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Aerodynamic question


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I posted it already at the other forum but I didn't get any response, usually here I get more feedback.. let's see..


Does a K-MAX require more than 300ft/min ROD before it really gets into vortex ring state?


The reason why I am asking the question - in my opinion and after being asked by one of my students - due to the position of the two main rotors, isn't there less chance for the upward airflow to overcome the downwash and create the vortices..?!


Any response greatly appreciated... :bowdown:

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Any response greatly appreciated...

...well you said it.


Given that the fuselage of the K-Max is very narrow and the rotors are pretty close together, I would guess that it would react to VRS pretty much like any similarly-sized helicopter. The recirculating air isn't going to know or care whether it's one big rotor or two intermeshing rotors.


I think that induced flow velocity has more effect on a helicopter's susceptibility to VRS than the setup of the rotor system.

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Again, with no hard data, just a combo of logic and intuition, I think that the key would be that it is a rotor systemb/b]. Just like a coaxial Kamov, the rotor system creates downwash. If the rotor(s) descend into the downwash, it won't make much difference whether there's just one, two stacked on top of each other, or two intermeshing.


This brings up a side question about a coaxial ship - do they lose a bit (or a lot) of hover efficiency due to the downwash from the top rotor passing through the lower rotor? I would guess that the lower rotor must be rigged differently collective-wise.

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my guess was that due to the inclined position of the two main rotor the downwash is more directed/tilted inward - or since the two main rotors besides each other.. and tilted towards each others center... there would be some sort of cancelation of reduced downwash - and then I came to that thought of there would be a need of higher descent-rate velocity in order to overcome the downwash and turn it around into vortices..


since I know/think that on VRS the vortices start on the inner section of the mainrotor due to the very small amount of downwash..


that's why I thought it should be a different number on ROD for VRS requirement



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dear CJ,


please could I get some info's on where to get those calculation.. please, please...  :bowdown:


that would be awesome.. you are the first one with some more information on it..


thank you...

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I dont want to jump ahead of CJ as I don' t know his exact calculations but I believe that the higher disk loading you have the stronger the downwash you will have...if you have a stronger (faster) downwash you will need to descent faster inorder to "catch up" with your downwash.  This being the case (?) I don't see why the V22 has had so many issues with it besides the fact they could get it on one side and not the other.
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You need to use this equation Sqrt(DL/2p)*60 to determine Vi.


DL in disk loading in pounds per square feet.

p is air density in slugs/cft.


Vi is your vertical index.

Vi *.66 gives you your minimum ROD to experience VRS at 0AS.

Vi *1.25 gives you your maximum ROD to experience VRS at 0AS.

Vi/101.4 gives you the speed at which you experience ETL.


A chart goes with the formula, but I don't have it on the computer.  If we write a new book here, it will be in it.

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The higher the ROD to achieve VRS, the faster the airspeed must be to achieve ETL.  The V-22 needs about 2,000 FPM ROD when approaching ETL which is about 44 knots.  So you can see that this is a realm of flight easily transition through on an approach.


I wish you could see the graph because Vi min is about 3,000 FPM, but as speed increases, the minimum ROD to experience VRS decreases to about 2,000 FPM right before ETL. Hard to explain online.  I will try and scan it sometime if I can borrow a scanner.

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