Optimizing Efficiency

[Jmann841]

First off, I realize there is about 10,000 variables at play, so this is more or less a theoretical discussion.

But, I am looking to optimize efficiency on an 800 and 900 class heli in terms of determining peak efficiency for headspeed and pitch required for forward flight from 8-12 m/s.

What would be really amazing is if the heli's onboard computers could make the calculations for peak efficiency between HS and pitch angle, then, continuously change the HS and pitch to meet the peak values during these current requirements of the flight profile. (I believe this is how boeing got 24 hours of flight out of the A160 Hummingbird)

But, keeping things simple, what is the general angle of attack range for peak efficiency for a basic symmetrical airfoil to attempt to optimize the headspeed to meet this angle of attack for the desired flight profile?

Or, how does one go about figuring this out? (I have a feeling it's extremely complex)

Also, how much does altitude change these optimal values? Are the values consistent independent of altitude, just, requires more power at high altitudes, or does the optimal pitch angle change with altitude?

[extrapilot]

Look at a parameter derived by dividing the rotor’s thrust coefficient (Ct) by its solidity. Ct is pretty easy to estimate- since it is really just rotor thrust divided by air density, rotor RPM, and rotor area; you should be able to find a calculator online, or at least the equation. Solidity is just the ratio of blade area to rotor area.

When you divide Ct by Solidity, you get a parameter which defines how hard the blades are working. Generically, a rotor is more power efficient in hover the closer this value is to about 0.125. Stall in hover begins near 0.17. The math for that is very complex, but you can do curve fitting if you have some free time to validate it.

When you start to look at this data, you will see why it is that a stock 700-class machine has such a huge improvement in hover/flight time with decreasing headspeed.

There are a bunch of problems which come with this conversation, because at the end of the day, you cant leverage info like this without understanding why every change you make is a compromise- often with multiple systems in play. Example, you want to alter rotor RPM dynamically- but that implies different power levels at different motor RPM. And, with fixed gearing, you may find that you lose more efficiency in the power system than you gain in the rotor at low RPM and high torque. Same for the TR. Etc.

As regards dynamic optimization, its not a huge problem for basic flight. You just map power vs headspeed for level flight at various airspeeds, and store that in a lookup table for future reference. The system would need a few sensors (altimeter/pitot/rotor tach/current/voltage) to do it. You will have huge problems getting this to work well in maneuvering flight or near edge conditions (max payload, max airspeed, RBS, etc).

[Jmann841]

As always, thank you.

I think airspeed will be the most difficult to get on a heli as you would need to find a place that you could sample the free stream airflow. For now I think just optimizing our setup theoretically for a specific condition (Forward flight at 10m/s or something similar to this) will get us within the ball park and we can do a bit of trial and error from there to try and achieve the best flight times.

I'll figure out the thrust coefficient over solidity and see what it comes out to be for this setup.

Thanks again!

[extrapilot]

Sure thing J-

Were you able to get some of this broken out into Excel etc? Where are you at with the problem? It will be interesting to hear what sort of results you are able to achieve.

Cheers

[Jmann841]

I've been super busy working on other projects, but, will get to this soon. I will let you know how it comes out!

Thanks again!