Jul 18, 2013, 10:54 PM
Registered User
Malaysia, Selangor, Kajang
Joined Jun 2009
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Quote:
 Originally Posted by Montag DP Autogyros using the spinning rotor to create lift, not drag. Yes, drag is created (as with anything producing lift), but the lift is much greater.
Lift is just drag in the vertical axis. For an airplane, the "lift" created by a freewheeling prop points backwards (in the same manner that lift in an autogyro points upwards). We therefore call this force vector drag in this specific discussion.
Jul 19, 2013, 12:14 AM
Sink stinks
United States, GA, Atlanta
Joined Apr 2005
4,463 Posts
Quote:
 Originally Posted by slebetman Lift is just drag in the vertical axis. For an airplane, the "lift" created by a freewheeling prop points backwards (in the same manner that lift in an autogyro points upwards). We therefore call this force vector drag in this specific discussion.
I'm afraid I can't agree with you there. Drag is the force parallel to the freestream, and lift is perpendicular to it. The mechanism by which an autogyro produces lift is completely different from the mechanism by which a freewheeling prop produces drag. Therefore, it is not appropriate to reference autogyros as evidence that a freewheeling prop produces more drag than a stationary one.

For the record, "vertical" doesn't have anything to do with lift or drag. For example, when you do a loop with your model plane, the direction of lift and drag relative to the earth is constantly changing.
Last edited by Montag DP; Jul 19, 2013 at 12:24 AM.
Jul 19, 2013, 01:37 AM
I bail out, anywhere, anytime
The Netherlands, OV, Almelo
Joined Nov 2010
2,512 Posts
Gents, added to my post about the Taurus, two more photographs, the control diagram and the inside of the Taurus.
Most important part is the controller in which the algoritm is stored.
This algoritm is continue under development because it's the most important, but also interesting part of this project.
Interesting of the brake funtion is, thinking about flying inverted with an airplane,
we can fly inverted with high speed (high RPM brake action) low AOA but we also can fly with low speed (low RPM brake action) and high AOA. In both situations the same weight of the plane is carried by the lift of the wings (braking force of the rotating propeller can be identic).
The algoritm (with the right parameters) takes care of this brake action to have optimum dynamic behaviour without stalling the propeller. The result even can be the engine is throttled up during the brake action but also fuel is consumed in the neutral situation between pulling and braking because the rotating propeller still generates drag even when the plane is in a descending flight path at exact the setpoint of the airspeed controller. Simple story with complicated solutions!

TF

# Images

Jul 19, 2013, 07:12 AM
Registered User
Joined Jun 2008
180 Posts
Quote:
 Originally Posted by Montag DP I'm afraid I can't agree with you there. Drag is the force parallel to the freestream, and lift is perpendicular to it. The mechanism by which an autogyro produces lift is completely different from the mechanism by which a freewheeling prop produces drag. Therefore, it is not appropriate to reference autogyros as evidence that a freewheeling prop produces more drag than a stationary one. For the record, "vertical" doesn't have anything to do with lift or drag. For example, when you do a loop with your model plane, the direction of lift and drag relative to the earth is constantly changing.

Isn't that exactly what Slebetman said? The force created by the prop is parallel to the free stream so it's called drag.

The auto rotational force created in a auto gyro rotor is identical to that in a free wheeling propellor.

Dave H
Jul 19, 2013, 09:42 AM
Sink stinks
United States, GA, Atlanta
Joined Apr 2005
4,463 Posts
Quote:
 Originally Posted by gerryndennis Isn't that exactly what Slebetman said? The force created by the prop is parallel to the free stream so it's called drag. The auto rotational force created in a auto gyro rotor is identical to that in a free wheeling propellor. Dave H
My point is that the force that slows down the airplane when the propeller is freewheeling is mostly from drag, but the force that an autogyro uses to stay aloft is mostly from lift.

A freewheeling prop is operating at a high negative angle of attack, and the relative freestream seen by a section of the propeller has a large component in the axial direction. Drag acts parallel to the freestream, so it contributes a large portion of the total force in the axial direction. Of course, the lift also makes some contribution to the force in the axial direction, but I would bet that for a freewheeling prop the drag dominates (especially if most of the prop is stalled, which I think would be likely but I'm not positive about that).

In an autogyro, the situation is different. You have the blades operating at a relatively low angle of attack compared to a freewheeling prop (certainly below stall) and the freestream air seen by the blades is close to perpendicular to the axial direction of the rotor. So, in this case lift produces most of the force that acts in the axial direction.
Jul 21, 2013, 01:13 PM
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Malaysia, Selangor, Kajang
Joined Jun 2009
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Quote:
 Originally Posted by Montag DP My point is that the force that slows down the airplane when the propeller is freewheeling is mostly from drag, but the force that an autogyro uses to stay aloft is mostly from lift.
You're missing something.

In a glide/autorotation, the autogyro only travels downwards with practically zero forward motion. In fact you can zero out forward motion completely by dropping a model autogyro from a given height. The direction of motion is downwards. The drag produced by the autorotating blades points in the exact opposite direction of the direction of motion.

And it is drag caused by the spinning blades. If you put some tape on the blades to stop it from spinning and drop the model autogyro it will fall at very close to free fall acceleration since there's very little drag produced by the model as a whole. But if you let the blades rotate the model will fall significantly slower. Almost as if the model has a parachute.

Now, if you still insist on relabeling the name of the forces we're talking about then fine. Let's discuss the similarity of an autogyro and an aircraft in a vertical dive with a freewheeling prop vs a stopped prop. When in a vertical dive the aerodynamic force produced by the wings generate forward motion so let's call that thrust and the forces generated by the spinning prop lies in a vector that points upwards so let's call it lift.

You will find the discussion is still the same: some aircraft, with the right combination of prop pitch etc. Will fall slower with the prop freewheeling as opposed to having the prop stop.
Jul 21, 2013, 02:00 PM
Sink stinks
United States, GA, Atlanta
Joined Apr 2005
4,463 Posts
Quote:
 Originally Posted by slebetman You're missing something. In a glide/autorotation, the autogyro only travels downwards with practically zero forward motion. In fact you can zero out forward motion completely by dropping a model autogyro from a given height. The direction of motion is downwards. The drag produced by the autorotating blades points in the exact opposite direction of the direction of motion.
Quote:
 Originally Posted by bjr_93tz Hey, in reality doesn't an autogyro in normal flight hang from a big freewheeling prop falling through the air???
I don't think I was missing anything. The discussion was about an autogyro in normal flight, not in free-falling flight. If you were talking about free-falling flight, you should have said so and we could have avoided this whole confusion.

Quote:
 Originally Posted by slebetman Now, if you still insist on relabeling the name of the forces we're talking about then fine. Let's discuss the similarity of an autogyro and an aircraft in a vertical dive with a freewheeling prop vs a stopped prop. When in a vertical dive the aerodynamic force produced by the wings generate forward motion so let's call that thrust and the forces generated by the spinning prop lies in a vector that points upwards so let's call it lift.
Sorry, but I wasn't relabeling the forces; just the opposite. I was trying to correct an inappropriate labeling of the forces. What I said was true: the drag force produced by a freewheeling propeller is not similar to the force produced by an autogyro in normal forward flight, which is what we were discussing.

Quote:
 Originally Posted by slebetman You will find the discussion is still the same: some aircraft, with the right combination of prop pitch etc. Will fall slower with the prop freewheeling as opposed to having the prop stop.
Of course, and that was what I was trying to bring to light. With some combination of diameter and pitch, a freewheeling prop will produce more drag than a fixed one. There is no doubt that a large rotor (like on an autogyro) may produce more drag during autorotation, provided the right pitch is used. That fact does not really shed any light on the drag of a freewheeling propeller, however, because the diameter and pitch are going to be completely different.
 Jul 21, 2013, 06:56 PM Registered User Toowoomba, QLD, AUSTRALIA Joined Jan 2008 597 Posts An autogyro rotor systems is alway falling through the air, ie air is coming up from underneath the system which keeps the blades spinning. The reason they need to spin is so that they meet the air coming up from underneath at some smallish angle so that they can generate lift. In very simple words, a stopped prop will produce simple flat plate drag. A slowly turning prop will develop "lift" but in the opposite direction if the airspeed through the prop and the rate of blade rotation conditions are met. This lift has the potential to be much larger than what can be achieved by flat plate drag, because we all know a stalled wing (stopped prop) generated less lift than an unstalled wing (slowly turning prop) Now if the prop just happens to be turning a bit too fast for the air traveling through it, it will produce zero drag and zero negative lift such as can occur with a completely freewheeling prop, but our electric motors (brake on or not) aren't completely free wheeling. Nobody who competes in F3A (where we strive to achieve a constant speed up and down) would stop the prop on a downline because when that happens they start building up speed. The right amount of braking (from the motor) to allow the prop to turn at the right speed to generate the braking effect needed is something guys spend hours on.....