not be complete.

To answer part of your question, HTL = High Thrust Line, HS= Horizontal Stabilizer and RTV=Rotor Thrust Vector. Also, when speaking of center of gravity, as it is referred to in the gyro world, people are usually talking about the vertical Cg but very few of us make that clear.

A short answer to your 2nd question is yes, it could, and maybe but I'm not the one to provide the in depth details.

[Udi]

...Also, wouldn't the angle of thrust, or the way/angle of the engine mount affect the position of the machine in flight?

Gary
Gary – Both the location of the engine and the angle of the engine affect how the engine thrust line is passing in relation to the gyro CG. If the engine is mounted 3" higher than the CG, but angled down, the thrust line can still pass thru the CG.

When the thrust line is passing thru the CG, it doesn't create any pitching moments, so it does not affect the angle of the gyro in flight. A high-thrust line (HTL) would push the nose down and a low- thrust line (LTL) would push the nose up.

A net nose-up pitching moment in a gyro is equivalent to a nose heavy fixed wing airplane. This is a statically stable configuration. A net nose down pitching moment is a de-stabilizing force, similar to a tail heavy fixed wing.

Udi

[ggfossen]

Dean,

Thanks. And now that I know I'm not old, I won't forget what you told me. I think Mark Twain also said something to the effect that, "if we always tell the truth, we never have to remember anything." I know he's wrong on that one.

Thanks,

Gary

[ggfossen]

Udi,

Thanks, also. It's been maybe 45 years since I went through ground school, but most of that actually makes sense to me. I guess there's hope!

Gary

[birdy]

Thanx Udi, just wanted understand why me RAF flys better with all the changes I'v made, and it hangs at only 4* nose down.

[ggfossen]

Birdy,

So now I'm confused again...or still. Udi says a Nose Down pitch is a de-stabilizing force. You say your bug flies better now, but it has a 4* down pitch?? Did I miss something?

Gary

[birdy]

and it hangs at only 4* nose down.

Only at the hang test Gary.
Keel is near 0* at SL.

[ggfossen]

Birdy,

OK, that would mean you have a bit of down thrust, or the engine is a bit below CG? Or maybe the drag on the blades, while in flight, would cause the aircraft attitude to shift a bit when in flight? Or maybe all of the above? I'm still puzzling over "SL," but I already warned you guys about my less than blissful ignorance.

How is the "hang test" preformed? Is it a simple matter of picking the machine up by the rotor mast?

Gary

[Udi]

Gary - the hang test is performed by suspending the gyro from the teeter bolt, with fuel in the tank and the pilot sitting in the seat with full flight gear. The teeter bolt is the bolt that is holding the rotor hub. A 2-blade type rotor is teetering in flight, thus the name teeter bolt.

A gyroplane rotor is normally flying with a back angle of 9-11 degrees at a cruise speed of 45-65 mph. The rotor thrust vector (RTV) is at 90 degrees to the rotor disc plane, so it is pointing about 10 degreed back from vertical (with the horizontal component of this vector being the rotor drag). Since the CG is aligned with the rotor thrust vector (to the most part), the airframe of the gyro hangs 9-11 degrees nose up in flight. We design the airframe with the CG located in such a way that the airframe hangs about 10 degrees nose down when statically hung from the teeter bolt. This ensures that it would fly level.

Don't ask me to explain why Birdy's ship is hanging at 4 degrees.

BTW, SL means straight and level flight.

Udi

[ggfossen]

Udi,

I do appreciate your effort. Although, at present, I'm only salving curiosity, one never knows where it might lead. I've always had an interest in flight, but never seemed to get there, other than a very little bit a very long time ago. The country in which we live is about as steep as a cow's face, and any strip is a fair bit of travel from the house. I have a fair dolop of pragmatism that sort of dictates that I don't spend much time or money on something that isn't really productive. Now that I am retired, however, I'm doing my best to beat that out of myself. I got as far as finding this forum, and that's a start.

I think the problem I'm having with understanding the gyros is that I keep think of powered rotors. I gotta' shift gears.

Thanks

Gary

[birdy]

Udi, the standard RAF minimum nose down at the test is 4 1/2*.
With all the changes I'v made to it,it FLIES best if it is set at 4* at the hang, one up.
Also, if I'v got it at the highest nose up setting one up, then when I'v got a fatty sit'n next to me, the one-way stab isn't applying much down force with the less nose down attitude.
I havent tested it with a 200 LBer in the seat coz there aint any 200 pounders living within a hunderd miles from ere.I'll have to use some bags of cement.Then I can set the stab so it'll be just off the stopper in SL two up.

[ggfossen]

Birdy,

I am a 200 pounder! Plus a bit. Maybe you need me as a Guinea Pig??

Gary

[PW_Plack]

Gary,

If you're interested in gyros, you need to spend a Saturday and come up to Scappoose for a meeting of PRA Chapter 73. The next meeting is March 12th. We'll start with a free two-hour ground school class hosted by Master CFI Dave Wiley. This one will be on weather. There will be coffee and donuts.

At noon, we have lunch...I'll buy yours if you come. The formal meeting (as formal as we get, anyway) is at 1pm. Afterward, we'll take you across the field to Sport Copter, which I guarantee will make the trip worth the drive, if you love mechanical and/or flying stuff.

If the weather's decent and the tandem trainer has its winter upgrades completed, we might be able to get you an introductory ride. One recent student was 6'-6" and 285, so you won't be pushing the limit!

[ggfossen]

Paul,

Although I will not be able to make it, I sincerely appreciate the offer. I have a few more irons in the fire than I probably should. I will print your entry, though, and keep it handy, just in case.

Thanks,

Gary Fossen

[PW_Plack]

Gary, good luck with the irons. We'll be at Scappoose every second Saturday except September and December, should you decide to put "an aluminum" in the fire!

[ggfossen]

Gary, good luck with the irons. We'll be at Scappoose every second Saturday except September and December, should you decide to put "an aluminum" in the fire!


Paul,

My wife thinks I should go. I have to rethink. I may, or may not. Where do you meet? I did, by the way, find Scappose on the map. A little "cross-country" exercise. Maybe what I need is a really fast pre-rotator so I can simply "jump" off the side of one of our hills.

Gary

[PW_Plack]

Gary,

I-5 north to Portland, US 30 west to mid-town Scappoose, right on Columbia, follow the signs. We meet during the winter months at the Northwest Antique Aircraft Club's building at the north end of the airport, off Moore Road. If you can leave early enough to make it to Hillsboro by 9:15am, we could carpool the rest of the way from here. If not, give me a call, and I'll get you more precise directions.

Your wife is right!

[ggfossen]

Paul,

OK, and I appreciate the offer, but I can promise there is no way that I would leave here early enough to be in Hillsboro by 9:15. I'm more inclined to leave a day early, and find a motel, or a good place to park the trailer. Of late, we seldom travel more than about 250, or 300 miles a day. We see a lot more that way.

Thanks,

Gary