aerodynamics questions...

[alltorque]

I am wondering where is a good place to ask for clarifications on some of the aerodynamic models for helicopters.

For example I have Disc Loading stated as "w=T/A" where T is the thrust per rotor, A is the rotor disc area. I do not know how to correctly determine the rotor disc area - ie is it the lifting portion of the blades as the radii of the disc or the entire disc from tip to tip (in a basic teetering head)? Ie do I exclude the blade roots, grips and hub/feathering area at the centre of the disc or is this portion considered a necessary part of the "rotor disc area"?

I am doing some comparative modelling and want to know where the best place is to ask basic (stupid??) questions.

TIA

[Dave_Jackson]

alltorque,

For a single rotor, when using Momentum Theory, the disk area [A] is area of the entire disk scribed by the blade tips. It includes the grips and the hub.

For twin rotor configurations, there is disagreement as to how to calculate the disk area.

[alltorque]

Excellent! Thank you Dave. I appreciate it.

[Vfrpilotpb]

Dave,

Does that disagreement on rotor area cover the Chinnock as well, or just the Kaman types?
PB

[Dave_Jackson]

Vfrpilotpb,

The first part of the web page OTHER: Aerodynamics - General - Disk Area (http://www.unicopter.com/0949.html) outlines the position of some knowledgeable aerodynamicists, regarding the disk area of twin rotor configurations. :ok:


The web page then deteriorates :uhoh: , as I try to find some basis for commonality.
This page is an attempt to determine the disk area of the intermeshing configuration, which is part way between the tandem configuration and the coaxial configuration.

[SEL]

For disc loading, the above is right. It is the rotor thrust divided by the entire rotor area. It may seem odd to include the hub and root cut-out areas but it is just a reference area and as long as you are consistent it doesn't matter.

As for mulitple rotors, I have always used the projected area. For co-axial rotors it is the same as a single rotor. For tandems I use the net projected area, ie dont include the overlap.

If blade loading is considered, you use the same approach. The rotor radius multiplied by the blade chord by the number of blades. I know this means some areas are covered more than once but as said above, as long as you're consistent it is okay. Plus, the inner parts of the blades/disc dont produce lift anyway.

Hope this helps, any more questions, drop me a line.

'Helicopters are like women, they're both complex things'
A.N.Modha, Rotorcraft Researcher

SEL

[Spaced]

Hey sel welcome to the forum, I think I ve got a paper by you on my computer. Need some more education before I can understand all of it:O

One thing which I did just read on the Kamov site was that the coax config behaved like a bi-plane in forward flight offsetting the drag of the extra rotor.
Dont know if its true, but it makes sense, looking at the figures for bi-planes.
For those interested here (http://www.kamov.ru/market/news/petr11.htm) is a link to the Kamov page, which states the benefits of the coax system. Bearing in mind they are promoting their system, its still an interesting read though.

Just to get back to the topic slightly, for calculation purposes, is the best method for coax calcs to treat it as a single disk with the appropriate no of blades?

[alltorque]

Hey Sel - that is very useful - thank you... (may I PM you?)

Clearly, if one is consistant the inclusion of non lifting sections of the disc becomes irrelevant, especially since those non-lifting sections are necessary components of the lifting disc.

I understand that the further extrapolation of the formula I described above becomes:

w=T/A where T==Weight of the helicopter rather than thrust per rotor...

[SEL]

Spaced

Glad it helped. As for paper do you mean A.N. Modha’s papers? If so it’ll be control which I have trouble with too!!

There is a beneficial effect with bi-planes that mean less drag is produced than doubling that from a mono but in the case of rotors there are a lot more factors. The lower rotor operates in the downwash of the upper which means it is way off optimum. Accordingly the power consumed is higher in co-axial than for other twin rotor configurations. The bi-plane effect may offset some of the drag but I doubt it would be enough to counter the above or the greatly increased hub drag. Then there is the weight of two rotor systems, two control systems, the transmission etc. Having said all that the Kamovs have been very successful but the down sides to the configuration kept western designers away from it until the Sikorsky S-69 with the Advancing Blade Concept (XH-59) which went some way to solving some of the co-axial problems.

As for co-axial analysis, if using momentum theory use the area of a single rotor. That way, you can avoid considering blades altogether. If you were going to use blade element theory, which would be tricky as you would have to consider the interference of the rotors on the induced velocity. It really depends what you’re trying to do.

Alltorque

Sure, you can PM me or email if you want.

You’ve got it, as long as you are consistent, it doesn’t matter what reference area you use. However, since practically all of the available data uses the entire disc area, that is the one to use.

As for the extrapolation, yeah, the general way to approach it, is that the rotor is in steady state i.e. a steady hover or climb, that way there is no acceleration and the thrust is equal to the weight.

Hope it all helps.

Steve

[Dave_Jackson]

The 'disk area' is a primary value, which is used in many calculations. For basic comparative purpose it is OK; as long as only one rotor configuration is being considered. Unfortunately, there are few helicopters with configurations other than the single rotor, and the current means of determining 'disk area' is invalid when making comparisons between helicopters with different rotor configurations.