Number of blades in a hover situation

[jedufa]

Following the thread thread6-3970.

Hi, I would like to know if in a hover situation, the number of blades will increase the total lift. From the last thread, i have understood that increasing the number of blades is done only to remove stress from each blades by distribuing the entire carried weight. My situation is a two-rotors (2 blades/rotor) system which is just meant to hover, removing the need for blade flapping. It is thus reduced to the "simpliest" case. But i would like to get more lift to carry heavier things, and adding blades would be a nice solution.

Another solution would be to increase the rotation speed, but I would have to stay under the speed of sound for the blade tips, right? For this last solution, is the result only a question of blade solidity? Is there any lift-related effect of going supersonic?

Any help would be appreciated.

Jean-Francois

 

[Johnnykool]

Was reading about an inventor who copied hump back whale fins design for extra stall lift. This does not answer your question but I thought it couldn't hurt.

http://www.pratt.duke.edu/news/releases/index.php?story=156

 

[GregLocock]

Drag rises rapidly as the blade goes transonic, and you need a different wing section for supersonic lift. I imagine that a traditional wing will still provide lift supersonic, but probably not very well.

If you add more blades the power requirement will increase faster than the increase in lifting capacity, I'd expect a 4 bladed rotor would lift about 50% more than a two bladed rotor, for twice the power consumption.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MikeHalloran]

Is this still a conceptual invention?

Experienced helo drivers have told me that twin- rotor configurations are basically unflyable without active flight control systems, because things go too wrong too fast for a human to perceive and correct.

Obviously the Flettner is an exception. I guess twin coaxial rotors would be, too, assuming you can solve the other problems. Still need torque control.






Mike Halloran
Pembroke Pines, FL, USA

 

[jedufa]

Hi guys, thank you for your answers. I had to redesign the quad rotor toy Draganflyer that you probably all know, to be able to fly on Mars which has a lower atmospherical density (0.015kg/m^3 compared to 1.224kg/m^3) and lower gravity field (3.71m/s compared to 9.81m/s) and, a speed of sound equal to 244m/s. All this together is the equivalent of having 31 times less lift than on Earth.

So first, to answer to MikeHalloran, yes this is an aircraft with closed loop control inside.

Second, I had to look at solutions to increase the lift in reaction to the lower air density. As you know, increasing the length of the blades will increase the area and the speed considered for the lift and increasing the number of blades should also increase the total area. I have read in many places that a higher number of blades will increase the lift but will lower the total efficiency of the rotor, which is the reason why we don't equip cesnas with 20 blades, if i may copy the example of my source.
So Greglocock is quite right by estimating the lift increase of 50% for a doubled power consumption.

 

[jedufa]

I have got the "Helicopter Theory" book from Johnson for only 30 dollars. How can such a book be sooo cheap? By reading it, I have found out some of the answers I was looking for.

First, Johnson states clearly that there are 3 different methods to calculate the thrust/power and other parameters:
1) Momentum theory
The momentum theory considers an infinite number of blades and gives the less accurate results of the three methods. But it is the simpliest one and gives a first approximation which can also be used for a first design. This method do not take in account the influence of the number of blades.
2) Blade Element Theory
This one considers the blade as an 2D element and the flow around it. It therefore takes in account the total blade area compared to the total disk area. Number of blades is thus considered.
3) Vortex theory
I have not read much about this one, as the blade element theory gave me most of my answers. Nevertheless, it seems to precisely calculate the vortices at the tips and root cuts, which translates into more drag.

What I am trying to say with this is that from my research on the web, I have not found such a clear distinction between the theories used. Or, they are stating the Momentum theory without remarking that there are other theories. My conclusion: Books are still ahead from the web ;-)

 

[MikeHalloran]

A book author has to convince a publisher to print his book. It costs money, so publishers tend to screen prospective authors for those likely to produce some net income for the publisher, or those who are capable of fronting the money for themselves.

Any idiot, including me, can get published on the Web.

Ahead, indeed.



Mike Halloran
Pembroke Pines, FL, USA