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50' span PRANDTL 4

Postby JoeF » Tue Mar 06, 2018 9:26 am

50' span PRANDTL 4 foot launched sailplane


50' span PRANDTL 4 foot launched sailplane initial flight testing at El Mirage Dry Lake November 7, 2016. Piloted by Steve Davey, and built by Erich Chase and Steve Davey. For more info contact Erich@chaseboats.com


Other source wrote:PRANDTL 4 foot launched sailplane initial flight testing at El Mirage Dry Lake November 7, 2016
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Re: 50' span PRANDTL 4

Postby wingspan33 » Tue Mar 06, 2018 11:49 am

Hey Joe,

That was a kind of boring video until I found out a bit more about the wing being tested. I'm not sure I understand the math and science behind the design, but I found some of it described in the following video -

https://www.youtube.com/watch?v=RoT2upDbdUg

Apparently this (fairly new/advanced) flying wing can roll without adverse yaw taking place. This wing has what Mr. Bowers calls "proverse" yaw. Rolling to the right causes the high (left) wing to want to fly faster - which is the opposite of adverse yaw.

Well, what gets me is that - despite what Dennis Pagen insist on - I have never experience adverse yaw in a flex wing (rudderless) hang glider. Does this mean that this fellow (and his team) with advanced degrees is discovering something that birds and hang glider wings have had in common for quite some time?

I suspect that flex wing hang gliders may involve some other aerodynamic feature(s) that cause our "proverse" yaw, but it's interesting to wonder about the differences between what Mr. Bowers is describing and what seems to happen when I fly my flex wing hang glider.

I'd love to sit down and talk with Mr. Bowers for a couple hours - at least. :thumbup:

PS - Ends up Mr. Bowers has had over 400 flights on early (1970s?) hang gliders. :clap:
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Re: 50' span PRANDTL 4

Postby Rick Masters » Tue Mar 06, 2018 1:18 pm



Al Bower's 2016 research paper on the Prandtl-D
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160003578.pdf

Citations
https://scholar.google.com/citations?user=CMlgP_wAAAAJ&hl=en

I have never experience adverse yaw in a flex wing (rudderless) hang glider.

Some hang gliders seem to have little noticeable adverse yaw.
This may be due more to the fact that flexwing hang gliders are very dirty, aerodynamically, rather than having "proverse yaw.
The stiffest, highest performing flexwings I have flown all had pronounced adverse yaw.
I would be interested in the observations of pilots who have flown topless racing hang gliders.

I remember to enter a thermal aggressively, one had to deal efficiently with adverse yaw.

1) in straight flight one had to bump his weight toward the intended outside wing to load it slightly and make it fly faster
2) shift his weight to the inside wing to tighten the turn to the preferred radius
3) then kick his feet toward the outside wing while sliding his torso to the outside as well as pushing out slightly to climb in a thermal.

Loading the outside (high) wing counteracted the adverse yaw.
I didn't do this from theory. I did it because it seemed to work best.
There was also more lift on the inside (thermal side), which complicates things.
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Re: 50' span PRANDTL 4

Postby wingspan33 » Tue Mar 06, 2018 10:32 pm

Rick,

I flew a Fusion SP from 2006 - 2012. It was bought second hand but was in very nice condition and was a topless glider. When it was new (in late 2001) it was Wills Wing's top comp glider. I include that glider when I speak of never experiencing adverse yaw in a hang glider. When I got the Fusion in 2006 I'd been flying HG's for 31 years and knew (and still know) what adverse yaw is.

I've describe to many people (mostly wuffos or students) that the way a hang glider turns is that shifting weight to one side makes that wing drop, along with causing that wing tip to billow/washout (more) while the other less loaded wing is pulled flatter (less billow/washout). The lower more billowed tip causes more drag and goes slower while the less loaded (flatter) wing flies faster. This creates a coordinated turn.

I honestly never recall shifting my weight to one side of any HG I've owned and having that lower wing want to speed up. I don't mean to imply you're wrong, only what my personal experience has been.
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Re: 50' span PRANDTL 4

Postby Rick Masters » Wed Mar 07, 2018 4:37 am

What do you think led to the side-slips that killed so many hang glider pilots when they fell out of 360-degree turns in the early days?
It was uncorrected adverse yaw.

I honestly never recall shifting my weight to one side of any HG I've owned and having that lower wing want to speed up.

This is not what I said.

In general, flying wings are notorious for adverse yaw.
This had always been my understanding, backed by my flying experience.
Since we both have flown many types of hang gliders, I find your argument to the contrary most bizarre.
The only explanation I can think of is that response becomes instinctual so you may no longer notice that it exists.

The observations and analysis of Steve Seibel coincide perfectly with my experience.
http://www.aeroexperiments.org/critiques.shtml
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Re: 50' span PRANDTL 4

Postby brianscharp » Wed Mar 07, 2018 10:17 am

The pilot was Steve Davy. Now that name sounds familiar.
https://newsline.kitplanes.com/2017/04/ ... -fuselage/
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Re: 50' span PRANDTL 4

Postby KaiMartin » Wed Mar 07, 2018 6:42 pm

You may call me overly sensitive. But I always cringe when concepts as the one by Al Bowers are pitched as 'bird-like'.

It is true that birds don't use tail planes to compensate adverse yaw like our traditional airplanes do. But they also do not use a carefully selected but fixed amount of twist at the tip of their wings either. Clearly, they don't follow Al Bowers recipe. Instead, soaring birds either use their large flat tail to compensate adverse yaw (e.g. birds of prey). Or they use the inherent dynamic yaw instability of anhedral in their wing configuration (e.g. sea gulls).

IMHO, both real bird concepts are worth a try for experimental hang gliders. Maybe I am just ignorant. But I don't think, either concept has been tried for foot launched gliders.

---<)kaimartin(>---
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Re: 50' span PRANDTL 4

Postby Frank Colver » Wed Mar 07, 2018 11:55 pm

Speaking of seagulls I wonder how Richard miller's diffuser tip hang glider (see photo) was in yaw characteristics? The day I watched him fly it he was making straight glides from a small hill.

Diffuser tip HG - Richard Miller Nov -76.jpg
Diffuser tip HG - Richard Miller Nov -76.jpg (184.22 KiB) Viewed 514 times
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Re: 50' span PRANDTL 4

Postby KaiMartin » Thu Mar 08, 2018 3:21 pm

Frank Colver wrote:Speaking of seagulls I wonder how Richard miller's diffuser tip hang glider (see photo) was in yaw characteristics? The day I watched him fly it he was making straight glides from a small hill.

The tips point down like the tips of a sea gull. However, unlike the wing configuration of a sea gull, the wing as a whole is still neutral to slightly dihedral. In addition, the wing is swept back like any traditional hang glider -- again very much unlike the wings of a sea gull. Both aspects massively influence yaw and roll.

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Re: 50' span PRANDTL 4

Postby Merlin » Sat Mar 10, 2018 11:16 am

Before buying my first Comet in the 80's I test flew two different 165's in longish soaring flights. Both handled very nicely. I bought a used one (which I had not flown) and found that it had significant and consistent adverse yaw. That is, I would initiate a turn, then the adverse yaw would happen, then the roll would start. So the yaw and roll felt decoupled. No doubt this occurs in a small way in most gliders, but this was dramatic. Actually it was the only glider I ever flew with any noticeable adverse yaw.
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