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Helicopter Max Efficiency

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dknguyen

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So for background, let's say that the higher the thrust a motor/propeller combination provides the less efficient it is.

I was sitting here with some charts that gave the thrust and thrust/W figures for a certain propeller-motor combination. I wanted a 2:1 thrust:weight ratio so it could actually maneuever, so I was about to take the voltage with the best thrust/W (ie. choose batteries that would provide said voltage at full throttle) and make everything weight 1/2 that max efficiency thrust.

But then I started thinking...hmmm it'd be nice to get more weight on the thing and maintain run-time. BUt that means higher voltage batteries which means more thrust but reduced efficiency, and also less run-time for the same batteries. So I say down and calculated it out and it seems that by increasing the batteries voltage (for more thrust) and capacity (for same run time as before due to increased current at higher voltages) I can get 8x as much thrust as the battery weight that will be added on.

But the efficiency is now less. But that's a moot point right? Because the max efficiency will now occur at around hover and only be reduced during big maneuvers (whereas before it occured during the most extreme of maneuvers since I took the max efficiency thrust and made the unit weight 1/2 the peak efficiency thrust).
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So basically, should a helicopter electric drive be sized so it is most efficienct at hover, or during hard maneuevers? If I wanted it to be most efficient during motion then I'd have gotten a plane right? (which is more efficient at its most demanding maneuever than an equivelant helicopter at its least demanding).

I'm just looking for people's opinions here.
 
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Hi dknguyen,

you can't generally say the efficiency decreases with increased thrust.

Let's look at two examples: The McDonnel F4 (Phantom) and the F-16 Fighter.

The F4 had a thrust to weight ratio of 1:2 while the F-16 has one of 2:1.

Expressed in performance this means the F4 can climb vertically to 10,000 feet and then will experience a stall sooner or later. The F-16 can climb vertically to service ceiling (50,000 feet) without getting close to a stall.

The fuel consumption of the F4 is four times higher than that of the F-16. (Of course the F4 consists of 24 tons of aluminum and steel with two engines - the F-16 weighs about 10 tons equipped with one engine)

Comparing these values you can't say the F-16 is less efficient.

It is just a matter of the right drive selection if the efficiency is higher or lower.

Aerodynamics are of minor importance for helicopters, except for straight forward flight (when the body resistance comes into play).

Hovering a helicopter certainly uses more energy than moving horizontally as well since the body will produce some lift, too.

Hovering near the ground the ground effect (similar to a hover craft) will decrease energy consumption.

Regards

Hans
 
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THe drive system is fixed in this case though, so at higher thrust outputs it is less efficient.
 
Very true!

If you can't change the propulsion the efficiency will decrease with higher thrust.
 
So for background, let's say that the higher the thrust a motor/propeller combination provides the less efficient it is.

I was sitting here with some charts that gave the thrust and thrust/W figures for a certain propeller-motor combination. I wanted a 2:1 thrust:weight ratio so it could actually maneuever, so I was about to take the voltage with the best thrust/W (ie. choose batteries that would provide said voltage at full throttle) and make everything weight 1/2 that max efficiency thrust.

But then I started thinking...hmmm it'd be nice to get more weight on the thing and maintain run-time. BUt that means higher voltage batteries which means more thrust but reduced efficiency, and also less run-time for the same batteries. So I say down and calculated it out and it seems that by increasing the batteries voltage (for more thrust) and capacity (for same run time as before due to increased current at higher voltages) I can get 8x as much thrust as the battery weight that will be added on.

But the efficiency is now less. But that's a moot point right? Because the max efficiency will now occur at around hover and only be reduced during big maneuvers (whereas before it occured during the most extreme of maneuvers since I took the max efficiency thrust and made the unit weight 1/2 the peak efficiency thrust).
=====
So basically, should a helicopter electric drive be sized so it is most efficienct at hover, or during hard maneuevers? If I wanted it to be most efficient during motion then I'd have gotten a plane right? (which is more efficient at its most demanding maneuever than an equivelant helicopter at its least demanding).

I'm just looking for people's opinions here.

Hope these links solve your problem:
Inside Story - February 2003 - RC Groups
Inside Story - February 2003 (Derivations) - RC Groups

Good luck
 
Well I did not follow the links above but can say with out a doubt that many helicopters throw away 30% in the tail rotor which is there only to prevent nasty old Newton from spinning it round.
 
Those links didn't have anything I didn't already know (but I had to take a couple courses and a bunch of self-study). Right now the thing I'm not really able to predict is the propeller.

THere seems to be arguments over the merits of a tailrotor vs multiple main rotors. I've heard figures from 10%-30% power consumption to the tail rotor vs multiple main rotors allow for all the power to go into lift. But I've also heard arguments that multiple main rotors (coaxial and tandem) have aerodynamic interactions (or rather interference) between each other which reduces their lift efficiency and that there is more helicopter body in the path of the downwash for multiple main rotors (tandem rotor like a Chinook) which can reduce the efficiency to less than that of a single rotor + tail rotor helicopter.

And then it also pointed out the increased reliability, easier/simpler design, reduced weight, reduced cost, and simpler control systems where all advantages that went to single rotor helicopters. It was also said that single main rotor helicopters have less drag and can therefore fly faster than multiple rotors but they also suffer more from retreating blade stall than multiple rotor craft and the reduced weight allows for better climb rates (whether or not the advantage also comes from other sources like better efficiency because of less rotor intefeference is not said).

**broken link removed**

And this probably also applies for full scale helicopters, but tail rotors definately give better yaw response than helicopters without tail rotors at the cost of crosswind sensitivity. I'm not sure how important this is for regular use like transport (which RC helis don't do) but it makes a difference in aerobatics.
 
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Check out the BO105, a German anti-tank helicopter. It was the first helicopter in the world performing a full looping.
 
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