ASK21 --- OR --

On Nov 7, 11:05*am, GK wrote:

*- Or Perkoz, with the 20 meter wing extensions it becomes
1/40 cross country trainer.
http://www.szd.com.pl/pdf/szd-54_perkoz_doku_en.pdf

A 40:1 best L/D with that big clunky fixed gear hanging down? That
calls for a larger grain of salt than I have handy.

I don't think 40/1 is remotely unreasonable for the Perkoz, even with
that "big clunky fixed gear hanging down." It has 20m in wing span and
a decent aspect ratio. Compared to the Duo Discus and the DG500/1000
series, it's about 4-5 pts less in best L/D or about a 10% hit in
performance. Seems very reasonable to this professional aerodynamicist.

On Nov 7, 5:10*pm, KevinFinke wrote:
I don't think 40/1 is remotely unreasonable for the Perkoz...

Ten bucks says that if the Idafliegs do a polar it tops out at or
below 39.

Thanks, Bob K.

On Nov 7, 7:51*pm, Bob Kuykendall wrote:
On Nov 7, 5:10*pm, KevinFinke wrote:

I don't think 40/1 is remotely unreasonable for the Perkoz...

Ten bucks says that if the Idafliegs do a polar it tops out at or
below 39.

Thanks, Bob K.

I'd be curious about the result. Our club Janus C has a 20 meter wing
and 'clunky fixed gear'. The manual claims 43:1. I know from
experience it's better than my Speed Astir.

On Nov 7, 8:10*pm, Uncle Fuzzy wrote:

I'd be curious about the result. *Our club Janus C has a 20 meter wing
and 'clunky fixed gear'. *The manual claims 43:1. *I know from
experience it's better than my Speed Astir.

I'm pretty curious as well. I don't hold Dr. Finke's qualifications,
but the calibration I get from comparing manufacturers' claims with
Idaflieg and Johnson results leads me to generally approach the
manufacturers claims with something between "probably not" and "no
nfbsk'n way." That skepticism has stood me in good stead so far.

The things I see in the photos and three-view of the SZD that make it
questionable in my mind are that the undercarriage does appear to hang
down quite a bit from the fairing, and is not the cleanly-faired and
mostly enclosed wheel of the Janus and ASK and even of the G103. Also,
the degree of taper of the wing leads me to suspect that there will be
a non-trivial amount of washout, which could tend to make the wingtip
extensions less effective than you might otherwise expect. I seem to
recall that the Slingsby 15/17m Dart is an example of such. To the
Perkoz's credit, it looks to be a nice clean and well-proportioned
machine, so I woudn't be too surprised to lose ten bucks on the wager.

Johnson's tests on the 18.2m Janus rang up an impressive 40.9:1, so it
seems within reason that the 20m variety could do 43:1. I had thought
that Johnson had done the G104, but it's not on the SSA Web Site. It
might well be in the hardcopy Johnson compendium, I'll try to remember
to check that at work next week.

Thanks, Bob K.
www.hpaircraft.com/hp-24

Couldn't the wing use multiple airfoils or varying thicknesses to get
around the need for (as much) washout, despite the moderate taper
ratio? Those wouldn't be immediately obvious from the visuals or 3-
views... *shrug*

--Noel

Bob,

I fly with Brad Hill, and I'm sure he'll vouch for me. I'll take you
up on the 10 dollar bet. My old club had a Jr and a Puchacz. Both
planes seemed well constructed and gave honest performance along the
lines of their published values. Back in college, I did measure our
clubs Ka7 using the techniques outlined by Dick Johnson. I still have
some of the equipment. Would you accept an independent measurement if
we every get one up here in the Northwest?

Kevin

The only time I've ever used Dr, was back in college when I did some
DJ'ing. I was known then as D.J. Dr. Finkenstein.

On Nov 7, 11:36*pm, "noel.wade" wrote:
Couldn't the wing use multiple airfoils or varying thicknesses to get
around the need for (as much) washout, despite the moderate taper
ratio? *Those wouldn't be immediately obvious from the visuals or 3-
views... **shrug*

--Noel

Multiple airfoils is a slick way to design the desired tip stalling
characteristics into a plane. I'm not sure if the Perkoz uses this,
but I have noticed it seems to have a good bit of forward sweep to the
wing. Forward sweep can certainly be used instead of washout, as
demonstrated clearly on the Sisu.

Based on independent tests I've seen done on other Polish gliders, I
have no reason to doubt the SZD figures for the 54. The Pols have
always been rather scientific and less optimistic about their polars
and such.

-Paul Hanson
SZD-59 driver, claimed at 40/1 by SZD (wet w/winglets...) and measured
by Johnson at 38/1 (dry w/no winglets...)

On 8 Nov, 03:51, Bob Kuykendall wrote:
On Nov 7, 5:10*pm, KevinFinke wrote:

I don't think 40/1 is remotely unreasonable for the Perkoz...

Ten bucks says that if the Idafliegs do a polar it tops out at or
below 39.

Perhaps, but if Dick Johnson were still around he could stick some
turbulator tape on it and suddenly get 45 ...

Ian

At 14:52 08 November 2008, Ian wrote:
On 8 Nov, 03:51, Bob Kuykendall wrote:
On Nov 7, 5:10=A0pm, KevinFinke wrote:

I don't think 40/1 is remotely unreasonable for the Perkoz...

Ten bucks says that if the Idafliegs do a polar it tops out at or
below 39.

Perhaps, but if Dick Johnson were still around he could stick some
turbulator tape on it and suddenly get 45 ...

Ian

Quite the reverse - Dick Johnson tended to find that turbulated gliders
performed better without the tape. He also usually found lower
performance figures than the manufacturer's (and Idaflieg) polars.

John Galloway