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Tandem rotor gyroplane

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joe nelson

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Over the last 6 months or so I've have been testing a scratch built model of a two rotor gyroplane. It has a struggle, at times, to guess what the problems are with this type of aircraft since there's not alot information out there. Last week, the model flipped over while testing in front of a large shop fan. It was broken but not too severly. I think the problem is one of center of pressure moves beyond the center of gravity. This allows the model to lift off at the front but remains ground bound in the rear. My theory is that the gimbaled heads allow the CP to have a large range of movement...to well in front of the CG. My solution is to change the rotors to swash plate control to keep the CP rather stationary.

I thought that I might share my project here on the forum with everyone and to see if there's any suggestions, ideas or chuckles.
 
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Grelly

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Surrey, UK
A two rotor gyro . . . I'm intrigued. Have you got a photo? I assume your avatar is something else.
 

joe nelson

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Cincinnati,Ohio
Grelly,

My avitar is just a cute little helicopter that I found on the web.

I do have some pictures but please understand that it's very crude construction. The model was just a proof of concept model to see if this was possible.P1200001.jpgP1200002.jpgPB300001.jpg
 

joe nelson

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Cincinnati,Ohio
Wow! The preview didn't look like this!

The first picture is the model just after the it flipped over. It is rather small so the detail is hard to see but the control links and the rotor blades are broken on the aft rotor mast. The next picture is a fully articulated rotor head that I make early on in this project. It will need to be reduced in size by about half to fit the scale of the model. This type of head will replace the gimballed heads in place now. The last picture is the model in front of a large industrial fan just moments away from the flip. Both rotors are up to speed (approx. 600 rrpm) with a small bit of vibration in the aft rotor.
 
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joe nelson

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This is "yarn tell tales" in front of the aft rotor studying the air flow effect between the the two rotors. Later, I found a 1954 NACA study on the airflow between rotors so I incorporated the dimensions on the model.

The study conclusion was there needs to have .3 to .5 times the radius of the front rotor in height between the rotors and .3 to .5 in overlap to avoid interferences...this is why the model looks like a patch work of fixes.
 

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joe nelson

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I guess that a better explanation of my project is in order. I want to build a gyro that is the Cessna 172 of the gyro world. It would be constructed of commercially available parts ie. blades, tubing, engines and standard flight instruments. Next is the construction would be simple enough for the first time homebuilder. It would have a fuel fraction to allow long x-country flight with an average load or larger loads for a short distance.

I have tested homemade blades with various aspect ratios and lengths to see what the best combination would be. Now I'm looking at configurations both conventional and tandem. Elastomeric hinges are of interest to me to keep the design simple. Finally, I want to build a full size test bed when I finsh my research.
 

jhausch

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Dec 10, 2008
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WI
Just out of curiousity - what affect does a fan in front of the model have vs a fan behind the model pulling air through a tunnel? Could some of the flow behavior be a result of the turbulent air off the fan?
 

joe nelson

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jhausch,

A fan in the rear in a wind tunnel is far superior than a front mounted fan for just the reasons you had indicated. The air is turbulent and would make a lot of difference if I were measuring performance data. I'm only interested in two things at this point, the ability to bring the rotors up to speed and if my control configuration works...it surfices for that. I had used a group of 4" PVC pipe to straighten the air flow but when I was testing with the yarn tell tails I saw that it wasn't working as intended so it was discarded...I didn't consider reducing the pipe size to maybe 1 inch. However, there was an episode of Myth Busters where they used soda straws for their wind tunnel...that is another idea. I will improve my setup when I get to testing BERP and other blade tips but that's down the road. You got to remember that my blades have such a small reynold's number that accurate data collection would be difficult without a method of cooling or pressurizing the tunnel.

The fan tubulence may be affecting the model but I have had yarn tell tales all around it with no indication of that. There is what I thought was wake turbulence behind the model but it may have been generated by the fan... I'll modify the wind straighter with smaller tubes and retest...thanks!
 
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joe nelson

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This morning, I put the wind straightener back in front of my fan. I put it as close to the fan as I could unlike the 6 inches previously. I used the same 4" pipe and attached it to the fan guard. I then place my tell tails in the path of the air flow at different distances to varify smooth flow. Eddies were found near my work bench on the left of the fan approx four feet out with the same off my machines on the right. There is an area directly in front of the fan at approx. two feet out where there is good smooth air.

Picture #1 is my new fan configuration and #2 is the indication that the air is smooth in front of the fan. #3 shows the eddies by the machines in my workshop.
P1230001.jpgP1230003.jpgP1230002.jpg
 

rtfm

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Brisbane, Australia
Hi Joe,
Fascinating stuff this... I love gyros, and although I can't fly one, I would love to learn. Two rotors? Sounds extremely interesting, and I hope your experimentation turns out well.

I particularly like your comments regarding the poor man's wind tunnel you've created. I think a zillion straws would be very efficient. I plan justy such a wind tunnel to test my Fowler/Zap flap arrangement, and to work out the optimum geometry for this.

Good luck, and keep the posts (and photos) coming,
Duncan
 

joe nelson

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Duncan,

Thank you for the kind words. The difference in my setup is before the straightener (4" pipe) wasn't attached to the fan but stood alone about 6 inches ahead of it. It appears that it needs to be as close as possible to the fan. At first, I thought that maybe a venturi effect might increase air flow by having a space between the fan and the straightener but there is no indication of this happening.

I will have to locate a source for soda straws before incorporating it into the straightener. Hopefully, they'll be 8" long so I won't need to buy so many...lol! I wonder how much restriction the straws will cause to the airflow? Well, we'll soon see.
 

rtfm

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Hi,
I was actually thinking of building a wind tunnel out of perspex so I could see through it, and then fit a great big leafblower to it. Enclose the leafblower so that no air escapes.

But I can see, this is WAY over-complicated. A good fan, a half dozen boxes of straws, and I'm in business. My main reason for building this will be to check what angle my fowler flaps need to assume as they extend rearwards. Might be fun to inject some coloured smoke into the stream.

Regards,
Duncan
 

Dan Thomas

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Sep 17, 2008
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Your flow-straighteners don't do much for the differences in airspeed off the fan at the various radii of the fan blades. Most of the airspeed will be from nearer the tips, much slower air from the hub area, and the conflict between the two speeds of the mass creates turbulence after the air leaves the straighteners. A long tunnel after the fan allows the air to mix, and after that it can be straightened.

Dan
 

joe nelson

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Dan & Duncan,

I had an opportunity to visit the wind tunnel at the Cleveland, Ohio's NASA facilities many years ago and it was exactly as you said. The straightener was down stream from the fans. I didn't understand the reasoning for that configuration but it makes perfect sense now. Unfortunately, space limitations in my shop prevents me from having a proper wind tunnel. I have seen several "school project size" tunnels on the web. One, if my memory serves me, was a down stream type fan with viewing windows. I think, Duncan's idea of having smoke is a good addition to the project but I haven't any idea of how to do it!

Dan, do you think the tunnel, as it is, would be OK for a dudementry testing? It has a "sweet spot" approx. two feet in front of the fan where the air is smooth.
 

joe nelson

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Today, I noticed, that while in front of the fan, the aft rotor rrpm fell off when I put my hand between the two rotors to adjust the pitch of the aft rotor. The first thing that came to mind was "airframe interference" when the airframe was at a high AOA. This condition would be catastrophic on a quick stop landing and a fix would be indicated. Two gear boxes and shafts would be too heavy of a fix for a model gyro...heli yes but gyro no.
 

joe nelson

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Today, I'm working through the coupling of the two rotors. The first attempt will be using music wire glued to small plastic gears held in place by UHMW plastic bearings. When I finish this mod I'll post a picture or two.
 

joe nelson

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My earily statement about changing to a swashplate configuration may not be nessecary! Saturday, I changed the rotor controls to give me the opposite control movement on the front and aft rotor longitudinal tilt...front tilts up and the rear tilts down ect. In this setup the CP stays relatively stationary...I think. Sunday, the CG was moved to coinside with the CP. The model was then placed in front of the fan and tested.

The results were very interesting. The model didn't rise up in the front as before but stayed level in attitude. It became very very light on it's wheels like a hovercraft when I moved it with a stick. I think that I've found the "differential collective" control input that I was looking for by changing to the differential tilt input.

Hopefully, the rotor coupling may not be required now!
 
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joe nelson

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My search for a larger fan hasn't been fruitful unless I buy a separate electric motor and fan. I feel that I need more volume to get a lift off with the model in it's present configuration. One thought was to mount a platform on the front of my truck. This would give me an acturate speed, by way of the speedometer, to make some calculations for future improvements.

It has been raining for a couple of days here so when it dries up enough to get under my truck I'll see if the platform idea is doable.
 

D Hillberg

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Now you sound like me, In 1998 I built a Gyro kite of a CH 47,foam body & plastic (Flexable) blades(3 bladed),= 3' span on the rotors, to test it I hooked a line to the hood and went down the street at 24-25 mph it lifted off and "hovered" above , I figured it needed a huricane to be a kite and gearboxs to sync the blades as they hit each other every few seconds,AoA was about 30 deg and used a nose and tail line to the main line ,(for adjustment of AoA)The flat belly helped in stability.
 

joe nelson

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Don,

I wish that I were like you...you know more than most people about helicopters. I recently made a few changes to my model that keeps the rotors apart and the heads now work opposite to each other for pitch control (differential collective). With these changes I hope that coupled rotors won't be needed. If I find that I'm getting airframe interference in flight, rotor coupling might be nessecary.

It's been raining here for two days so crawling under my truck to mount my model platform isn't high on my to do list...lol. Maybe by the weekend I can start testing!
 
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