Angles and dangles

[Spaced]

Hey guys, Ive got a few questions reguarding the relative angles on a gyro. Sorry if the questions are a little basic, Ive got my haed around helos, and am trying to do the same for gyros.
Generally speaking are the hang angle, mast tilt, and prop down thrust all related?
So say the gyro has a hang angle of 8deg nose down, if the fuse was at 0deg, would the mast be raked back 8deg?
And if this is the case what angle is the prop at relative to the fuse?
Thanx

[Doug Riley]

In a static hang test, the gyro behaves like any other object that has mass: the CG dangles directly below the hang point (the teeter bolt). The "hang angles" of the various parts (be they keel, mast, rotor head cheek plates or other bits) are chosen by the designer so that these items are in their correct positions during flight. There's no particular "correct" angle for the mast or keel. Each item is arranged around the line formed by the hang point and the CG.

The key info the designer needs when setting hang angles is the cruising angle of attack of the rotor. This angle varies with rotor size and design. A line that's perpendicular to the rotor disk when the rotor is at this cruise angle of attack will be the rotor's thrust line. In a properly designed gyro, this line will fall just behind the CG at cruise. Hence, in cruise, the line that connected the CG and the hang point during the hang test will have slightly more rearward inclination than the rotor thrust line.

During cruise, the rotor head should be in the midpoint of its travel. Therefore, we want the neutral angle of the rotor head to be tipped just a bit forward of level during hang: this guarantees that, when the rotor is at its cruise angle of attack, the head will be in the middle of its travel.

The angle of the mast is best set to match the angle of the rotor thrust line. This puts the mast in pure tension, with no bending, during cruise. (Designers do fudge this rule for reasons of convenience.)

The angle of the engine and keel are best set to be parallel to the flight path in cruise. The prop thrust line should, of course, be through the CG.

The horizontal stab should be set so that it has the necessary angle of attack to create the down-force (if any) required for stability, when the gyro is in its cruise stance.

[GyroRon]

Hey Spaced! How are you going to design and build or fly a gyro or helicopter when you still can't figure out how to sign your name to your posts???? This is a rule on this forum, it is simple to do, Please do it.

[Spaced]

Sorry Ron, I forgot to fix it, is it OK now?

Thanx Doug, that cleared up alot.

[GyroRon]

Yes it is. Rules are rules, and signing your real name either on your posts or in your profile is Rule Number 1. Thank you.

[CLS447]

Doug , thanks again for an excellent answer! I always wondered where these angles came from.

[Bill Freiburger]

Doug,

When designing for a Horizonal Stabilizer you have said in the past that the information needed was,

1. Thrustline
2. Total Thrust

Is there a formula to calculate the download required? And what about the 12 to 14% rotor volume / surface area of HS numbers that are sometimes are quoted? Where do that fit in?

Bill

[Doug Riley]

Bill: The "Cierva Rule," relating HS volume to rotor volume, is so rough an estimate that it's not much use in designing gyros in general. Cierva himself alluded to the possibility of changing the HS size with different control mechanisms. There are half a dozen other factors that go into sizing and angling the HS. Some of these can make the difference between a craft that's stable and one that's crashy (a cabin that produces a powerful nose-down "wedge" effect at high speeds can be your worst nightmare, for example).

Certainly, as a starting point, if the thrustline is at all above the CG, the HS must be set to counter that effect at all throttle settings and airspeeds. The PPO moment created by a high thrustline is measured in foot-lbs. The moment at wide-open throttle will equal the maximum engine thrust times the height of the thrustline above the CG, in feet. E.g., 300 lb. of thrust and 3" above the CG yields 75 ft.-lb. of PPO moment (300 x 1/4 ft.).

The HS must generate a counter-moment to this PPO moment. The HS uses a lever arm equal to the distance from the CG to a point 1/4 of the way back from the HS's leading edge. It comes out to 4-4.5 ft. on most Bensen-style gyros if the HS is mounted as far aft as possible. To counter the PPO moment mentioned above with a 4-foot HS lever arm, we'd need the HS to create a down-load of 75/4, or 18.75 lb.

To know how big the HS must be and what incidence to give it is a bit more complicated. Standard airfoils have known "lift curves" -- graphs that relate various angles of attack to pounds of lift and airspeed. Once you know the airspeed the HS "sees," you can pick a HS size and incidence. The HS is likely to have to be immersed in the prop blast to work at low airspeeds.

I don't know of a standard formula for HS size or down-load in the case of perfect centerline thrust. Something on the low end of the Cierva formula is probably enough for an open-frame gyro. In the case of centerline thrust, I'd give it a token negative incidence, no more than -1 degree. The aerodynamic effects of a cabin can throw off this number, however, requiring a much more powerful HS.

The process is to use these methods to get your HS in the right neighborhood. Specific flight tests for pitch stability are needed to see if your estimates are correct. Greg Gremminger has outlined a good, objective test series in the PRA mag.

[Bill Freiburger]

Doug,

Thank you for your explanation, its great to have you as a resource.

Bill ;D