Polar for 304C?

[Moshe Braner]

Does anybody have a polar for the HPH 304C - the model with 15m
unflapped wings? Either a graph or just some numbers will do. The
"Glasflugel 304" polar built into XCsoar (span not specified) seems
wrong (STF too high, much higher than, e.g., LS4). The "304CZ" built-in
polar also calls for a high STF, but not quite as high. Is the "H-304"
line in Max Kellerman's polars spreadsheet - or the more mechanized
polar10.xls - appropriate? It seems to be (at wing loading 35.8) very
similar to the LS4 (at 34.4) up to about 85 knots. Above that speed the
304 does better, according to that spreadsheet.

[Britton]

I have it, including flight test data (digital) from Dick Johnson. I dug
into this polar a lot when I had my 304C. Great ship!

Will send this to you today.

Britton

[Britton]

File sent

[Justin Sinclair]

On Tuesday, 1 June 2021 at 12:20:11 am UTC+10, Britton wrote:
> File sent
Hi Britton,
you would have a copy of the Johnson test would you ?
Justin

[Britton]

You bet. Will send tonight.
Britton

[Britton]

File sent, enjoy!

[Bob Gibbons]

In the event others are interested, the full flight test report was
published in Soaring magazine, January 2010, page 22.

For those as confused as I was about finding the magazine archieve in
the new "improved" SSA website, see https://magazine.ssa.org/

Bob

[Moshe Braner]

The January 2010 report is for the 304S "Shark" 18m glider. The report for the 304C (15 meters, retractable, club class) is in the April 2003 issue. There's also the 304CZ (15/17.4m) report, in the July 2000 issue.

[Moshe Braner]

I am still puzzled about the performance of the HpH 304C "Wasp". Our
club has one, so I'd like to know. On the HPH web page about the 304C:
https://www.hph.cz/products/hph-304c-wasp/
they claim best L/D of 43 in the text - but the only polar they display
is the graph from the Dick Johnson flight test, that shows a max L/D of
about 40, and compares it (rather unfavorably) with the LS8 and D2 at
higher speeds. I can't find any other polar for it.

I put numbers from that flight test into Paul Remde's polar-computation
spreadsheet, and I get that the STF at MC=3 is only about 68 or 69
knots. That seems too slow to be true? (Even my 12.6-meter Russia
clocks at about 63 knots in that sense.) That's similar to an ASW-15,
and inferior to an ASW-19. Why would the 304C, a much later design,
with same span and retractable gear, glide so poorly at moderately
higher speeds (75+ knots)?

[Mark Mocho]

"MANUFACTURER'S POLAR: An exercise in creative mathematics, embellished with artistic prevarication; a gaudy package of high hopes, bound with a red ribbon of fantasy; proclaimed as Gospel by the perpetrators, accepted as such by the feeble-minded and soaring pilots."

(Gren Seibels, from his book "After All)

I would tend to believe the Dick Johnson report. His methods were quite involved and accurate. Several "Manufacturer's Polar" figures were shown to be rather optimistic.

[Papa3]

One of the old-timers in our club used to repeat the story (probably apocryphal but still amusing) about the time he confronted Wolf Lemke on the polar of the LS3 while at the SSA Convention. If Lemke really believed the polar (so the story goes), he'd agree to be towed 30 miles out to sea at dawn on a calm day with a zero margin glide to Block Island.

P3

[Mark Mocho]

> One of the old-timers in our club used to repeat the story (probably apocryphal but still amusing) about the time he confronted Wolf Lemke on the polar of the LS3 while at the SSA Convention. If Lemke really believed the polar (so the story goes), he'd agree to be towed 30 miles out to sea at dawn on a calm day with a zero margin glide to Block Island.

I heard that story, but it was attributed to George Moffat. Still a valid test of designer's integrity.

[Moshe Braner]

OK, but I still would like to know the real performance of the 304C.
Maybe it is as poor (at higher speeds) as Dick Johnson measured it, but
that seems unlikely to me? Has anybody flown it on long glides
alongside other gliders at speeds around 80 knots and can tell us how
they compared?

[Christoph Barniske]

If you want to get a good understanding of your 304C performance, I suggest you use the LS4 polar for your final glides, do >100h of XC gliding and check if your glide performance is better or worse than the LS4 reference polar.

The 304C is estimated to be of similar performance like an LS4. So without ballast and at the same wingloading of say 35kg/m², you will notice a slight disadvantage compared to an LS8 or Discus 2 at higher speeds. The higher the wingloading, the bigger the advantage of LS8 and Discus 2 gets. Both are proven models in ~25 years of Standard class competitions with about equal performance. The latest upgrades of the LS8 nullified the slight advantage the Discus 2a had compared to the original LS8-a at high speeds and in dynamic conditions like cloudstreets.

The 304C wing is based on the original 304, with a thickness of ~16.5%. The profile for the 304 was developed under the constraint of using the existing wing of the Glasflügel 303 Mosquito as a base for the new wing. The thinner profiles of LS8 and Discus 2 handle higher wingloadings much better and in case of the LS8 are less affected by contamination (bugs, rain).

Btw, many of Dick Johnsons measurements have not been very accurate, which becomes obvious when looking at the scattered data points in the polar graphs.

[Moshe Braner]

Thank you Christoph for the useful information. That was the local
guess too, to use the LS4 polar as a first guess to put in the 304C
glide computer.

If you look at Figure 4 and Figure 5 in that Johnson report, he made 3
measurements of the polar, and there are jitters in them (the points
don't fall on a smooth curve), but those jitters move together between
those 3 measured glides. Which hints that there may have been some
systematic errors. Or is there a reason why the 304C would have a
significant drop in L/D at 80 knots, relative to the interpolated smooth
curve, but then a bump back up at 85 knots, a bump down at 90, and up
again at 95? Again, not in one measurement, but consistently in all 3
repetitions? Figure 7, comparing with the LS8, shows the 304C as having
a "bucket" of low L/D in the 80-90 knots range, but it's much closer
(although somewhat inferior) to the LS8 from 95 knots and up.

[Tango Whisky]

Dick Johnson's measurements have the serious systematic error that atmospheric mouvements cannot be separated from the sinkrate of the polar.
While this is not a big problem for bricks like 1-26s, it becomes a huge problem for a glider with 40:1 or more.
With typical best L/D speeds around 90-100 km/h, an atmospheric mouvement of 2 cm/ will introduce a measurement error of 1 point.

Measurements are typically done under anticyclonic conditions (i.e nice weather, early in the morning) but even there you frequently have a wide-spread atmoshperic sink rate of 5 cm/s (in a high-pressure area, the overall airmass movement is sink).

The only way to eliminate this is to have the glider to be measured flying alongside a calibrated glider, and measuring the relative sinkrate. This is done by the Idaglieg in Germany, togehtherwith the German DLR, and it is a huge effort. It takes a year to calibrate a glider (formerly an Open Cirrus, now a Discus 2 18 m), and it takes two tows in paralle to 10-15'000 ft.

This gives quite good results, and as a consequence, Dick Johnson's measurement are entirely disregarded in Europe.

[Mike Carris]

Moshe,

Tim McAllister flew a 304C in the 2013 Club Class Nationals, coming in third. He might be a good source of info for you. Can probably get his contact info on the SSA website.

Mike