Ground effect aircraft efficiencies

[autogyro]

Interesting question.
Without spin up the full size aircraft needs about 25 yards in still air.
For a model it should be much better because of the light weight.
With a very light rotor it would pick up rotational speed very fast.
A few inches probably.
Dont forget the airscrew helps force air through the rotor as well as forward motion.

[machin]

@Arunus; its not perpetual motion; its akin to the fact that a yacht can travel at speeds faster than the prevailing wind by taking advantage of the properties of an aerofoil section...

[autogyro]

@Arunus; its not perpetual motion; its akin to the fact that a yacht can travel at speeds faster than the prevailing wind by taking advantage of the properties of an aerofoil section...

Thats right, it is very efficient.
Also safe. If the engine thrust stops it simply glides down using gravity instead at a sink rate comparable to a sailplane. Far far safer than a powered rotor.

[autogyro]

The air near ground level is of the highest density and is also subject to surface friction that in effect increases its density.
The result is more lift than at altitude but with added drag.
Ground effects aircraft can fly on less thrust and at lower speed because of this.
I do not believe this to be as efficient as autogyro flight however.
At a higher altitude the autogyro rotor will create less drag with similar if not better (lower) power.
Conventional fixed wings create higher drag than either, even when at high aspect ratio and high lift to weight ratios.

[wrcsti]

so what exactly is the challenge? farthest who makes it or first to 10 laps? I personally would go with a parkzone vapor like design.
http://www.youtube.com/watch?v=Y86wLJpJ ... re=related

i had one, flies on a 3.7v lipo with a geared 6mm motor. so light that the AC in your house will knock it off the sky.you can build it from balsa sticks and plastic bags for covering. undercambered wing ofcourse.

[riff_raff]

Minister,

If your course is 10 laps around a pole, then you'll probably want some sort of lateral asymmetrical wing geometry, since the aircraft will be constantly yawing, with the outboard wing making more lift than the inboard wing. Designing your propulsion system to create the correct yaw angle without using rudder trim would also help.

A large diameter, slow turning, 2 blade, tractor propeller would likely give the best propulsive efficiency. And most importantly, at small scale accuracy of your airfoil shapes is paramount. So craft them with great care.

Good luck.
riff_raff

[Arunas]

@Arunus; its not perpetual motion; its akin to the fact that a yacht can travel at speeds faster than the prevailing wind by taking advantage of the properties of an aerofoil section...

Thats right, it is very efficient.
Also safe. If the engine thrust stops it simply glides down using gravity instead at a sink rate comparable to a sailplane. Far far safer than a powered rotor.

Excuse me for being off-topic, just want to be sure, have I got it correctly: autogyro aerofoil has better efficiency (higher lift/drag ratio) due to higher relative speed in air (comparing to static aerofoil)?

[machin]

Isn't the L/D ratio of a low incidence aerofoil better? A higher speed means a lower incidence angle can be used for a given lift and that means less drag?

[autogyro]

No, speed through the air is not the reason.
It is because the teeter head allows the blade angle of attack to vary around the disc to give the most efficient lift to drag ratio at any position around it, the result is an even distribution of lift over the complete disc, even at airspeeds way below fixed wing stall. It is impossible to stall such an unpowered rotor.
Maximum airspeed is limited by blade length and available power.
I would like to experiment with telescoping rotor blades at some point.
Maximum ceiling is also limited by blade length although this limit has yet to be reached.
The fully proven W116 has an airspeed of 135+ mph and a ceiling of 20,000ft.
The limit to airspeed is the 90 hp engine and the limit to ceiling is the lack of turbocharging. There was no lack of rotor lift at this altitude unlike a helicopter. Shortening or lengthening the rotor will easily exceed these figures as will a turbocharged engine.
A model of course suffers from the scaling effects and it will be difficult to beat a well designed fixed wing for the project. I believe it can be done.

[autogyro]

Isn't the L/D ratio of a low incidence aerofoil better? A higher speed means a lower incidence angle can be used for a given lift and that means less drag?

For experimental use and models a suitable aerofoil section and angle can be designed into an unpowered rotor with ease (unlike a powered rotor).

Of course the angle will vary around the disc, depending on airspeed, which cannot be achieved with fixed wing other than in a crude sense with pitch angle.

It should be possible to design an aerofoil section to give a constant high efficiency through a range of rotor airspeed and airflow angles.

The problem is the lack of suitable computer simulations containing the research data done at BAE Bedford by Ken Wallis using the Concorde laboratory, using a real autogyro in flight.
The CAA used Glasgow University who had nobody available who had ever flown or worked on autogyros. They constructed computer models that are frankly wrong and the CAA based their limited regulations on this incorrect data. A joke.

[DaveW]

I shall be having lunch with Ken this week and will bring up the idea then.

Auto: What did W/C Wallis think about the idea of a man-powered autogyro?

Thanks.

[autogyro]

I shall be having lunch with Ken this week and will bring up the idea then.

Auto: What did W/C Wallis think about the idea of a man-powered autogyro?

Thanks.

Sorry Dave, I have had a nasty chest infection for over a week now, just clearing up I hope. So I have not as yet had a chance to see Ken.
He is definitely interested in an electric Wallis.

[machin]

I'd completely forgotten about this when we were discussing auto-gyros a while back:-

Whilst tidying out my flat I found an Auto-Gyro "kite" which I bought a few years ago... it features a very light wire frame silhouette "body", and a rotary aerofoil wing mounted to a "teeter-totter" bearing. The technique to get it airbourne is to spin the rotar blade by hand, then suspend the "kite" from about half a metre of string with your back to the wind and slowly walk backward....

....At least that's what the instructions say. In reality I found moving backward very briskly is needed to lift the "kite", the wind needed to be strong and constant to sustain flight for even a small amount of time....

I'm not saying I'm doubting the benefits of auto-gyros (I've seen "Little Nelly" on James Bond after all!), but I guess the point I'm making is that it is far simpler (and therefore more likely to succeed) to make a model-scale fixed wing aircraft than a rotary winged one... so the original poster might be best off persuing his ground effect design....

[autogyro]

The full sized W116 Wallis autogyro with 350 rpm pre spun rotor speed will take off with an airspeed of 5 to 7 mph at average AUW.
A brisk trot, with engine off.
Tethered flight in wind speeds above this is easy without engine on and the aircraft can be flown safely in 70 mph winds.
It is difficult to scale the aircraft to model size it is true.
However, there are far to many variables to compare either an autogyro for this project or a manpowered ag to your ag kite.
The wartime U boat tethered man carrying kite, used successfuly by the Kriegsmarine, used an articulating head but was far less efficient than the Wallis with half a century of development. This kite flew in a wind speed of around 6 mph. I do not think your model kite is the most efficient ag by a long way.

[machin]

As I said; I wasn't doubting the full size autogyro... Just pointing out that building a small scale version which out performs a fixed wing aircraft could be very difficult...