[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: More on Argentavis

Comments inserted below.

----- Original Message ----- From: "Tommy Tyrberg" <tommy.tyrberg@norrkoping.mail.telia.com>
To: <jrccea@bellsouth.net>; <gerarus@westnet.com.au>
Cc: <dinosaur@usc.edu>
Sent: Wednesday, July 04, 2007 1:11 PM
Subject: Re: More on Argentavis

At 05:37 2007-07-04, jrc wrote:

Note that many insects, dragonflies for instance do have rather effective aerofoils. The veins are oriented to force the airfoil shape under load.

It is a common misconception that a wing has to have "airfoil shape" to work.

That's true. as we all know, even a flat plank will work pretty well up to an aoa of about 11 degrees, albeit at high drag. A decent camberline is capable of increasing the stall aoa substantially while reducing the drag. As I've often mentioned, avian feathers don't have to be cambered and asymmetric to work -- the asymmetry just reduces the required cross section of the shaft and the biological cost of the moult.

Many aircraft have symmetric wings profiles which work perfectly well provided the wing has a positive angle of attack.

Also true. And cambered wings can work pretty well even when upside down. The Reed Clip-wing Cub is quite a good example of that. Has to fly the USA 35(B) airfoil at a pretty healthy aoa when upside down though. And cambered airfoils often cruise at negative angles of attack. The Cherokee 150C that I fly uses a 65(2)-415 airfoil that has a zero lift aoa of almost a negative 4 degrees. In cruise at normal weights, the aoa is still quite negative, by a degree or so. It only becomes positive when climbing slowly and at high weights. But, occasional oddities are seen from time to time. Like most planes, the Boeing Stearman PT-17 that I sometimes fly generates a tail download during cruise to balance the pitching moment of the wings -- but the asymmetric horizontal stabilizer is cambered for creating an upload. So, the tail has to be flown substantially more negative than would otherwise be the case. Durned if I know offhand why they did that. It may have to do with the interaction between the flow off the upper wing with the fuselage and tail, but that's just speculative -- I've not bothered to sort it out in my mind.

> Remember that a wing works by deflecting the airstream downwards. Any
shape that does this will work as a wing provided the speed is high enough.

It'll work even at slow speeds. Some just have to go faster than others to balance the weight of the craft and the tail download (if the craft is one that generates a tail download for stability).

Of course some shapes are better than others in specific cases (gnats, dragonflies, teratorns, pterosaurs, 747's, XB-70's, space shuttles etc).

Well said, Tommy. Have a happy 4th JimC