The Oz Report

World Speed Gliding Championships

David Glover, <footlaunch>, writes in response for a request for Mpeg videos of the World Championships:

There is much video being shot.  We will let you know.  Some people are talking about making it available somewhere on the web.

And in response to a request about when the video will be shown on TV:

Short programs about the event start August 7 on Discovery Wings channel.  There will be 10 different ones of these.  The hour-long documentary will air in winter 2000 or early 2001.

Otherwise, even though I've held the Oz Report awaiting word there is still no word from Panos or David on how things are going in Greece.

Rigid spins – it's all in how you twist it

(This topic is in: 4.149 4.148 )

Dave Swanson, <davidrsw>, writes:

(editor's note: This material comes to me in a number of e-mail messages.  I have edited it to be a bit more coherent.)

While I don't have the actual twist distribution of the ATOS, I do for the Ghostbuster.

(editor's note: See below for ATOS twist.)

The ATOS twist does seem to be similar however.  The problem is that it appears that both do *not* have the proper Irv Culver Twist for a flying wing.

In Feb.  '86, Irv wrote a paper discussing the stability of tailless flying wings.  He discusses tumbling, pitch and yaw damping, poor span-loading, and aero elastics.  The section of span-loading is very much related to the current problem of spins.  He uses a unique twist distribution to achieve an optimum span-loading and reduced induced drag.  This is the Culver Twist.

Current rigid wings seem to have the conventional tailed-aircraft twist distribution.  They don't even come close to the twist distribution of a flex wing.  So without a tail and rudder behind the conventional wing twist distribution, it would seem that spins are expected rather than a rare problem.

The solution to the rigid wing spin problem is to retwist all rigids to the Culver Twist.  The spin problem will go away.  Any gates or fences will add more drag and don't fix the underlying problem.

A readjustment of the ribs will solve the problem!!  Culver Twist = No spins

Irv Culver was the chief aerodynamicist at the Lockheed Skunkworks for probably 30 - 40 yrs.  He gave the place its name.  He helped design the shape of the SR-71.  He designed many sailplanes including the Screaming Weenie in the 1930's and the Carbon Dragon in the '80's.

This fabulous man retired to Tehachapi, CA.  This is where I met him.  Dan Armstrong and Les King can tell you so much more about him, but he "wrote a paper" covering just about everything aerodynamic.

One of these papers is "Tailless - Flying Wings".  I plan on scanning this paper and posting it for all to see.  Irv passed away about 1.5 yrs.  ago.  What a brilliant man!!! This is the reason that Don Mitchell calls this twist idea the "Culver Twist."

Don used the Culver Twist on his last ship, the Stealth II.  So please call this twist distribution the "Culver Twist".  BTW, Irv was humble and wouldn't like to see his name on it, but he's gone and I do.  ;-) His paper talks about some other flying wing subjects that will be of importance to all of us.

Dave Sharp writes:

On the ATOS, I do know that about on the rib # 5 it's about 1 degree and moves up progressively to near or at 7 degrees at the last rib.  One could take a digital level like they do at the Atos factory and get an exact measurement and I'm sure with some basic math calculate the total twist with respect to span.

Dave Swanson replies:

That's what I'm looking for :-) This info seems to match what most rigid wings have for twist (washout). Something like this seems to be typical: Root (keel) = 0° angle of attack (aoa), 1st - 4th ribs = 0°, 5th rib = -1.25°, 6th rib = -2.5°, 7th rib = -3.75°, 8th rib = -5°.  Even if we increase the twist to say -7° at the 8th rib, we still have all the twist on the outboard portion of the wing span.  This is what we see with conventional wing washout of a tailed aircraft.  Tailed aircraft have a rudder to get out of a spin.

So how much and where should the twist distribution be in a flying wing?  Conventional aircraft (w/ a tail) have all their twist toward the wing tip and it works very well.  The current crop of rigids seems to do the same thing.  Unfortunately this is the worst place to put it if one is trying to prevent a spin on a flying wing.  There must be twist in a flying wing but the Culver Twist solves the spin and stall problems, all at the same time.

So what is the Culver Twist you ask?  A flex wing has >20° of washout (from root to tip).  Rigids have around 5-7° of twist (washout).  Now what is the distribution of this twist across the span?  This is where the Culver Twist comes in.

Flex wings just happen to have the Culver Twist as a result of their flexible sail cloth.  The most efficient twist distribution and the one that prevents a spin (not a tumble) is to have a semi-elliptical twist distribution.  Most of the twist must be done at the root (in the first 1/3rd of span).  Notice the difference:

[Say the 1st rib is near the root and the 8th rib is the tip.]

Notice how there is more twist at the root in the flex and Culver twist?  The Culver Twist gives the stability to prevent tip stalls and spins and gives great efficiency with reduced drag of low twist.

All flying wings should have their twist towards the root.  No current rigid wing does this, so this leads to a tip stall and a spin.  Here's how:

A rigid is in a slow turn.  The pilot pushes out until the root of both wings are stalled.  The pilot adds some spoiler to the inside wing, which stalls some of the tip area.  So we have the root area (say about 1/3rd of the wing area) of the inside wing and the tip (say 1/4th of the wing area) of the inside wing stalled.

The entire wing loading of the inside wing is now carried by the remaining half of the wing area (1/3 root + 1/4 tip = 7/12 =approx 1/2).  So what happens to the stall speed if we cut off half of the wing area?

The wing loading goes up, which results in a higher stall speed for the still flying wing area.  This still flying area may or may not stall.  If the glider speed is higher than the local flying area stall speed, then no stall and no spin.  But what if the stall progresses from the root out through the still flying wing area?  This is where the wing twist comes in.

If the mid-span still flying area has the *same* or almost the same angle of attack as the root and coupled with the higher wing loading, then the mid-span area will stall too.  Here's your spin!!  An entire wing is now stalled.

In the typical rigid mid-span area, the local angle of attack (aoa) is either the same as the root or within 2° of the root, so it will be much more prone to stall and lead to a spin.  The mid-span area of a Culver Twist wing is a full 5° from the root stall aoa, thus it won't stall.  No spin will occur.  The same happens for a flex wing.

The numbers that I chose to illustrate the Culver Twist are for a thermalling glider and not a racer.  A racer Culver Twist would be something like:

Notice how the majority of the twist is within the first 30% of the span?  5° of twist in the first 30% of span (let's say) and 1.25° in the remaining 70%.  This is how we can prevent tip stalls and spins.  This should be fairly easy to do with new rib placements.  No large redesign is needed.

I'd like to thank Felix and Mike for kick starting the rigid wing movement!!  I had a blast flying the Exxtacy when you gents brought it down to Torrey Pines way back there in '97.

Dave Sharp's under the control bar bridle

Dave Sharp,<flysharp1>, writes:

One day when it was blowing 20 mph plus on the ground I hooked up my Mason three string release to my lower strap on my Carbon CG and attached 10 feet of line.  Next my friend Steve handed towed me.  Yes, it pulled up on the base tube.  But on my Atos at the bar is so far raked forward it was a simple matter of pulling in on the uprights.

Since then that's all I used.  It works on static tows, Pay out and Aero tows (over the bar).  It works in no wind, light wind and cross wind, it works when the rope binds on my Flytec mount on the base tube.  So far it has worked without a single mishap.

Dave has written in about his static line (and modified reel towing) bridle.  I got to see it in action at Zapata, and was quite impressed.

First of all, it is a simple standard bridle that happens to be connected to his harness at the waist (or lower chest).  Two straps form a V about 18" out to a three ring circus. Another short line pulls the pin from the circus.  We attached our standard weaklink to David's three-ring circus and attached the towline under the control bar.

The cool thing is that Dave can run with the towline under the control bar and as soon as he is up, there is no bridle line over the top of the control bar, getting into your instruments, or hitting your helmet.

The uncool thing is the question of how much control David has in the first few feet of the tow.  After all the towline is under the control frame, and pulling up on it trying to raise the nose of the glider.  David has to make sure that he has pulled back on the downtubes while running, and that once in the air he is pulled in enough to keep the angle of attack low.

At Zapata it looked just fine.  We were towing early and there was only light thermal activity on the runway.  With a good breeze from the south, we weren't required to run very far, and consequently didn't have to keep the bar pulled in as long a time as we ran.

Dave responds:

In no wind it takes a bit of strength to hold back a bit longer while building up speed.  As mentioned, with a keel helper to push the keel up (rear) and push a bit forward, it's a breeze.  Almost immediately once airborne I experience the same pressures as someone has just left the truck (payout style) and the rope angle is non-issue.

I can't say I recommend this technique for everyone, it requires good control of the nose angle in the first few steps, until the wing has enough lift to support itself, but it seems very reasonable for me at the moment.  It is a real treat to have a single short bridle to stow.

CIVL Official threatens Oz Report

Sarah Fenwick, <civl>, taking exception to how I characterized her written statements to me and to the Brazilians, has stated that she may be forced to take some action against me.  I suggest arm wrestling her former boy friend in a Swiss bistro the next time I get to Europe.

I also suggest that Ms.  Fenwick realize that she is a public official and here in the land of the free and home of the brave, a public official's comments are fair game.  I have neither slandered nor libeled her, and in fact have only dealt with her comments, not her person.

I also suggest that CIVL, if it doesn't want their officials looking bad, have them make their comments reasonable.

Lovely Roswell

Looks like I will leave Roswell, New Mexico tonight or tomorrow, after my new truck is cosmetically repaired.  I have loved dealing with a number of the people here, including Steve McMurtry at McMurtry Auto Body and Glass (http://www.roswell-record.com/mcmurtry/autobody.html) (his family seems to own half the auto dealerships in town).  Steve was a State Farm claims agent for 14 years, and he treated me great when I came to his shop trying to get my truck fixed fast enough so that I could get to Zapata.  He turned my truck over to Ricard Lovato at Bodycrafts, who he knew had a bigger frame straightener, after ordering the new radiators.  Richard has been great to deal with also.  He got me to Zapata, the next day.  Thanks also to State Farm (Teresa Connell ,709 Jones Avenue, Haines City, FL 33844-4341, Phone: (863) 422-4901, Fax: (863) 422-1248,    ) for processing my claim instantly.  Thanks goodness for un-insured drivers insurance (the other guy got four tickets).

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Davis Straub
co-author of Windows 98 Secrets, and More Windows 98 Secrets, IDG Books
"The Jaws of Life for Windows 98 Users"
<davis>
http://www.davisstraub.com/secrets