MPR Helicopter Blades

[PedroTheRocketNerd]

I am in the process of designing a 2.56" internal-rotor helicopter. I am planning to get some helicopter experience first with an Apogee Gyro Chaser, so by the time I build this I will not be a total helicopter noob. However, I have a few questions about the blades.

First, will 1/4" balsa be strong enough for the rotors, or should I use something like basswood or even thin plywood? I am basing this off of BABAR's Whopper Flopper Chopper, https://www.rocketryforum.com/threads/build-thread-for-whopper-flopper-chopper.28887/, which was 380 grams, flew on Estes E9s, and used 1/4" balsa for the rotors. My design, with the 1/4" balsa rotors, is ~500 grams and will fly on composite E through G motors, more specifically 29/40-120 reloads. For reference, the rotors will be ~22 inches long.

Second, should I go with four 2"-wide blades or three 2 5/8" blades? I calculated those sizes based on the interior circumference: 2.56*3.1416/3 = 2.681, and 2.56*3.1416/4 = 2.011. I am going to be using Gyro Chaser-style blades, with a curved leading edge and straight trailing edge. I will bend them using the Apogee method, softening them with ammonia and curving them on a mandrel, so their circumference will equal the internal circumference of the tube. Which blade count would be better?

Third, does anyone know of any resources on the aerodynamics of helicopter rocket blades? I would like to understand how they work, so I can better design my own.

 

[Zeus-cat]

What you are proposing to build is very complicated. Two people in my club have built G powered helicopters and it took them several iterations. They are both Level 3 high powered certified and have built many, many rockets. I'm pretty sure they used plywood as balsa will shred if the deployment of the fins doesn't occur almost exactly at apogee. The rotors were completely enclosed in the body tube as speeds with MPR motors will cause a lot of problems for exposed rotors on ascent. Be prepared to build and/or rebuild many rockets as this is a very tough challenge and damage is very common on flights. A lot of things have to go right for a successful flight.

 

[PedroTheRocketNerd]

Thank you for the advice and cautions. I will be sure to take all this into account when designing my rotors.