Analysis of the Influence of Particular Autogyro Parts on the Aerodynamic Forces

Jean Claude

Junior Member
They write : "The impact of the operation of the main rotor and propeller blades on the airflow around the gyrocopter construction was ignored"
It seems to me amazing that the effects of the rotor on the angles of attack are totally neglected. Not to mention the engine cooling drag.
In this case, I wonder what is the interest of this study which neglects the main effects ? Sans titre.png
 
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kolibri282

Active Member
It is common practice in helicopter design, Jean-Claude, to investigate the fuselage characteristics separately and then combine the force and moment coefficients thus obtained in your complete numerical model. You calculate the rotor down flow to obtain the resultant wind direction with respect to the fuselage. Then you can use the fuselage data to calculate the fuselage contribution for a given flight condition. That is actually what I currently do in my program. I am using the coefficients from Padfield to simulate the Westland Lynx.

This practice is also explained on the Polimi site, here:
http://www.windtunnel.polimi.it/roto...-aerodynamics/

The wind tunnel tests of helicopter are usually split in tests of the fuselage (including rotor hub) and tests of the rotor.
There is a picture of the AW 139 fuselage in the wind tunnel that shows that only the hub is present in this case while there are also wind tunnel models which do not even have a hub attached.

You might also want to have a look at the paper below, describing similar tests in Australia:
http://www.icas.org/ICAS_ARCHIVE/ICAS2012/PAPERS/070.PDF
 
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Jean Claude

Junior Member
Juergen,
I guess this common practice is acceptable when the shapes are very bumpy, and with small horizontal surfaces of the helicopters.
In my opinion the flow arched by the rotor makes this computation much less reliable when the horizontal surface is large and the forms very streamlines, because the drag and moments are much more sensitive to the angle of attack. Sans titre.png




Also, I do not understand that lift, drag and moment coefficients are linked to the radius of a ghost rotor.
Would not it have been more logical to choice the frontal surface of the studied fuselage as reference?

Thank you for your opinion, Juergen
 
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kolibri282

Active Member
the drag and moments are much more sensitive to the angle of attack
Too true, Jean-Claude, the whole procedure stands or falls with the accuracy of your calculated rotor wake. Fortunately today's Computational Fluid Dynamics (CFD) programs are extremely powerful. Time and again I am amazed at what kind of problems our CFD chaps are able to investigate successfully (I'm currently working at a Siemens steam turbine plant). It is therefore today no problem to calculate the angle of attack of any fuselage part immersed in the rotor wake to great accuracy. (of course my program is hopelessly outdated in that respect, using analytical expressions for the rotor wake)

(why) lift, drag and moment coefficients are linked to the radius of a ghost rotor
As you see in the picture of the 139 the fuselage is cluttered with all sorts of things: a rescue winch, an antenna running the length of the tail boom, something looking like a mount for a depth charge and so on and so on...
Therefore in helicopter design the frontal area of the aircraft is almost never used, instead the drag is calculated based on the so called "flat plate area". This basically means that the frontal area and the drag coefficient of the aircraft are combined into one number. For a fixed wing aircraft the drag force is:
fD= 0.5*rho*cDfus*Afus*V^2
where rho is the density of the air, cDfus is the fuselage drag coefficient and Afus is the fuselage frontal area. V is of course flight speed.

For a helicopter that almost always is
fD = 0.5*rho*fPA*V^2

here fPA is the flat plate area and is equal to cDfus*Afus

As we have seen the aircraft drag depends on various parameters that might change so it is a good idea to normalize coefficients with respect to rotor diameter, since that one does not change and is readily available.
I am currently struggling with the Lynx coefficients and would be very happy if these were normalized with respect to the rotor diameter, unfortunately they are not...;-(
 
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Jean Claude

Junior Member
As we have seen the aircraft drag depends on various parameters that might change so it is a good idea to normalize coefficients with respect to rotor diameter, since that one does not change and is readily available.
The airflow angle of the rotor depends on its load, while the calculated coefficients do not depend on it.
The drag, lift and moment of the fuselage being independent of the rotor diameter, this idea seems to me as bad as to normalize with respect to the diameter of the pilot's hat, Juergen
 
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