# Nose down tendency at speed!

#### Vance

##### Gyroplane CFI
Thank you Doug

Thank you Doug

I would not be too concerned if it was 1.5 degrees of change we were talking about.

I feel that my rotor disk angle of attack will vary through more than 30 degrees just going from 20kts to 100kts.

Is this an incorrect fantasy?

I feel that the rotor disk angle of attack changes less as the speed increases so I may be jousting with windmills.

I hope to decrease the drag above the center of gravity significantly over the Predator with less drag reduction below the center of gravity so I feel this may have an effect on the angle the body flies as the speeds go up.

I am trying to avoid that anemometer cup experience as the speeds rise.

Thank you, Vance

#### Doug Riley

##### Platinum Member
Vance:

In my experience, disk AOA's of 30 degrees aft are only reached or needed during landing flares (if then) -- not in any flight regime, even behind the power curve. Ditto disk AOA's of zero.

IOW, the flight range of disk AOA's is only a fraction of the available tilt in the controls. The extremes at both ends are used only for ground ops.

But, in any event, the frame angle is not a simple function of the rotor angle once you have a HS. The bigger the HS, the smaller the change of frame angle for a given change in rotor angle. We ought to think up a name for that ratio. Sorry I don't have time to draw you a picture.

Sketch out what I posted above and you'll see how things interrelate. A good starting place is Chuck's cannonball model; then attach an airframe rigidly to the cannonball. You might even print the drawing and manipulate it as a physical model.

Gotta go make a living! Cheers.

#### Vance

##### Gyroplane CFI
A doodle to see what is happening.

A doodle to see what is happening.

Thank you Doug,

I did a doodle as you suggested and found that my rotor thrust vector moves aft of the CG eight inches when I accelerate from 50 mph to 100 mph.

The horizontal stabilizer has an area of 22 square feet and has a moment arm of 6.4 feet.

The gross weight is projected to be 1,400 pounds.

I was not successful at applying your math so I don’t know what the effect of the horizontal stabilizer has on this. If I assume it is large enough to fly level; is moving the rotor thrust vector eight inches behind the center of gravity a problem?

Thank you, Vance

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#### Doug Riley

##### Platinum Member
Vance, don't take my guesstimate about rotor AOA at 100 mph as gospel. The actual AOA depends on the rotor's lift curve -- something that can be approximated based on past experiments, but only really nailed down by testing.

Suppose that 5 deg. is the number at 100 mph, though. Then 8" aft is the displacement of the rotor thrustline at the CG if the frame is going to fly level. Eight inches is 0.67 ft. 1400 x 0.67 is a tail-up moment of 939 ft.-lb. Your HS must be able to generate a counter-moment, tail-down, of the same amount.

On a 6.4-foot lever arm, the stab will need to make a total down-load force of 147 lb.

A small airfoil H-stab makes about 1.5 lb. of lift per sq. ft. for every degree of AOA at 100 mph. (This number comes from the truck-boom test that I've reported here in the past.) The relationship of lift to AOA is roughly linear.

To create a total down-lift of 147 lb., a 22-sq.-ft. H-stab needs to make 147/22, or 6.7 lb. of lift/sq.ft. 6.7/1.5 = 4.46. Therefore, your H-stab would have to have an AOA of -4-1/2 degrees to maintain the body in a level stance. There's your need for a trimmable H-stab. Maybe.

But...

You may not care to tolerate that much added load on your rotor, since your stab's 147 lb. of down-load acts just like dead weight. It piles on a little induced drag as well.

You could compromise and design to allow the nose to drop SOME. Try a drop of 2 degrees. The rotor thrustline is then offset aft at the CG by 2/5 of your 8", or 3.2". Your tail-up moment is then reduced to 373 ft.-lb. That's 2.6 lb./sq.ft. on your H-stab. The stab can make that number at 1.7 degrees -- so you don't need to change its incidence at all. The 2-deg. nose drop gives you more tail AOA than you need to balance things out, and the actual nose drop will be slightly less than 2 deg.

All of this assumes that the pod is not causing trouble. That is, it's not either (1) ruining the free airflow to the HS or (2) not adding the nose-down moment. You can't explore those issues on paper very well; you need to do some actual testing.

#### RotoPlane

##### Gold Supporter
Vance - .....is moving the rotor thrust vector eight inches behind the center of gravity a problem?
Doug's excellent post #38 explains the RTV and HS relationship well. The Predator's RTV at high speed may also be close to 8" aft of the CG and is the main cause of its nose dropping. A more negative pitch of the HS would help that, whether permanent or adjustable. Is there any wonder why it is possible for a gyro with no HS to be susceptible to tumbling?

#### Vance

##### Gyroplane CFI
Thank you Doug!

Thank you Doug!

That helps a lot.

Is the rotor thrust vector 8 inches behind the CG a bad thing?

Would it be worthwhile to split the difference and have her fly a little nose up at low speeds?

I am more comfortable with nose up than nose down as far as the bathtub catching air and the windshield letting go of the air while converging.

The horizontal stabilizer on the Predator is set at zero angle of attack so this is where my experience comes from.

I am trying to stay out of trouble with the rotor thrust vector.

My horizontal stabilizer will have trim adjustment so I will have the opportunity to learn.

What you are describing is just what I want to learn about.

I suspect that adjusting the pitch of the body may have a significant effect on drag.

Thank you, Vance

#### Jean Claude

##### Junior Member
Vance, to keep your frame in the horizontal position, HS will give a lift: zero to 50 mph, 250 lbs to 100 mph. So, with an Rotor ratio L/D = 5, the drag increase by about 50 lbs: 25 hp engine added.
Jean Claude

#### Doug Riley

##### Platinum Member
Vance, if you had no HS at all, there would be no 8 inches offset.

The body would simply change its "dangle angle" the same amount as the rotor's AOA had changed and you'd fly 5 deg. nose-down. As you start to mix in HS power, the rearward offset of the rotor thrustline begins to appear. To obtain your 8 inches, you must have a very hard-working H-stab. The performance compromise may well be unacceptable to you.

The simplest solution may well be to mount the pod on the frame in a rather nose-up stance relative to the frame's overall datum line. That way, you wouldn't have to worry about the cup effect unless the nose angle beacome drastically low.

As to whether 8 inches would be too much from a stability viewpoint, I don't know. It is quite a lot compared to gyros such as the Dominator, though.

#### Vance

##### Gyroplane CFI
I feel confused on a higher level.

I feel confused on a higher level.

Thank you Jean Claude,

Part of why I would like a pitch trim on the horizontal stabilizer is to find at what attitude it flies the fastest for a particular power setting so I can achieve the best range. I realize that at some point I will be creating more drag with down force from the horizontal stabilizer than I am saving with the angle of the body.

I suspect that as the Predator pitches nose down the drag from the open body increases. The top indicated airspeed in level flight is around 95kts, 109 miles per hour solo and 90kts, 103 miles per hour with Ed in the back seat. I feel that this may be in part to flying slightly more nose down rather than just the extra weight.

We are considering making the mast on Mariah Gale adjustable fore and aft and we feel that one of the benefits is to be able to experiment with this. She will also be designed for our weight specifically so we can choose the angle the body flies.

This thread has been helpful for me in beginning to understand the relationship of rotor thrust vector to center of gravity. We are not stepping out very far from the Predator because she seems to work so well over her entire range of flight and I don’t know enough about gyroplane stability to design a gyroplane.

By working the numbers and trying to understand the posts I have come to understand that the pitch change is much higher at low speed. I already knew this but it is clearer to me now and helps me to understand the compromises.

Thank you Doug,

As always I learn a great deal from you.

I am headed in the direction of optimizing the pod angle for the higher airspeeds because it makes more difference there. This is clearly a much bigger issue on a two place tandem than on a much shorter machine.

What is the offset of the rotor thrust vector to the center of gravity on the two place dominator you had as the speed neared 100 miles per hour?

Thank you, Vance

#### Doug Riley

##### Platinum Member
Vance, you might learn a lot of useful information by "flying" a scale model of your new gyro (with pod, of course). Since you are simply trying to minimize drag, you won't care about the actual numbers or how to scale them up. The angle of min. drag will the same on a model as on the real thing. Put the model on an axle that passes through it from side to side through the presumed CG. Make the axle's rotation lockable, and measure drag at various angles while "flying" the model out a car window, etc.

I don't have accurate numbers to give an exact answer on the Dominator, and its numbers are complicated by the LTL. Speeding up from 50 to 100 mph moved the RTV aft by about 4.5", I think. At 100 mph, the gyro did not lower its nose noticeably. This tells us that the H-stab was not creating much down-load. Instead, the LTL is what was holding the nose up. The aircraft felt very, very solid --almost stiff.

In contrast, my low-rider Air Command, before I added the H-stab, rode very nose-low even at 80 mph, and felt ready to pitch the rest of the way over. The stick was extremely sensitive and required back pressure to maintain pitch attitude and speed. I attribute this reversal of control pressure gradient to the nose-down stance: it slacked off the trim spring.

I'm trying to think of a real down-side to a far-aft RTV, balanced out by high H-stab download. There's the trim drag created by the H-stab, of course.

I fall back on analogies with boats, approximate as they are. Vessels with very low CG's are extremely stable and are said to be "stiff." They also have a reputation for unpleasantly quick rolling motions, because the restoring moments so large. Similarly, when the Dom. hit an up- or down-draft, it weathervaned in the pitch axis rather abruptly, even at 60 mph. I would think that, as you cranked in more and more stab trim, these reactions would become more and more abrupt. I'm not sure that would amount to a comfortable ride. But that's a guess.

#### j4flyer

##### Member
Vance a lot of aircraft fly nose heavy with the pilot and fuel. This allows for the change in CG when a passenger is placed in the rear. Even in my Beechcraft I pick up 20 mph when I place max weight in the rear seat as it shifts the CG aft. Make certain if you make a craft with neutral CG that you don't have an aft CG problem at gross. The closer you can get the CG to the aft envelope, the more speed you will see.

#### Vance

##### Gyroplane CFI
Cornucopia of knoledge and ideas.

Cornucopia of knoledge and ideas.

Thank you again Doug,

You continue to be a cornucopia of knowledge and ideas.

Before I lowered my thrust line the Predator used to fly less nose down at speed. If I would pull the power back the nose would drop to where it is now under power. Now the power doesn’t seem to make much difference.

I am a terrible model maker.

Maybe there is someone on this forum that would be willing to make a scale model of the proposed design I could learn a lot.

We used to test drag in a scale model by hanging it from two threads and measuring how far back it went in a given wind. It was time consuming but we learned a lot.

The nice thing about a gyroplane model is we don’t have to simulate the ground plane.

Thank you Bob,

That is interesting information and it certainly validates the warnings of drag from the horizontal stabilizer trying to pitch the aircraft.

As you know the Predator has the CG slightly behind Ed’s CG so Ed makes her fly a little nose low compared to solo. A heavier passenger makes the Predator fly more nose low. The fuel is also slightly ahead of the CG so full tanks make her fly nose low.

I did extensive hang tests on the Predator before I started training in her to try to establish the CG limits. I do CG calculations to establish an acceptable range of loading for fuel and passenger.

With Mariah Gale I am going to attempt to get Ed and the fuel closer to the CG so the extra weight will have less effect on the attitude of the aircraft. 40 Gallons of fuel weighs almost 240 pounds so this becomes more of a concern. I am going to have to address weight shift of the fuel somehow.

Thank you, Vance

#### Friendly

##### Platinum Member
Vance,
Richard Kennedy has a tall tail that is tube and fabric construction. He has adj. elevators on his HS.

#### Vance

##### Gyroplane CFI
Elevators are part of the tail Package.

Elevators are part of the tail Package.

Hello Mark,

I am planning using an aluminum tail from a fixed wing so it comes with elevators that I will use for trim unless I decide to re-skin the tail without elevators.

I like the horizontal stabilizer on the bottom because on the Predator it kicks the tail up when it gets ground effect and it is time to flair. It is a way I compensate for my lack of depth perception.

I lose rudder authority when I do a power off vertical descent because the horizontal stabilizer blocks the rudder. I find it is a nice way to find which way the wind blows.

I want an airfoil shaped horizontal stabilizer because in my opinion it will be effective over a wider range of angle of attack.

We are not doing a tall tail unless you consider 55 inches tall. I hope to use vanes to achieve torque compensation.

I am at the point where I need to stop dreaming and start building. We hope to make sparks and chips starting in July depending on Smokey’s schedule.

Thank you, Vance

#### StanFoster

##### Active Member
Vance- I am a big fan of yours and hearing you setting a time for chips and sparks to start flying is exciting. I can't wait to see your dream turn into a true story . I expect many sequals after that. You are and you should be commended for what you add to this planet. Stan

#### PW_Plack

##### Active Member
...I want an airfoil shaped horizontal stabilizer because in my opinion it will be effective over a wider range of angle of attack...
Vance, how wide a range of AoA are you considering? Generally, the advantage of an airfoil-shaped H-stab over a flat plate is most pronounced at low angles.

An airfoil H-stab can be expected to stall at about 17º, and lose effectiveness suddenly. Flat plate H-stabs can retain much of their effectiveness even when stalled.

You probably still want an airfoil, because you're on a mission to conquer drag, but wider AoA tolerance probably won't be part of the deal.

#### j bird

##### Gold Supporter
Tall tail on my Dominator single 58".
Duel seat Dominator 68".

#### Vance

##### Gyroplane CFI
Thank you Stan,

Thank you for the kind words.

So far the July start date is just a fantasy.

I wanted to start July of last year but the funds faded and I found the deal on the IO-320 B1A so I made an interim move that was instructive but drained funds from Mariah Gale and ultimately will limit her scope due to lack of funds for the TSIO 550.

Hello Paul,

At the risk of seeming contentious, in my opinion an airfoil shape has a more progressive response to changes in angle of attack. The airfoil shape will probably make no noticeable difference in how she feels. I expect there to be very little drag reduction at the speeds I will be flying. The rudder may be a different story.

Hello Jay,

Thank you for the information. I almost always carry a tape measure and I often measure different aircraft to try to learn things. I knew it wasn’t a tall tail although it is certainly not a short tail.

I like a tall tail; I am not building a tall tail for structural reasons.

I have a longer moment arm on the empennage and much more area for both the vertical stabilizer and the rudder than a Dominator. Mariah Gale is almost 17 feet long and I expect her to weigh 700 pounds dry. I would like her to fly something like the Predator so I am using close to her volumes and basic layout.

I am not a gyroplane designer. I was fortunate with the engine installation and attendant modifications. The wonderful results have made me overconfident and willing to try things.

Thank you, Vance

#### Jean Claude

##### Junior Member
I suspect that as the Predator pitches nose down the drag from the open body increases. The top indicated airspeed in level flight is around 95kts, 109 miles per hour solo and 90kts, 103 miles per hour with Ed in the back seat. I feel that this may be in part to flying slightly more nose down rather than just the extra weight.
Vance, why you suspect this? If we assume 100 hp engine with propeller efficiency = 0.7, then it gives (to 103 mph) 220 lbs thrust propeller. Weight 850 lbs, the rotor drag is 70 lbs. This means frame drag is 220 - 70 = 150 lbs to 103 mph.
Solo now: to 109 mph, the propeller thrust is 210 lbs. Weight 700 lbs, the rotor drag is 380 lbs. This means frame drag is 210 - 38 = 172 lbs to 109 mph. This shows just that frame Cd is approximately constant. (frame drag increase as squ. speed)
Jean Claude

#### Resasi

##### Gold Supporter
Seems to me the optimization of fuselage drag for any given speed can be achieved by the method Vance is proposing with his all flying HS being trimable in flight.

There is another way. In the Vickers VC10 they had a vertical stab fin tank. The flight engineer could pump fuel in and out of this to achieve, for a period of time untill he needed to use the fuel, a means of creating a rearward CG in flight.

This rearward CG required forward stick on the elevators creating lift in the tail section. This decreased the amount of lift required from the wing and therefor a lower angle of incidence, less induced drag and a greater speed for a given power setting, or less power for a given speed.

This method might be a little too complex for Vance's needs. It also leaves you with the possibility of a pump/valve failure and fuel you can't use and where you do not want it, depending on your provisions for redundancy. Our VC 10's being transport category had triple redundancy.