Kawa rough landing?

On Saturday, September 21, 2019 at 6:27:31 PM UTC-7, BobW wrote:
On 9/21/2019 2:30 PM, 2G wrote:
On Saturday, September 21, 2019 at 6:06:18 AM UTC-7,
wrote:
The main reason motorgliders like the Arcus see such a large decrease in
performance with the mast up is that the engine bay doors remain open,
and they are about six feet long. That's a huge amount of drag. When we
installed jet engines in the Tst-14 and four Arcuses, we had the main
doors close over the engine bay and two small "sub-doors" open around the
engine mount. With the engine extended, we measured the L/D of the Arcus
J (jet) at 38:1. The Arcus M gets 13:1 with engine extended.

The principal source of drag is the prop; the engine bay doors are aligned
with the slipstream, and doesn't matter how long they are, the frontal area
remains the same (very small)...
Because I've long been fascinated with aerodynamic drag, this particular topic
fascinates me, and the above exchange reminds me of a factoid my brain thinks
it has retained which - if retained accurately (too lazy to look it up just
now) - may surprise many a RASident. But first...

If the expression "there's devils in the details" applies anywhere,
aerodynamics fits the bill. And for the Truly Anal (or simply Seriously
Interested), I recommend the late Sighard F. Hoerner's masterpiece book,
"Fluid Dynamic Drag" which can be used to actually do a great job of assigning
numerical relative estimates of the drag contributions being discussed above.

I don't expect to actually make such an attempt, but I'm also not gonna bet
the retirement slush fund on "prop drag uber alles" in this instance, while I
*would* be willing to bet some actual money *against* the broad-brush
statement following the semi-colon of the shorter excerpt above being correct,
especially the "...doesn't matter how long they are..." bit.

Shape matters...a *lot* when it comes to aerodynamic drag. In drag-reduction
terms, whether it's more useful to (say) streamline the front of a
motorcycle/rider combo, or fair the rear may surprise many people. Consider a
theoretical, round, 1"-dia lift strut (think 2-33) vs. a faired version of the
same strut, both operating normal to the airflow, at pattern speeds. How many
RASidents would guess the drag coefficient of the former shape vs. the faired
shape is ... wait for it ...

... == 8X == HIGHER?!? And - like hands in lowball poker, where you only
get worse - it (drag) all adds up!

If I ever get an Arcus M and the POH tells me it has an attention-getting sink
rate with the mast extended and inop engine at pattern speeds, I'm definitely
gonna incorporate some personal testing of that configuration early-on, with
gobs of altitude, in my getting-to-know-the-ship phase...and, in the purely
nut-behind-the-stick sense of things, I don't care *where* the drag sources
may actually be located!!! :-)

YMMV.
Bob W.

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Your "analysis" boils down to "I've read a book on aerodynamics so my gut feeling is better than yours." It would actually have been helpful to do actual calculations. The bottom line is that the total drag of the 26e prop+mast+radiator+engine bay doors drops the glide from 50:1 to 17:1. At best, what you are arguing about is the relative contribution of these elements - my bet is on the prop being the largest. But the only thing that matters is the total drag.

Here are some actual Cd figures for various shapes:
https://www.grc.nasa.gov/www/k-12/airplane/shaped.html
Notice that the flat plate (prop) has a Cd over four times that of a bullet (engine bay door) - at the SAME cross sectional area. A prop is going to have MANY TIMES the frontal area of a set of engine bay doors. Conclusion: your "analysis" is wrong.

Tom