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#1 alfoot Oct 08, 2006 01:27 PM

Monotwirl Electric Autogyro

An early video (23Mb) of the Monotwirl during one of the many development flights, this one in May of this year. I put a clip of it here previously, but this is a whole flight. I was trying out more rigid rotor blades (not such a good idea...), and it had a rather forward CG and too little aft rotor tilt angle. It has gone through many iterations since then, but I don't have any movies - yet.

Spot the designer BS at the end!!

Monotwirl Electric Autogyro - Early Rotor (4 min 28 sec)



#2 alfoot Nov 04, 2006 12:20 PM

Well just for a change we have had some decent weather in the UK, so here is the latest movie (10Mb) of Monotwirl with the latest tweeks which are included in the forthcoming mag article.

Click here to watch Monotwirl-Autogyro-29-Oct-06



#3 alfoot Nov 05, 2006 02:49 AM

2 Attachment(s)
Here are a couple of stills from the latest movie. It is interesting to note the shape of the rotor disc, you can see that the blades flap up at the front and left of the disc (CCW rotor direction).


#4 JochenK Nov 05, 2006 03:43 AM


very good. Nice, stable flight. Congratulations!


#5 minifly Nov 06, 2006 01:03 PM

C'mon, PLANS ! :D

#6 boberos Dec 30, 2006 01:37 PM


Congratulations on your Monotwirl.
In the second video, the machine appears to be much more stable & controlable.
What changes did you make to the rotor head to achieve this?


#7 alfoot Dec 30, 2006 04:22 PM

Hi Bob,

In the first movie, the rotor mast angle was at 5 degrees aft. The C of G was about 1 inch forward of the rotor axis and the 3mm thick depron rotor blades were covered in paper top and bottom, along with a double spar made from lollipop sticks - very stiff indeed. You can probably see that the rotor speed was quite low, round about the 200 rpm mark, and it needed almost all of the rotor tilt control to the right when coming in to land.

In the second movie, the rotor mast angle was changed to 10 degrees aft, the autogyro now balanced level when held by the rotor pivot and the rotor blades were now simple 3mm depron with a 3mm x 3mm spruce spar. Additionally, the blade pitch was slightly more negative, achieved by adding 1/16 balsa plates underneath the inboard trailing edge of the blades. The rotor has built-in flexibility/flapping and, as a result, needs little or no lateral correction throughout the speed range and is very easy to build.

In both cases the rotor rotates CCW as seen from above.

There were a number of iterations in between the two movies of course, but I have not made any further changes since the second movie.

Hope this answers your questions,


#8 JochenK Dec 30, 2006 06:17 PM


this behaviour while landing is cuirious. You were using a CCW rotor and were pulling when landing. In my theory this would make the front ends of the blades move upwards, making it happen 90 later. This should have tilted the rotor plane to the right, and you should have tilted the rotor to the left in compensation.

You're sure about the right tlit?


#9 alfoot Dec 31, 2006 02:36 AM


Yes, in the first movie I was having to use about half of the available lateral control to the right throughout the flight, and even more ( ie full right) when landing. Even then it still landed "left wing low" which contributed to the left undercarriage coming off.

On some parts of the first movie you can see that the rotor is level but the fuselage is leaning left ( ie right rotor tilt applied).

I'm not certain of the reasons, but I'm sure that the low rotor rpm had a lot to do with it - I had a number of "roll left at liftoff" incidents when I attempted to do take-offs at that stage of development. Once I changed the blade angles to a more negative value, the rotor rpm almost doubled and the left roll disappeared.


#10 leadfeather Dec 31, 2006 06:23 AM


Of all of the Twirl variants the MonoTwirl is my favorite. All of your creations look really good, but the extra measure of simplicity of the MonoTwirl makes it stand out nicely for me. :) It also lives up to the slogan in your avatar quite well!

I would love to know why the left roll problem occured. I wonder if at the lower rotor rpm, there was significant ASL; and being a semi rigid rotor, maybe there was large enough component of the resulting moment in the lateral direction to cause this roll???


#11 alfoot Dec 31, 2006 06:56 AM


I think that you are probably right. The breakthrough was getting the rotor flexibility right. I must also say that I was lucky too, because I had, until one of Mickey's posts on coning and the resultant effects, completely misunderstood the concept when related to autogyros.

Fortunately the modifications that I did resulted in success but I might have got there quicker had I understood the theory a bit better. It was (and still is) a good learning process though :) .

I hope your New Year weather forecast is better than mine.


#12 JochenK Dec 31, 2006 07:09 AM


I think the right trim you need while flying is simply a compensation of the motor torque. What really bothers me and what I have no explanation for is that you need more right trim when landing. Because when you're landing, you're cutting down the motor speed, which should mean less right trim. Your rotor speed will go down, too, and you'll have more coning. Which will cause a roll to the advancing blade, and with a ccw rotor, this means less right trim again. I'm thoroughly stuck.


#13 alfoot Dec 31, 2006 07:38 AM


I agree with you. Unfortunately I was not rigourous in documenting the effect of each modification individually, and sometimes I made more than one modification at a time.

The differences between movie 1 and movie 2 were:-

C of G moved aft so that the autogyro balanced level when held by rotor spindle.

Rotor mast angle increased from 5 degrees aft to 10 degrees aft.

Rotor blades made more flexible and 3mm x 3mm leading edge spar added. This effectively gave a limited flapping hinge.

Pitch of rotor blades more negative.

With these modifications, I have not noticed any motor torque effects even though I have, on occasion, applied coarse amounts of throttle. The motor, by the way, has 5 degrees of downthrust but no sidethrust. There is now no requirement for lateral trim throughout the speed range.

I feel lucky that we are discussing why the Monotwirl flies well rather than why it flies badly :)


#14 boberos Dec 31, 2006 12:35 PM

Hi Al,

You explained your blade & rotor set up just fine, & I am glad it works for you.
But I am amazed that your blades flex just the right amout to simulate a hinged or flapping head. I am not sure I could trust myself to exactly duplicate your blade set up but will try.


#15 alfoot Jan 01, 2007 10:48 AM

1 Attachment(s)

I wouldn't worry about the construction of the rotor - it is made in exactly the same way as the Twirl rotor. I used UHU Por glue throughout which is flexible, and allows easy removal/replacement of individual blades should the need arise.

Although it might appear to be weak, it has stood the test of time and the abuse that I have given it during the development.

I would avoid the use of "hard" glues such as epoxy as this would affect the flexibility, and make it more difficult to replace blades.

The picture shows how much the blades flex in flight - the rotor is built with the blades flat.


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