Aerodynamics - De-cone the Pre-cone
PPRuNe Enigma
Joined: Feb 2001
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From: Scotland
4-6000 fpm <img border="0" title="" alt="[Eek!]" src="eek.gif" /> <img border="0" title="" alt="[Eek!]" src="eek.gif" /> <img border="0" title="" alt="[Eek!]" src="eek.gif" /> . .. .I'll stick to a nice safe R22 thanks.
Thread Starter
Joined: Nov 2000
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From: Vancouver, BC, Canada
The <a href="http://avia.russian.ee/vertigo/ka-22-r.html" target="_blank">RA-22 Vintokryl</a> is very similar to the <a href="http://avia.russian.ee/vertigo/bvertol_osprey-r.html" target="_blank">V-22 Osprey</a>. The primary difference seems to be that the RA-22 has separate rotors and propellers, whereas the V-22 combines them.. .. .Superficially, it looks like a modern version of the RA-22 could give the V-22 a run for its money, if the RA-22 was to use the advancing blade concept or the CarterCopter concept for its rotors. . .. .The advantages of a modern RA-22 might be;. . 1/ a cleaner transition between hover and forward flight. . . 2/ it can land in either mode. . . 3/ it is probably not as susceptible to the vortex ring state. . . 4/ its propellers and rotors are optimized for their own flight realm.. . 5/ it has a slower rate of descent in autorotation. . ._______. .. .Just 'food for thought', which is being served on the dinner plate, which Nick wouldn't buy from me. <img border="0" title="" alt="[Wink]" src="wink.gif" />. . . . <small>[ 06 March 2002, 01:26: Message edited by: Dave Jackson ]</small>
Iconoclast
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From: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
To: Dave Jackson. .. .Your Russian version (as you say) of the V-22 was built in 1959 and appears to be a more complex copy of the Fairey Rotordyne which was built in 1957.
Iconoclast
Joined: Sep 2000
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From: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
To: Dave Jackson:. .. .I agree with you about the center of pressure moving on a cambered blade. That is why I stated there are problems relative to stability. See below.. .. .Quote from Nick Lappos above:. .. ."Only Lu can mis-remember a lecture from 1956, and then blame the instructor when he screws it all up".. .. ."Give us a break, no aerodynamicist gave you that wrong info Lu (the aerodynamic center is outside of the blade), but you would NEVER, EVER admit a mistake so I give up".. .. .To: Nick Lappos. .. .You were totally correct in saying I had misinterpreted what was presented to us in the classroom back in 1956. I could sluff it off by saying that I suffer from old-timers disease but in truth I just got it wrong. The basic premise about cambered airfoils on helicopters was correct and it reflected the conventional wisdom at that time. I had to dig to get the reference but I finally found it. The words are those of Ralph Lightfoot.. .. .…..as the angle of attack changes, the distribution of the individual forces over the surface of the airfoil changes. This causes the resultant to act at a different location or center of pressure. With airfoils generally used in fixed wing aircraft, changes in angle of attack result in a considerable amount of travel of the center of pressure. This is not a very serious problem with fixed-wing aircraft because longitudinal stability can be achieved in a number of ways. In the case of rotor blades, which must change angle (feather) about a fixed axis (feathering axis), the problem takes on more serious proportions.. .. .The camber itself causes a diving moment which is constant, while, with respect to a rotor blade spar within the airfoil, the lines of action through the aerodynamic center cause increasing stalling moments. It can be seen that an airfoil whose center of pressure changes with angle of attack would not be suitable for use in a rotor blade where constantly changing pitch is necessary for most directional flight speeds. Such instability would cause many complications and undesirable control forces. These complications are essentially eliminated by a symmetrical airfoil section whose aerodynamic center and / or center of pressure is always located on the median line and whose feathering axis and chordwise center of gravity are located at the same point.. .. .…..It should be apparent that an airfoil with many of the important factors concerning its’ performance located at one point would be relatively free of any inherent moments which might be detrimental to its’ use as a rotor blade.. .. .My words:. .. .I do not know if Sikorsky ever adapted cambered blades for use on their ships but just about everybody else has. I guess, mainly to generate greater lift. However in 1956 the above words or in effect the same meaning was inferred in the classroom presentation. If in fact Sikorsky has adapted cambered blades then there is a point of confusion as the above material is still being presented to pilots and mechanics attending Sikorsky Service School.. .. .Whether you feel that the so-called Blue Book is nothing more than a comic book it is still what is presented to students at Sikorsky. I would suggest that you get a copy of the Blue Book and read pages 10-11 and if you disagree with what is said then take it up with the service school.. . . . <small>[ 08 March 2002, 04:21: Message edited by: Lu Zuckerman ]</small>
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Lu,. .Good post, thanks for clearing it up. Clearly you wee off when you discussed aero centers outside of the blade.. .. .All manufacturers of modern helicopters use assymetrical airfoils. They are not just cambered (that means the chord line or a symmetrical airfoil is curved) but rather the whole airfoil is different top to bottom. This is for better lift and less drag.. .. .The Blue Book is a nice comic book, kind of like a Classics Illustrated, but could not be considered an aero text. It is admirable in its purpose - to acquaint non-engineers with enough understanding to allow them to repair helicopters.. .. .Thanks for the post Lu. I was wrong, you can admit a mistake! Keep up the good posts.
