Nose down tendency at speed!

Vance

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I struggle with the basics of gyroplane flight and I would like to expound and seek knowledge on one of the many things confusing me.

I want to build a somewhat faster open tandem gyroplane.

The Predator tends to lower the nose just a bit at speed.

She has a very large horizontal stabilizer set at zero angle of attack and is very near center line thrust with full tanks and Ed in the back. As the 22 gallons of fuel are exhausted she becomes slightly low thrust line.

My fantasy is that as the speed picks up the rotor drag goes down and she is balanced to fly level for the rotor drag at about 60kts, 69 miles per hour.

My second fantasy is that all the rest of the frontal area increases its drag by the square of the increase in indicated airspeed.

I feel that her windshield also produces some down force.

I feel that an open tandem at some level of nose down will have the drag go up dramatically, sort of like a bucket with the open end into the wind.

I feel that this might exacerbate the nose down tendency and could become a non recoverable event.

I feel it would be good to have a pitch trim adjustment on the horizontal stabilizer but I seem to be alone in this.

I suspect I am missing something because the fast open tandems do not seem to be falling out of the sky.

Thank you for your input, Vance
 

Friendly

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Vance,
I am curious, What is the speed of your gyro now?
How fast do you want to go?
 

Vance

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I find joy in running up on the fixed wings in the pattern.

I find joy in running up on the fixed wings in the pattern.

Hello Mark,

The Predator will fly around 95kts, 109 miles per hour indicated airspeed straight and level solo, and 90kts, 104 miles per hour two up.

It is easy to see 115kts, 132 miles per hour by heading down hill.

My GPS has seen 140kts, 161 miles per hour of ground speed.

I spend most of my flying hours between 60 and 70kts but every now and then I find an excuse to cut loose.

I know that given my personality that I will at some point see how fast Mariah Gale will go. I wouldn’t want it to be my last flight.

I expect Mariah Gale to be able to see 110kts, 127 miles per hour straight and level and it wouldn’t surprise me if I saw 140kts, 161 miles per hour indicated airspeed downhill.

I don’t want to wait until I can afford an IO-550 so she will have an IO-320B1A so I can start building in July.

Thank you, Vance
 

okikuma

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Hi Vance,

With high power settings (higher RPMs and greater amount of thrust) and higher speeds, it is to my understanding that cyclic inputs in a gyroplane becomes very sensitive, mostly because of the decreased amount of drag from the rotor system because of the lower angle of attack of the rotor disk.

I can see in some ways having a gyroplane that will cruise near C-172 speeds will give one greater range with the same amount of fuel burn, but is it worth the higher level of cyclic sensitivity and the ease of diverging from normal cruise flight with the possibility of a fatal result? The work load flying at the higher speeds with a sensitive cyclic would take the fun out of an otherwise a relaxing flight.

I often wondered how sensitive the Little Wing "Woodstock" cyclic was when Andy Keech was performing his speed runs for the records?

For all practical purposes Vance, you already have a high performance gyroplane with the Predator. Your gyro now outperforms majority of the gyros out there including certified ones.

Wayne
 

RotoPlane

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Vance - I feel it would be good to have a pitch trim adjustment on the horizontal stabilizer but I seem to be alone in this.
You aren't alone....I also agree with an adjustable pitch trim and my ship will have it. An HS with no in flight adjustment is balanced to a small speed band. If you go faster than that, the RTV moves further aft from the CG and the nose will drop. Other things that you mentioned can also aggravate this nose drop but an adjustable HS could bring it back to level. It is more important for a fast gyro like yours, than a slower one….
 

Vance

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Imaginary challenge.

Imaginary challenge.

Hello Wayne,

The faster the predator goes the more stable she is.

I have heard about the twitchiness at speed and it is not hard to imagine but I haven’t experienced any of that. If anything the controls of the predator get a little heavy over 90kts and she is almost heavy on the controls at 100kts..

To achieve this higher performance I have to knock 27% off the drag of the Predator.

I feel that the most import part is the back where I have no streamlining.

A simple airfoil shape can knock 85 percent of the drag off a round tube.

There are a lot of unfaired round tubes on the Predator.

The external fuel tanks alone are 1.1 square feet of frontal area and the mast is another square foot of frontal area. The suspension adds up to over a foot of frontal area and my windshield is diverging when it ends. It will be fun trying and I feel I need to build the aircraft myself for the book.

It sounds challenging to most people to get together with your friends and design and build an aircraft out of junk yard parts in your garage and fly it across the country particularly without an engineering education.

As long as I am building I might as well do the best I can and I don’t want to have a problem when I explore the edges of the envelope.
I also want to have at least 40 gallons of gas on board.

This nose down tendency I imagine seems to be only in my mind because the Predator doesn’t do it much and so far no one has validated my hypothesis.

Mariah Gale will have pitch trim because I expect to be using an airplane horizontal stabilizer with elevators so I don’t have to design and build an empennage from scratch. I expect to use them only for pitch trim and probably electrically.

Thank you, Vance
 

Vance

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We are in the minority Ed.

We are in the minority Ed.

My response to Wayne contains the reason I will have an adjustable pitch trim.

I learned long ago that if you can buy something it is much less trouble than building one from scratch. I doubt if Smokey and I could build a lighter stronger empennage. I have trouble imagining the forces on an empennage, particularly at speed in rough air. The propeller throws a resonance into the tail that I don’t begin to understand. An empennage failure could be challenging and I don’t want to build it too heavy because of my ignorance and fear of the unknown. I also have trouble knowing how to go from a tube structure to a sheet metal structure.

I feel that a slightly down loaded stabilizer may be good for a wider speed range but my only experience is with the zero pitch of the Predator.

Thank you, Vance
 

dabkb2

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Danger Danger Danger, I don't think the fuselage or anything else on the gyro will be as much drag as the rotor. To go any faster than you are already going in a pusher gyro would mean putting the AOA of the rotor blade at near 0. I think if you want to go faster, build a tractor gyro with wings.

The only time I have been truly scared in a gyro is when I pushed the top speed, the ASI was pegged at 100 in a dive and I will never do that again.

Just a thought from an uneducated gyro pilot
 

GyroDoug

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What about the Carter Copter's Gyro that broke the MU1 barrier? It went a lot faster than 100 mph and did it safely.
 

JAL

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I preface this post with "just thinking out loud" as I have no knowledge of aerodynamics.

Has anyone ever considered a canard on a gyro. I was thinking you could have the tail to help stability at lower speeds and high power settings immersed in the prop wash and and canard optimised for higher speeds to offset the rotational forces (around CoG) applied by the pod being dragged through the air. Obviously a canard would create more drag (overall) as you would have more flying surfaces with different AoA set for each purpose. You could also have the canard as all flying and maybe act a as a trim device?

Jordan
 

Arnie Madsen

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What about the Carter Copter's Gyro that broke the MU1 barrier? It went a lot faster than 100 mph and did it safely.
Hi Doug.

Carter used wings to unload and slow the rotor (I dont think Vance is using wings) .

But what I really wanted to say is I am sure Jay Carter published exhaustive figures and studies about (standard) rotors at high speed (and their limitations) and I am thinking that information might help Vance.

Back during the years when Carter was doing his testing I followed it minute by minute. I will take a look and see if the pertinent link is still there.
 

C. Beaty

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Without moments of torque about the CG of a gyro, its CG will always lie on the on the line of rotor thrust so it is normal for a gyro to nose down with an increase of speed.

A horizontal stabilizer tends to prevent the natural nose down effect but the rotor doesn’t know that and always flies at the same angle at a given airspeed. The net effect is that the rotor thrust vector moves aft with speed increase, creating a nose heavy aircraft. Nose heavy aircraft are stiffer in pitch since the restoring forces are stronger.

The attached plot illustrates the general trend. It was calculated for an open frame gyro of 500 lb. AUW with a 23’ rotor. Drawbar power is the power based on towline pull.
 

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Arnie Madsen

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Vance

Not sure is this is helpful:
Pie chart - pylon drag , hub drag
http://www.cartercopters.com/gt-hub_drag.html

Same as above with Detailed presentation:
(Georgia Tech Hub Drag Calculation )
Factors affecting fuselage attitude
Faired hub drag improvements etc
Index here:
http://www.cartercopters.com/gt-hub_drag-compatible.html


Mast fairings (Bottom of page article #8)
http://www.cartercopters.com/mu-1.html


2 Place Personal Air Vehicle
(claims to be Pure Autogyro but I see stub wings)
http://www.cartercopters.com/images/pav_12g_gyro_canopy_closed_iso.jpg

Same as above with article: (cruise 120 - 130 mph) (gross 1300 lbs)
http://www.cartercopters.com/pav_12g.html


I have not come across what I was mostly looking for. I am sure there was at one time a spread sheet showing all the variables affecting the rotor as forward speed increased . (standard rotor)
 

Alan_Cheatham

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Vance,

Since retreating blade stall really sets the limit to max safe speed in a gyro I'm not sure how realistic it would be to design a gyro to attain speeds much higher than what you have in Predator, however streamlining an aircraft has many benefits other than just higher speed, fuel burn and range for one.

There's no reason you couldn't design a new ship, just do your homework and be able to do the structural analysis and I also suggest you research the patents for the old tractor gyros and see how they dealt with trim issues, there are many interesting ideas there such as a trimable head (moves fore and aft) to move RTV forward for high speed flight, trimable tail surfaces, and water ballast tanks to shift the CG.

Having said that, first do a preliminary weight estimation of your ideal gyro and then see if there is a rotor out there that will fly that weight, with 40 gallons of fuel, two people, heavy engine, airframe, kitchen sink, etc. sounds like you dream ship will be Super Dreadnought Class and there are limits as to how much a conventional gyro rotor can take.

.
 
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Vance

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High speed is not the goal.

High speed is not the goal.

Thank you for your concern David,

I have not had that experience.

The Predator seems to get steadier the faster she goes.

I don’t want a tractor and I particularly don’t want wings.

I don’t want to surprise myself.

Thank you Doug,

High speed is not the goal; it may be the product of reducing drag.

Thank you Jordan,

When I considered a Canard on a gyroplane it seemed to me to be destabilizing in gusting conditions.

Thank you Arnie,

High speed is not the goal so devices to unload the rotor are not on the table.

Estimating drag for different shapes is something that we did a lot of on the unfaired motorcycles we ran at Bonneville. We used to do elaborate coast down test culminating in my unfaired Harley Davidson that went 200 miles per hour on 140 horsepower in 1985. The same bike did 164 miles per hour on 90 horsepower in 1997.

Thank you, Vance
 

Vance

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Thank you Mr. Beaty.

That is just what I am talking about.

I am glad to know why the Predator seems more stable the faster she goes.

It seems to me if the Predator got too nose down that the body itself would become unstable because it is open at the top.

I don't want to change that.

Thank you for the help, Vance
 

Vance

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Happy rotor and longer range.

Happy rotor and longer range.

Thank you Alan,

It appears to me that the rotor I have now would be happy going 160 miles per hour in a shallow dive without having a problem with retreating blade stall. It is the rest of the aircraft that concerns me.

I don’t feel that there are a lot of special needs for a gyroplane going less than 160 miles per hour in a dive.

We are not going very far out on a limb structurally and I feel it is inside our capabilities.

We are not really stretching the gyroplane performance envelope that hard and again top speed is not the point of the exercise.

The point of the question is to avoid an unpleasant surprise.

The Predator weighs 800 pounds with a 50 pound body. I feel I can reduce that weight by at least 50 pounds.

That makes Mariah Gale weigh 750 pounds. If I figure 250 for me and 150 pounds for Ed and 240 pounds for 40 gallons of fuel that still comes in with a gross weight around 1390 pounds which is less than many two place gyroplanes flying now. I have flown the Predator with a 250 pound person in the back seat and 16 gallons of fuel. She grossed out at 1,400 pounds and flew fine.

I feel this 40 gallons will provide Mariah Gale with a more useful cruising range than the Predator’s 22 gallons. I like to land with an hour reserve so with the Predator that gives me a 110 mile range. I feel with the reduced drag and 40 gallons of gas will get me closer to 300 nautical miles at 70 kts cruse. This seems to me to be a worthwhile improvement.

Many people have suggested that the Predator is much too heavy for a gyroplane but I like her fine.

It is my intention to make Mariah Gale have the same gross weight with a longer range at a slightly higher cruising speed.

It is an example of my arrogance that I feel I can improve on such a lovely design. I have an advantage in that I have a starting point much closer to the goals than Mark Givans had.

Thank you, Vance
 
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StanFoster

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Vance- This is going to be the neatest adventure to watch unfold. I have no doubt you will achieve your goals. You are an amazing person to watch.


By the way....my SprarrowHawk weighed 950 pounds. I flew it will full fuel..120 pounds....myself at 220 pounds...and my son Jeff at 270 pounds....and it flew with abundance of lift. I dont recall the rate of climb, but it got up to pattern altitude very quickly. That was 1560 pounds gross and it was near 80 degrees.

This was with the stock 2.5 Subaru. I never did get him up when I had the cam grind done and had another 15 horses that would have all been in rate of climb increase.

You are a rare personality Vance...and my life has been enhanced knowing you.


Stan
 

Doug Riley

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Vance, of course Chuck is correct that the aircraft will gradually trim nose-lower as the rotor's disk angle flattens out. It's confusing to think of this as resulting from "less drag." You're less apt to err if you think of rotor thrust as always perpendicular to the rotor disk (or tip-path-plane).

A H-stab with good access to clean free-stream flow and the right amount of negative incidence will help counteract the nose-low effects of both the pod and the flatter rotor angle as you speed up. The H-stab's download will, like drag, increase as the square of airspeed. It also increases in direct proportion to increasing nose-down frame angle.

You're right, though, that an open pod, at some nose-down angle, turns into an anemometer cup. Its parasite drag will, beyond that certain angle, incease exponentially. That's simply the nature of an open-topped bathtub shape.

Obviously, you can minimize this problem by making the opening as small as possible. More elegantly, you can build a hinged windshield that forms a fairing around and over the crew once they're seated.
 

Heron

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Better rotors and better aerodynamic is the way to go and that includes some lift from wings.
That stubby thing in the PAV is the root for the wing that will lift the machine in another config. It will fly on wings just using the rotor for take off and landing . .. compound gyro.
The Predator is right there at the best it can be . . .120 miles cruise is good thing.
Heron
 
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