Wanted, Rotax C-Box with 2.0:1 gear

AirCommandPilot

Just a fledgeling
Joined
Feb 24, 2014
Messages
617
Location
Houston
Aircraft
Air Command Elite #003
Total Flight Time
88
Does anyone know where I can get a rotax c-box with a 2.0:1 gear ratio?
 
No such thing Bobby.
You will have to use a B-Box for 2:1 ratio.
That will also only be good with a short prop.
Smallest C-Box is 2.62: 1.
What are you trying to accomplish ?
 
I've just put an EA82 on my Air Command Elite with an Airtrikes PSRU with 2.58:1 gear. My 68" three blade IVO is not turning fast enough to give me the required 400lbs of thrust for my 800lbs of flying wieght.
Using the prop calculators and other known thrust test, I've come to the conclusion that I need to speed the prop up. The EA82 should have a max static rpm around 5400. That makes my prop turn around 2100 rpm. With this, I'm only getting about 325 lbs of thrust.To get the best thrust numbers, it looks like the prop needs to turn around 2800 or so.

Does it look like a "B" with a 2:1 ratio will work?

IVO wants to sell me another matching 3 blade to stack with mine making it 6 blades. ~$700

Airtrikes wants to sell me a 4 bladed Kool Prop. ~$1300

I would like to not just "make it work", "but make it work right".
Will adding blades add thrust, or should I change the gear ratio of my box?
 
How much twist is in your Props blades now?

Adding blades is a solution, only if the problem is the current prop is maxed out pitch wise.

If you have say, 80hp and to absorb that Hp with the prop you have to add pitch to the blades to the point of holding RPM to where you want it. At a very high amount of pitch, the prop can actually be stalled and not able to make good thrust, especially at zero to 60 mph airspeed. If your having this issue, the solution is to do one of three things.... 1. Turn the prop faster ( by changing the redrive ratio ) 2. Add diameter to the prop ( not always practical on a gyro due to keel and rotorblade clearance ) 3. Add more blades.

I will say that Ivo makes different props for different power and applications. They have skinny chord, low HP, ultralight type props.... then they also have thicker chord magnum props made for higher HP engines.

My advise would be to find someone to loan you a 3 blade 68 inch warp drive and try that. I had a 69 to 70 inch 3 blade warp drive on my last gyro and it was powered by a 120 hp yamaha sled engine, and I tried several gearbox ratios with it. I can't remember if I ended up with 2.62 to one or 3.47 to 1. In either case, the warp drive was able to absorb the power without needing too much twist and therefore it was not stalled statically.

I am not a fan of Ivo props at all. I would sell it and buy a warp drive. and try your current set up.
 
GyroRon;n1121206 said:
How much twist is in your Props blades now?

Adding blades is a solution, only if the problem is the current prop is maxed out pitch wise.

If you have say, 80hp and to absorb that Hp with the prop you have to add pitch to the blades to the point of holding RPM to where you want it. At a very high amount of pitch, the prop can actually be stalled and not able to make good thrust, especially at zero to 60 mph airspeed. If your having this issue, the solution is to do one of three things.... 1. Turn the prop faster ( by changing the redrive ratio ) 2. Add diameter to the prop ( not always practical on a gyro due to keel and rotorblade clearance ) 3. Add more blades.

I will say that Ivo makes different props for different power and applications. They have skinny chord, low HP, ultralight type props.... then they also have thicker chord magnum props made for higher HP engines.

My advise would be to find someone to loan you a 3 blade 68 inch warp drive and try that. I had a 69 to 70 inch 3 blade warp drive on my last gyro and it was powered by a 120 hp yamaha sled engine, and I tried several gearbox ratios with it. I can't remember if I ended up with 2.62 to one or 3.47 to 1. In either case, the warp drive was able to absorb the power without needing too much twist and therefore it was not stalled statically.

I am not a fan of Ivo props at all. I would sell it and buy a warp drive. and try your current set up.

My IVO can get just enough pitch to bring the RPMs down to max at full throttle. The prop just isn't turning fast enough to get me the thrust I need. I've been informed that my 95 estimated HP is too much for a "B" box. I've got a club mate here that has a belt drive with the ratio I need from his old Air Command Tandem with a EJ22. I might try to adapt it to my EA82. I'll make check it out this Saturday.
 
You do NOT need to spin the prop faster. You need a better prop.
 
I tested it with a 70" warp drive and still only got 360lbs. What prop do you suggest?
 
I would suggest double checking the scale for accuracy, then possibly looking into the engine itself to insure its producing the proper amount of power.

In my experience, you won't gain much thrust by turning the prop faster, not when you have the prop at a pitch setting that is within the range suggested.

A rotax 582 ( 65 HP ) will usually produce about the same amount of thrust as what your subaru is producing. They tach out at 6500 rpm, with the various gearbox ratios your looking at, 4 to 1 = 1625 prop rpm.... 3.47 to 1 = 1873 prop rpm.... 3 to 1 = 2167 prop rpm.... 2.62 to 1 = 2481 rpm.... 3 to 1 is the most popular and common ratio used in the C box. Your turning about the same prop rpm with your subaru and airtrikes gearbox as the 582 and 3 to 1 box would turn. My guess is your engine needs some loving.... possibly a better carb, exhaust, intake, check timing, etc...
 
Warp drive prop is one of the best btw. You gained 35 pounds of thrust just switching props.... Keep the warp drive and get someone who knows subaru engines to help you get some more power out of it. You can change gearbox ratios, but I highly doubt that will result in any noticable amount of extra thrust. It will just make it louder!
 
I ended up buying a new 68" 4 bladed Warp Drive prop. I installed it Saturday and did some thrust testing. I got 450 lbs of thrust at 5200 RPM! I took it out around the pattern and boy does it climb!
I was skeptical about having another blade on the prop solving my problems, but damn if I was wrong. I'm more than happy with the outcome.
 
That is remarkable Bobby; thank you for sharing the adventure.
 
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