Tandem gyro plans

joe nelson

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Monday, I received plans for a tandem rotor rc gyro. Being a big fan of the H46 and 47 I was excited to see how the designer controlled the rotor. Unfortunately, they were fixed. All control was by engine power, elevators and rudders on the tail. The rotors, themselves, were set at -4 degrees...no lift here!

The designer has the mast angle for both rotors set at +4 deg. and the blades set at -4deg. making flight questionable. If it flew at all it would be from engine power and maybe a small amount of newton's lift from the rotors.:rant:
 

joe nelson

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This morning, I cut the heads out of 1/8 inch ply and drilled an 1/8 inch hole for the rotor shaft. This was sleved with a 5/32 brass tube and glued in place. A flairing tool was then used to lock the sleve in place on the head.

The plans didn't specify the type of airfoil but it looks as if it was an 8H12 or Clark Y...both being flat bottomed and 12 "in length. I went to the r/c webpage to poach a few ideas on rotor blade construction. Building blade is the next task list.
 

Jazzenjohn

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Do the rotors intermesh or are they completely separate? side by side or front and back? do they spin in opposite directions?
 

joe nelson

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John,

In the plans there is overlap of about 30% and the rotors turn in the same direction. I intend to have them turn in the opposite direction and like the Chinook an overlap of 34% which agrees with the formula in Stan Dzik's book. The front rotor, in the plans, is set at 4 degrees but I'll set them at 9 drees like a real gyro and the rear set with two or three more for the airframe interfrence ( that's Joe's best guess). The rotors are separated in height by the height of the pylons.

Yesterday, I found a rotor blade at the hobby store which might save me some time in construction but they are symetrical rather than the flat bottom blades in the plans. They look like that they may be 0012 airfoils. This will save a lot of work in building blades that turn in opposite directions...just turn them updown for the counter-rotating rotor. The front rotor turn ccw and the rear turns cw... I don't known if that makes any difference?
 

joe nelson

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Sorry John,

I didn't answer your question. The blades overlap but do not intermesh. There is no linkage between rotor for separation. The rear rottor is about two inches higher than the front.
 

joe nelson

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Today, I'm using an idea from Chuck B. in using a small door hinge to allow the blades to cone a few degrees. I'm thinking of attaching them at a 20 degree angle as an offset but I'm not sure if I have the best understanding of this process.

Over the weekend I'll begin work on the airframe.. it'll be as simple as possible. Most likely, just a few sticks of blasa and some control surfaces (elevator and rudders).
 

joe nelson

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Well, the first spin tests were failures! The rotors mast angles were set at 4 degrees(per plans) with no success, then 9 and 12 degrees...no luck. The blades are set a zero pitch and they would'nt even rotate. I'll try adding a few degrees of pitch and try again. I thought they would autorotate at zero pitch but I guess I was wrong! In the plans, the blade angle was set at -4 degrees with a 8H12 airfoil which 2 degrees below zero lift. I'm using an 0012 airfoil set at zero pitch which, I thought, would give me the same result...wrong.
 
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HobbyCAD

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Joe,

Go to AeroBalsa, look at the gyro rotorblades here. They have a perfect little reflex trailing edge, you won't find that anywhere else. Dirt cheap for a set of 4 blades.

They are SUPER DOOPER model autogyro blades, simply the BEST. When I used to play with R/C autogyro's, I only used these. They spin up in an instant, very durable as well. They come in different chord size and lengths.

Also go to a very informative R/C gyro site here.

Dont forget this site here.
 

joe nelson

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Thanks Francois,

I've been a regular visitor to the r/c website...lotsa good info there. I will order a couple of sets of blades on Monday morning with luck.

The model that I'm working on is a way to educate myself on varios types of rotorcraft without the large expense of building the real thing. My goal is to build a gyro with 1000 pound payload. Now, I'm working on a tandem (chinook type) gyro configuration.
 

joe nelson

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Today was a big letdown. The aerobalsa blades were all sold out! However, I've been working on a new rotorhead. This is a fully articulated one with pitch control only. Keep your fingers crossed.
 

GrantR

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Joe,

The symmetrical blade will not hand start. You can make blades out of house insulation foam using a hot wire and airfoil template to guide the wire. These will work pretty good but not as good as the AB blades.

You can also hand sand balsa planks into blades.
 

joe nelson

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Thanks Grant,

I had two large fans trying get the 0012 airfoil rotors to turn but with no luck. The diameter is about two feet and I'd even considered mounting them on my truck to get enough relative wind.

Do gyro blades need some amount of reflex to behave normally?
 

joe nelson

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Bingo!!

Finally, it works as it should even though with low rrpm. My guess is that I still have to much pitch in the blades. Presently, I have zero pitch and later I'll reduce this to -2 degrees.

The head, that I've just finish, has two hinges but only one works as of now. The drag hinge is locked down until I can make it work as it should. This will be addressed when the blade pitch issue is resolved.
 

joe nelson

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The pitch was changed to -4 degrees as the plans indicated...who's the dummy? The blades turned up without effort in front of my large fan. Later, I try to add some washout on the last 10% of the blades. I'll try setting the root at -4 and the tips at -2 degrees for my first attempt...wish me luck!
 

joe nelson

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Brett,

The model gyro is a way to educate myself on several things that I'm having a problem with. First will be the problems of a sas-less tandem. Could I mount a rudder on the aft pylon with a horizontal stabs to minimize the need for a sas? Second is the amount of airframe interference with the blades around the airframe itself? Does the rotors have to turn in the same direction as on the model? Both turn ccw!

The rotor blades are one inch in cord and 12 inches long giving it about 72 sq. inches of lifting surface and 3-4 pounds in weight it should fly.
 

brett s

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The biggest issue on a tandem is lots of body surface area ahead of the cg & nowhere to place stabilizers on a long arm, and there's not a lot you can do about that.

A real CH-47 is a handful in yaw in particular with the artificial stability off, the blunt aft pylon & the strakes on the lower fuselage & ramp that were added starting with the B models supposedly helped out quite a bit.

Having the rotors turning in the same direction seems like it would be asking for a blade strike if they overlap at all, it'd be hard to have sufficient clearance to make it impossible. I suppose you avoid the gearing & driveshafting that way though.

That weight sounds high for a model gyro that size - my helicopter should be less than 4 lbs ready to fly, with 2 ~28" 3 bladed rotors (blades are 24x320mm). You sure it'll weigh that much?
 

joe nelson

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Brett,

I'm not sure of the weight. The weight range was on the plans and dependant on the engine used.

The blades are separated by two inches by the height on the front and rear pylons. I also thought the rotors should be counter-rotating but there's no torque to contend with.

Do you think horizonal stabs like on the H-21 and H-25's would help?...That was my first thought. On the plans, to climb or descend is by the application of power and left or right is by rotor tilt on the aft pylon. It also said to land with power... maybe it's a rock when unpowered.

I don't want to get ahead of myself here. I still have to build the front rotor. One thing I've discovered is the 1/4 inch aluminum shaft is too weak and I'm switching to music wire for now.
 

brett s

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If there's only throttle + yaw for controls that would explain why they say to land power-on, you've got no way to flare - landing without power probably only works once :)

That's how they get away with so little clearance between the rotors too.

Seems like you'd learn a lot more from the experiment if you set it up with full controls, but that sure does make it a lot more complicated. Be easier to start off with r/c helicopter parts, steal the whole drivetrain!
 

joe nelson

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Brett,

You're probably right about my approach to the gyro! I have'nt built the pylons yet so it's not to late to put full controls on it.

Today, I have to disassemble the aft rotor to use it as a pattern for the front. I have heavily modified the aft head so I can't use the plans for this. Besides, I have a few ideas how I can make them fully articulated... cross my fingers lol
 
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