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No Auto Rotation

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Keith Blom28/06/2019 00:13:12
1 forum posts
4 photos

Greetings. I saw some good help with autogyros being given so I thought I'd see if my problem could be resolved.

I built a Gyro Shtick from the plans on outerzone. The build went well but it will not auto rotate. I hold the model into the wind with the rotors tilted well back and nothing happens. I did this with a good 20 - 25 kph wind. The rotors spin freely enough on their oilite bushings when I turn them by hand.

On my first set of tests, I used the 1/64" shim described by the plans to get negative incidence in the rotors. When that did not produce auto rotation, I hacked a much larger shim, perhaps 2 - 3 times the negative incidence.

The one thing I haven't done yet is put a good airfoil on the blades. But I did not think that was necessary for auto rotation to occur. PLEASE correct me if I'm wrong on this as that would be an easy explanation for my problem.

I'm attaching some pictures to get started. Let me know if other shots are needed. Note that these pictures are with the hack for large negative incidence.

2019-06-20_gyro_shtick-modified_tail.jpg

p1090855.jpg

p1090857.jpg

p1090858.jpg

Richard Harris28/06/2019 09:44:48
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2171 forum posts
2023 photos

Keith,

Welcome to the forum.

Generally an autogyro which has blades struggling to rotate is down to the their Aerofoil.

I would shape them as per the plan then go out and try again, any problems call back in.

Rich

John Tee28/06/2019 14:00:55
877 forum posts
73 photos

I know very little about autorgyro's but I would have thought you need positive incidence not negative. The rotor would need to generate lift which I can't see happening with negative incidence.

John

Levanter28/06/2019 14:34:39
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883 forum posts
437 photos

John

It is definitely negative incidence. I have built Richard Harris's Atom Special where it is explained in a number of build blogs. I think you will find that it is the spinning disc that has the positive incidence to create the lift and not the individual blade's relationship with the hub.

Opposite to helicopters where the rotor is driven rather than impelled. 3D helicopters can vary the pitch either way.

I am sure Richard will be back to explain much better than I.

Levanter

Simon Chaddock28/06/2019 16:17:32
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5729 forum posts
3034 photos

kieth

You have got the same incidence (negative)and in the same sense on all the blades? wink 2

Does it rotate with the blades held like a windmill? But only do this in a gentle wind. If it still does not rotate then something is seriously wrong. smile o.

John Tee28/06/2019 17:12:55
877 forum posts
73 photos

I stand corrected.laugh

John

Russell Aisbitt13/01/2020 08:33:58
1 forum posts

It’s a complex subject how airfoils generate lift. But you need an airfoil shape to generate lift efficiently and as the lift vector is perpendicular to the relative wind a little negative incidence helps to tilt enough of that lift vector forward enough so that It pulls the rotors around.

Peter Christy13/01/2020 09:39:12
1829 forum posts

Even if the pitch is set negative with respect to the hub, it will still be positive with respect to the *airflow*, which is the important bit!

Remember, the disk is tilted back so that the air flows up through it.

My experience with autogyros is limited, but I have a lot of helicopter time! More negative pitch will accelerate the blades to speed quicker, but the rotational speed will be slower. As you increase the pitch, the blades will spin faster, but take longer to get there - bit like using a higher or lower gear on a car. Eventually, if you increase the pitch too much, the blades will slow down - like over-gearing a car.

At our sizes, autorotational forces are small, and need all the encouragement they can get! Airfoil section is fairly important, and a flat plate is unlikely to work well!

--

Pete

 

Edited By Peter Christy on 13/01/2020 09:39:38

Martin Harris13/01/2020 10:18:30
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9399 forum posts
253 photos

It seems sensible to follow the designer's recommendation on how to shape the blades but I have had 2 different twin rotor fixed incidence gyros that flew with flat balsa blades - a DB one back in the 70s and Cyril Carr's Kestrel which I built from an RCM&E plan. I did find it interesting that the Kestrel's rotors would actually stop in fast flight with the tiny aerofoil sectioned rotor support/winglet obviously providing sufficient lift to support the model.

Autogyro

Experience with both mine and a clubmate's version was that the blades were prone to failure at the roots - on several occasions I landed with less blades than I took off with...

Former Member13/01/2020 23:26:23
3577 forum posts

[This posting has been removed]

Piers Bowlan14/01/2020 07:54:36
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2166 forum posts
53 photos

Having only built one (unsuccessful) autogyro 'centuries ago' I am not qualified to comment - but that won't stop me devil.

I had a look at the Gyro Shtick plan on Outerzone and your blades Keith seem to copy what is on the plan. However, I notice that all of Richard' s rotor blades have an aerofoil which is quite different. Namely, the leading edge of Richard's blades is quite sharp and only the upper service is rounded and the lower surface is completely flat. It is, in effect, a very high lift aerofoil. The Shtick blades have a completely rounded leading edge, which clearly work, judging by the photos in the magazine article, but they may not be optimal. The other observation I made is, is the GRP sheet used for the rotor head a bit thin and flexible, allowing the blades to twist so reducing the negative angle of attack? Looking at various build blogs of Richards auto gyros it looks like slightly thicker sheet. I appreciate that the sheet needs to allow the blades to flap up and down as they rotate - but not twist.

Just a couple of random thoughts! Good luck.

Edited By Piers Bowlan on 14/01/2020 07:59:35

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