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Old Jan 02, 2013, 07:07 PM
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Northumberland, England
Joined Jan 2007
644 Posts
Raf_Bob, there's nothing driving the rotors that's fixed to the airframe, therefore there's no force for the rotors to react against.

Gordon, bearing drag would act in the same direction as rotation, therefore not a reaction.

I think the need for any corrective trim can only be due to rotating wash from the rotor.
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Old Jan 03, 2013, 03:11 AM
So I'M meant to be in control?
Colonel Blink's Avatar
Ilkley, West Yorkshire, UK
Joined Nov 2008
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Excellent that an autogyro is being entered for the Build Off!! I have looked at the Ro-Dart several times, but I've never had the bottle to even decide to attempt it - and still haven't.

Re the torque reaction - what GeeW said re bearing drag makes a lot of sense. Now I may be exhibiting an astounding lack of knowledge here, but isn't the greater problem the asymmetric lift - ie the forward travelling half of the rotor disc is travelling through the air faster and at a greater AoA than the rearward moving half? Am I right in thinking that the swash plate mechanism on jellychoppers counteracts this by increasing the pitch on the rearward moving half of the rotor disc? But with contra rotors, that issue should be negated.

Another thing (admittedly based upon a single full size experience of flying a Rotax powered 3 axis microlight alongside a similarly Rotax powered gyrocopter) is that you may find that you will need more power to fly an autogyro than a similar size and weight aeroplane. I remember that at similar cruise speeds, climb rates etc the guy in the gyro was caning his engine whilst we were happy below the redline at all times (and using a lot less fuel)...!
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Old Jan 03, 2013, 04:13 AM
Light and floaty does it
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Cambridge, Great Britain (UK)
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Originally Posted by Colonel Blink View Post
isn't the greater problem the asymmetric lift - ie the forward travelling half of the rotor disc is travelling through the air faster and at a greater AoA than the rearward moving half?
Morning, Colonel. You're absolutely correct - this applies to all rotating wing aircraft, whether the rotor is powered or unpowered. As soon as the whole craft starts to move bodily through the air, i.e. when it has net airspeed rather than simply being in a hover, vertical climb or vertical descent, then it has an asymmetric effect because the forward-moving blade has higher airspeed, and generates more lift, than the rearward-moving blade.

Whereas the reactive torque of a helicopter, or the bearing drag of an autogyro, act in the yaw axis, the advancing / retreating blade effect acts in roll. Without some compensating mechanism the single-rotor autogyro will roll towards the retreating more and more strongly as the forward airspeed increases.

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Originally Posted by Colonel Blink View Post
Am I right in thinking that the swash plate mechanism on jellychoppers counteracts this by increasing the pitch on the rearward moving half of the rotor disc?
Yes, up to a point. This factor is usually what determines the never-exceed speed for a rotary wing craft - as the forward speed increases you eventually run out of ability to keep it flying straight.

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Originally Posted by Colonel Blink View Post
But with contra rotors, that issue should be negated.
Again, yes, up to a point. Obviously you solve the problem of asymmetry, and thereby you solve the roll control problem. But the retreating blade still remains a speed limiting problem. As the aircraft increases forward speed, more and more of the retreating blade starts to experience backwards airflow and it all becomes rather messy, aerodynamically.

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Originally Posted by Colonel Blink View Post
you may find that you will need more power to fly an autogyro than a similar size and weight aeroplane. I remember that at similar cruise speeds, climb rates etc the guy in the gyro was caning his engine whilst we were happy below the redline at all times (and using a lot less fuel)...!
Very true. Autogyros are not particularly efficient - even the best, which are the Wallis designs. They do, however, excel in terms of the useful load that they can lift (fuel, plot, payload) in comparison to the empty weight of the airframe.
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Old Jan 03, 2013, 04:57 AM
Sic itur ad Astra
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United Arab Emirates, Abu Dhabi, Abu Dhabi
Joined Aug 2009
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This thread is such an education, you guys are great.
sparks
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Old Jan 03, 2013, 06:46 AM
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United Kingdom, England, Kent
Joined Aug 2007
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This is going to be a fascinating build. Good luck Bob.
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Old Jan 03, 2013, 12:25 PM
SB-28 UK Display Pilot
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ENGLAND
Joined Jul 2001
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Power wise, for a model A/G, work on about twice that you would expect for a fixed wing.
Orientation is one of the biggest issues whilst flying R/C A/Gs so make sure it has a high contrast colour scheme. It also helps enormously to have the top of the rotor blades a different colour from the bottom.

The C of G shown on the plan does not look ideal to my eyes having had a few A/Gs. DO NOT let me put you off, A/Gs are great fun and can be flown (once you've had a few flights) in quite small spaces.
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Old Jan 03, 2013, 12:53 PM
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United Kingdom, England, Lancs
Joined Sep 2006
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I seem to have stimulated a bit of response by taking on this model. As I stated in an earlier post, my knowledge of these machines is almost nil so your various post are very welcome and enlightening. I suppose this model is not a true a/g as the head is not articulated! That would be a step too far at this stage and, of course, the whole idea is to build something that fits the "Britplan" formula. Is it torque or gyroscopic precession + bearing drag or all three or any other combination (sounds like echos of Twenty Questions) that affects these flying machines?? Whatever it is I think the challenge is intriguing. I've machined the spindles for the head and will show the photos when I can get time (being called for dinner). I'm aware of the need for extra power so I expect only short flights but time will tell.
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Old Jan 03, 2013, 02:04 PM
Light and floaty does it
Work in Progress's Avatar
Cambridge, Great Britain (UK)
Joined Sep 2004
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Oh, it's an autogyro, all right. It's just a fairly simple one.
Personally I like the two-rotor solution as used on the DB Autogyro: there's a good example built by a German modeller here:
http://www.helioldie.de/tragschrauber2.htm

These fly very similarly to a rudder/elevator/throttle trainer, but with the low-speed benefits of the rotary wing added on.
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Old Jan 03, 2013, 07:15 PM
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Upper Arlington, Ohio
Joined Dec 2007
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Quote:
Originally Posted by GeeW View Post
Power wise, for a model A/G, work on about twice that you would expect for a fixed wing.
Orientation is one of the biggest issues whilst flying R/C A/Gs so make sure it has a high contrast colour scheme. It also helps enormously to have the top of the rotor blades a different colour from the bottom.

The C of G shown on the plan does not look ideal to my eyes having had a few A/Gs. DO NOT let me put you off, A/Gs are great fun and can be flown (once you've had a few flights) in quite small spaces.

Is that a PT Gyro?
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Old Jan 04, 2013, 03:37 AM
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United Kingdom, England, Lancs
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Originally Posted by GeeW View Post

The C of G shown on the plan does not look ideal to my eyes having had a few A/Gs. DO NOT let me put you off, A/Gs are great fun and can be flown (once you've had a few flights) in quite small spaces.
Thanks GeeW for your posts. Could you let me know what you consider the correct balance point. Flying is a long way off, of course, but I'll probably start with the suggested position and see how it goes. Nevertheless, it would be interesting to experiment, so any observations from experienced A/G fliers would help.
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Old Jan 04, 2013, 09:43 AM
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United Kingdom, England, Lancs
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Contra-Gyro under way with the rotor head bearings. Mast is 14swg wire as original but the bearings are turned from delrin rather than fabricated from tinplate and brass tube. Next job is to bend the blade attatchment arms, again from 14swg wire. I always struggle with wire bending! getting clean, sharp bends eludes me somehow. I know one can get bending jigs but I'm relying on a home made one and a medium sized hammer
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Old Jan 04, 2013, 10:13 AM
So I'M meant to be in control?
Colonel Blink's Avatar
Ilkley, West Yorkshire, UK
Joined Nov 2008
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Originally Posted by Raf_Bob View Post
...the bearings are turned from delrin...
POM tiddly POM POM......!!

Nice work!!
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Old Jan 09, 2013, 04:02 AM
NSWFFS
Richmond NSW Oz
Joined Apr 2005
706 Posts
I'm a bit late on this thread but thought I'd jump in as one of our older (88) flyers builds anything oddball, this being one of them.

I would offer two pieces of advice.

1) When building your rotor blades, build at least two sets. Sooner or later you WILL break one or two.

2) To launch, give both rotors a touch to set them on their way, run like blazes into the wind and give the model a healthy heave, preferably with the model at the correct attitude at release. When running, angling the model upward will result in (desirable) increased rotor speed at launch.

Landings were quiet reasonable affairs when preceded by flight, those immediately after launch rarely resulted in a quick flick and a second attempt. This is illustrated in one of the lower rotor blades in the larger, lower quality pic but he's still smiling!

Given Brian's age, getting the required launch speed was a problem for him with this model but the flights were always predictable and as long as you cared to put fuel into the tank for.



Cheers,
Rob
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Old Jan 09, 2013, 04:19 AM
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United Kingdom, England, Lancs
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2) ..................... run like blazes into the wind ..........................

Rob
Wow, Rob!! I can't run the length of my shoes, never mind blazing into the wind!!!

I'm having second thoughts about it now. Perhaps I'll finish it as a "hangar queen" and hope flying is not required for the build-off. I live in a windy area so it may be possible to let the wind run to me. We'll see.
Thanks for you post and the pics. I've done a bit more work on the rotor head and will get some photos up soon.
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Old Jan 09, 2013, 12:54 PM
SB-28 UK Display Pilot
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ENGLAND
Joined Jul 2001
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Are you converting the head to Direct Control or is it staying fixed and using rudder and elevator?
In answer to a previous question 1/2A PT Gyro.


The trick in hand launching A/Gs is not to run....you do not need to. The rotor heads are slow to accelerate into full autorotation and it takes several seconds to get there. Most days if there is say 5 mph of wind I will just hold model at 45 deg nose up and wait. You will clearly hear when the heads pass from wind-milling and into autorotation and really start to accelerate. Once head speed stabilises start to walk forwards listen for heads further accelerating ...walk a bit faster for a couple of seconds then rotate model to nose only 10 degrees nose up and push the model forwards.
For ROG you spin up head (s) using fingers to make sure they are going the right way round (don't ask) and then taxi slowly forwards...give it a few seconds then a fast taxi then give it a couple of seconds the add a bit more power. Normally it will come airborne just as you add the final bit of power. The PT comes off the ground using about 1/2 power and is travelling at a medium walking pace. I then add more power to climb away.
http://www.photoavia.org.uk/AutoGyro%20Video.wmv

Gordon
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