RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
Excellent reference; a lot of of what's described in this paper confirms what I'm currently seeing with the Contra Drive that I'm now flying.
One thing to note however, is that the 10% increase in propeller efficiency that I cited is what I calculate I'm getting by increasing the prop diameter from 21" to 22", and dividing the available power between two props. Currently I am not making any attempt to estimate the potential increase in efficiency from the recovery of energy from straightening the spiral airflow.
I had hoped to see this show up as a reduction in power draw from my battery pack, but I'm not seeing this. For all intents and purposes my power draw appears to be unchanged from when I was flying my single 21X14 prop on my old Integral. However, this doesn't mean that that the spiral airflow isn't being straightened. It just means that the energy recovered is either too small to show up, or it's being used up in some other way. For instance, it might be lost because the prop pitches are mismatched. If this is the case I'll see it as either an increase in performance, or a reduction in battery power draw when I try a 22X20 prop on the rear.
As far as the spiral airflow is concerned, there is some indication that it is being straightened. This is because of the very significant increase in rudder effectiveness that I'm seeing. In order to reduce this effect and make the plane more flyable, I've had to reduce the rudder travel to less than 30 degrees, and increase my rudder expo to 75%, and I might have to do more.
As far as noise is concerned, I believe that the concerns that are expressed in the literature are comparing two counter rotating props to two contra rotating props. In this case the power loading for the props in each scenario doesn't change very much.
In my case I am going from a single prop to a pair of props that each have half the power loading of the original single prop, and what I'm experiencing is really a change in sound quality, rather than in increase in sound power. Regardless, I still have to be able to pass the sound check at the Nats, so I can't have a system that is dramatically louder than what we currently fly.