expert advice needed!

[chuter]

As I’m proceeding with the flight testing of my gyro, I’m finding a characteristic that is troubling: I’m practicing descents at gradually lower power settings, working towards simulated engine-out landings. At lower power settings the nose goes down a lot.

I taped a digital level to the top longeron; straight and level at 5900rpm it’s about 2-3 degrees nose up. When I throttle back to 4500rpm the nose is 6-7 degrees nose down.

My concern is that if the engine should ever quite it may nose over in a bunt.

Determining the thrustline/cg relationship by the 2-picture method and by calculation gave mixed results: the picture method showed the thrustline about 4-5 inches below the CG, the calculation method showed the opposite.
From the way if flies I believe the thrust line is below the CG. Giving it throttle causes the nose to go up, reducing throttle and the nose goes down.
Also; when banking into a turn I have to put back-pressure on the stick to keep the airspeed from increasing.

The HS is pretty large; about 9 sq ft.

My thoughts are: as thrust is removed, the forces left acting on the airframe are aerodynamic forces and rotor thrust.

Just looking at my machine it looks like the vertical center of drag could be pretty low.

I’m going to try and determine the approximate vertical center of drag on my machine using the picture cut out method.

I’ve taken a picture from the front of the machine, I will cut the paper around the gyro part of the picture, then balance it on an edge horizontal to the gyro.

Questions:
How should the rotor be handled?
Should I cut it out so it’s not part of the picture?

Other thoughts: giving the HS some negative incidence could possibly help, I haven’t thought this through completely yet to think of any negative consequences to this change.
I plan on increasing my fuel capacity, so determining the best placement for this extra fuel to counteract this nose-down problem could also help.
Reducing drag below the thrustline is another option, the large Hegar wheels and tires being the obvious target. I sure like the disc brakes setup I’ve got so I hate to put wheel barrow wheels on it. Wheel pants are another option.

Any comments appreciated. Am I thinking straight here?

Thanks much,

[WHY]

High Michael
Just read your post about your nose droping when the engine is reduced in power and have the following thought, the drop has to come from either a situation of CG or a matter of drag, at least thats what I would think but then I'm not expert by a long way. Although you have a windshield for protection it wou;ld astound you how much drag that open fuselage has, have you thought any about covering any of the basic frame work, this would reduce the drag a bunch, also Ron Herron mignt have some comments.

Tony

[gyro19xl]

Mikey,

You will always have to add back pressure on the stick when in a turn, other wise the nose will desend and as a result your airspeed will increase.

Gary

[chuter]

I was thinking that a high thrustline machine without a correctly configured stab would result in the rotor thrust vector being ahead of the CG, resulting in the nose coming up while banking.

A low thrustline will cause the nose to rise under power so the rotor thrust vector is behind the CG, causing the nose to drop when banking.

It could be that my thrustline is high but the stab causes the rotor thrust vector to be behind the CG, causing the nose to drop while banking.

Either way, the nose dropping in a bank is a sign of positive stability, at least the way I understand it.

(howz that for a buncha fancy tawkin'?)

[Alan Coats]

Hi, Michael.

Glad you are getting some more flight time on your cool gyro. I still want to see it flying myself!

You asked for expert advice, and I am only half way to soloing so, as usual, take this answer as an opportunity for me to be corrected and learn more about gyros;

I don't think you are going to bunt over if you are at low throttle. I believe that bunting is synonymous with power-pushover. No power, no pushover.

As I recall, when we did the hang test, your gyro balanced at something like10 or 12 degrees nosedown (or more). If you are hanging from the rotor in the air with low power the hang angle shouldn't be too much different from when you were hanging from the hoist, should it?

The horizontal stab becomes less effective at low airspeeds and with low prop wash, so negative incidence wouldn't help much on low power approaches.

Alan

[chuter]

Toni,

Thanks for posting. I had earlier thought of the open fuselage as being draggy, but I hadn't thought of it in relation to this problem. That's a possibility.

I really like the open fuselage for ease of inspection and repairs, and just for looks, so I hope I don't have to give it up. But if that's what it takes I could put some aluminum skin on as removable panels.

Alan,

I was thinking about the hang angle and how it plays into all this. I figure in a vertical descent it will hang at the hang test angle. The nose-down when throttling back happens between 50-60mph (I haven't tested it any faster, too scary).

I would like to think that the large stab would prevent it from going over, but it's pretty alarming when the nose moves down that far and that quick.

Keep the thoughts coming guys; this is great stuff.

Thanks,

[PW_Plack]

Michael, I'm no expert, but from what I can tell so far, your "problem" is what many pusher gyros wish for. It's called "airspeed stability." Your machine will tend to hold trimmed airspeed regardless of changes in the power setting.

Unless the fuselage nose-down motion pulls the rotor disc so far nose-down that it acquires negative angle-of-attack in the relative wind, I don't believe you'll be in danger of a self-sustaining bunt.

Just from eyeballing your pictures, it seems hard to imagine that you'd have a center-of-drag below the center-of-thrust. But, imaginations aren't measurements. :)

[mceagle]

At lower power settings the nose goes down a lot.

I taped a digital level to the top longeron; straight and level at 5900rpm it’s about 2-3 degrees nose up. When I throttle back to 4500rpm the nose is 6-7 degrees nose down.
My concern is that if the engine should ever quite it may nose over in a bunt.
Thanks much,Chuter, your posting suggests that you have not had much prior gyroplane experience. It matters little what the angle of the fuselarge is to the earth, as long as the drift flag is straight and the control stick is still within its normal range.
The 6 to 7 degrees nose down is nothing as most dirty gyroplanes will be 45 degrees or more nose down in an engine out glide.
If your continual throttling back leads you towards running out of stick travel then quit your tests. The balance, thrustline or H/S need looking at.

[GyroRon]

I think Tim has hit the nail square on the head

[chuter]

Tim said, Chuter, your posting suggests that you have not had much prior gyroplane experience. It matters little what the angle of the fuselarge is to the earth, as long as the drift flag is straight and the control stick is still within its normal range.

You're right Tim, this is my first gyro. The only thing I can compare it to is my training in Ron Menzie's gyros. Mine just seems to go more nose down that what I'm used to with his.

My concern is that it will run out of rear stick travel. I've measure some angles as follows:

With the top longeron level, stick all the way back, the rotor head is pitched 18 degrees back (I have one of Ernie's heads with lots of extra travel).

I figure the rotor head flies with the rotor about 9 degrees back, so this should leave me with about 9 degrees of back stick travel at the rotor head with the fuselage level.

This means if the fuselage goes nose-down 9 degrees I will run out of back stick.

Does that sound right?

I'm really hoping someone will say this is normal, nothing to worry about!

[skyguynca]

I have to agree also with Tim. I have had a engine out in my gyrobee and the frame angle during the descent made me feel as I was going to fall out of the seat to maintain airspeed for the landing, but it was very uneventful (that is because it is a great design). The few clt I have flown did do just what you are describing though. As power was lowered the nose automatically lowered to maintain airspeed, I did not have to push the stick forward. During the descents the stick had plent of range of travel regardless of the angle of the airframe for the rotor angle to relative wing stayed the same to maintain airspeed.

[pwendell]

Michael,

If your gyro has airspeed static atability, which it probably does, it will tend to return to the same, 'trimmed' airspeed no matter the power setting. The only way for this to happen when the power is reduced is for the disc to fly at a lower AOA. In other words, the disc will flatten with respect to relative wind and the nose will follow. When you reduce power, it's not just the fuselage's nose that drops, but the disc also. The relative angle between the fuselage and the rotor tip plane path should not change that much if you are near CLT and have an adequate stab, whether you are at full or idle power.

At least that's how I understand it. I know that the CLT AAI modified RAF's and the Sparrowhawk I'veflown all drop their noses when power is reduced. If they didn't they would slow dramaitically and go into a rapid decent.

[mceagle]

With the top longeron level, stick all the way back, the rotor head is pitched 18 degrees back.Assuming that the top longeron is the horizontal flight datum, then that is correct.
the rotor head flies with the rotor about 9 degrees back, so this should leave me with about 9 degrees of back stick travel at the rotor head with the fuselage level.That is also correct. as previously stated, as the nose goes down, the rotors maintain approx the same angle to the fuselarge, so the joystick should still remain in roughly the same position (center of travel).

[Mayfield]

I figure the rotor head flies with the rotor about 9 degrees back, so this should leave me with about 9 degrees of back stick travel at the rotor head with the fuselage level.

This means if the fuselage goes nose-down 9 degrees I will run out of back stick.

Does that sound right?

I'm really hoping someone will say this is normal, nothing to worry about!

Michael,

You are essentially correct. The rotor will fly at a disc angle of about 9 degrees (or so) to the relative wind.

During your descent, the relative wind is a product of your horizontal and vertical movement. Even if the nose feels like it is at some god-awful nose down angle; the disc angle is still pretty close to 9 degrees or so at the same airspeed that you were at in level flight.

It sounds like your aircraft is doing what it should.

Jim

[chuter]

OK, thanks for all the replies. It sounds like it may not be so bad. (duh, I hadn't thought of the rotor staying in the same relation to the fuselage as the nose tilts down.)

My plan is: get plenty of altitude, pull the throttle back to 4500 rpm and establish 60mph. Then I'll see if I can pull the stick back and "flare" it at altitude.

I'll do this at gradually decreasing power settings.

I just want to be sure I can flare it for landing from this nose-down attitude.

Any more thoughts still appreciated.

What a great place.

Thanks,

[C. Beaty]

Michael, the amount a gyro should nose over when power is removed depends upon the overall lift/drag ratio (rotor+airframe).

The glide path is –27º at 2:1 L/D; -18º at 3:1 and -14º at 4:1.

Open frame, draggy gyros are in the range of 3:1.

All is well if airspeed remains nearly constant with locked stick when power is pulled.

[gyro19xl]

Mikey,

Anything other than wings level will cause a loss of lift requiring slight back pressure. The steeper the turn the more aft stick required.

Gary

[PW_Plack]

..and when you're sitting in the seat, especially if you're used to fixed-wings, that -18º will feel like -45º. More than one fixed-wing pilot has freaked out during his gyroplane intro ride when the power-off descent was demonstrated.

[chuter]

OK, so if my glide angle is -18 degrees, there could also be aerodynamic forces on the airframe tilting it even more nose-down; is that right?

Also, I have a centrifugal clutch and I've heard that at low rpm's with the clutch freewheeling, the prop can be pretty draggy too.

Agggghh!

[C. Beaty]

OK, so if my glide angle is -18 degrees, there could also be aerodynamic forces on the airframe tilting it even more nose-down; is that right?


18 degrees or so is the flight path, Michael. If you have adequate horizontal stab, the airframe ought to maintain its alignment with the flight path whatever the drag of the prop.

Of course, a draggy prop can make the flight path steeper if it reduces your overall L/D below 3:1.

[chuter]

Thanks Chuck and all. I'm feeling better about this.

But I'm still going to approach with extreme caution.

Just got to get used to that angle.

[Doug Riley]

Mike, you might investigate that VCG/prop thrustline relationship a little more closely. It's true that the nose-down attitude of any gyro in a power-off descent is pretty thrilling until you get used to it. However, if the prop thrustline really is 4-5" below the CG, the thrustline offset is excessive.

Too much undersling of the prop thrustline below the CG makes the nose swing more than necessary with power changes. This will tend to cause the gyro to "overshoot" past its correct nose-down position and airspeed when power is cut -- at least initially. This is pointless and adds to the pilot's workload.

What's more, you correctly point out that an idling prop can be a drag-maker. Whether it will be or not in a particular case will depend on the prop pitch, the idle RPM and the aircraft's airspeed.* The drag is most likely to be noticeable at high glide speeds. If the prop's center is significantly below the aircraft's CG, the prop drag will tend to pull down on the nose. Your HS will probably take care of this, but, again, there's no reason to set up this situation in the first place. (Nose-down drag effects are one of the things that Greg Gremminger refers to when he theorizes about non-PPO "buntovers.")

I'd suggest re-checking that VCG and, if the prop centerline is below the CG, moving the engine up so the prop C.L. is on, or within an inch or two of, the CG.

*For example, my Dominator gets significant thrust from its prop in an idling glide. You can tell this because it glides quite a bit farther when idling than when the ignition is off.

[chuter]

Thanks Doug, I'll do some more checking.

I keep thinking of that accident in Colorado a few years ago in a gyro that supposedly had a very low thrustline. I think there was some talk that it may have bunted over.

After thinking on this for a few days it seems like the ideal gyro might have the thrustline a little below the CG, and the center of drag a little above the CG (might be hard to do).

That way when to motor quits the center of drag above the CG would help keep the nose up............?

[PW_Plack]

Mike,

The nose dropping is what's supposed to happen. If you keep the nose up without power, you start a rapid vertical descent, ending in a hard vertical impact. Maintaining airspeed is the first priority, while you look between your knees for a suitable landing spot, so that you'll maintain enough energy to flare when you get there.

If you've somehow developed an instinctive fear of the nose dropping, you'll need to train through it. When the engine quits, descending is not optional, only the speed at which you do it. Your best glide ratio, which means the most ground covered before you land, happens at a pretty steep nose-down angle.

[chuter]

Paul,

I think you're right about just getting through it. I didn't have a problem with simulated engine outs in Ron's AAI conversion. I'm just used to sitting on the ground in mine at the nose-up angle. Even straight and level feels a little weird.

I think I'm going to prop the tail up at about 20 degrees while on the ground and sit in it for awhile.

Since mine is unique and I didn't have anything like it to train in, I'm approaching this with a lot of caution. It is an untested design after all.

Thanks,

[PW_Plack]

Mike,

"...prop the tail up at about 20 degrees while on the ground and sit in it for awhile..."

Bingo! The first passive, ground-based, engine-out simulator. :)

Caution's nothing to apologize for.

[Doug Riley]

To add to the realism, you can sit in it and NOT make engine noises.

[Canadian Rhino]

:D Whistling would be good! :rolleyes:

[chuter]

OK, I went out to the hangar and propped up the tail at 19 degrees, then sat in it for 5-10 minutes. I did NOT make engine noises.

I looked straight ahead and picked out a point on the mast that was about even with the horizon.

Went out and flew it and pulled the throttle back to the point I was getting scared last time; NO PROBLEM! Not even close to the point I picked out on the mast, and it was quite comfortable.

I still haven't pulled the throttle all the way back to where the clutch disengages, but I'm not nearly so worried.

Thanks to all the input and encouragement from you guys, and to Chuck for putting an actual number to the angle.

Hell, this gyro stuff might even be fun!

[PW_Plack]

Mike, does it come close to holding trimmed airspeed when you pull off the power? Remember, level is not "comfortable" in this circumstance. Maintaining enough airspeed to flare when it's over is what should bring you comfort.

If the machine doesn't naturally nose down enough to maintain trimmed airspeed, you're going to have to remember to do it with the stick, at a stressful time when your workload is already rising. Yours won't be the first gyro to require this, but you need to experiment, to see how steep you'll have to nose it down to maintain safe speed.

[chuter]

Paul, yes it does maintain airspeed. In fact it seems I might be holding a little back pressure on the stick so it doesn't increase.

When I get a little further along I plan on doing some of the tests that Greg G. has posted, but so far I'd say it looks pretty good.

[WindRyder06]

Mike,
Glad to hear that you are doing what you have wanted to do for so long now!! It is a great feeling. I will tell you that when I first trained with Rusty Nance I was fresh out of helicopters from the Army. The engine out approach was one HELL of an "E" ticket ride for me. In the Huey for landing you would look through the chin bubble to see the numbers at the end of the runway. It was very uncomfortable to look at he numbers over the top of my toes. I got the fixed wing training and followed it up with gliders to give me the sight picture that I needed to feel good about power offs in a gyro. It sure is fun now!!!! :D

Keep us posted on your progress.

Fly Safe,
Chris

[Heron]

This thread is "collector's item"
thanks
Heron