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

2G wrote on 9/22/2019 1:43 PM:

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.

Don't forget the massive landing gear that puts the wheel and tire almost entirely
outside the fuselage (and it has doors with a frontal section nearly as big as the
engine doors)!

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

On Sunday, September 22, 2019 at 8:15:41 PM UTC-7, Eric Greenwell wrote:
2G wrote on 9/22/2019 1:43 PM:

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..

Don't forget the massive landing gear that puts the wheel and tire almost entirely
outside the fuselage (and it has doors with a frontal section nearly as big as the
engine doors)!

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

That 17:1 figure was with the gear retracted.

Tom

2G wrote on 9/22/2019 9:21 PM:
On Sunday, September 22, 2019 at 8:15:41 PM UTC-7, Eric Greenwell wrote:
2G wrote on 9/22/2019 1:43 PM:

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..

Don't forget the massive landing gear that puts the wheel and tire almost entirely
outside the fuselage (and it has doors with a frontal section nearly as big as the
engine doors)!

I'll have to redo my measurements of years ago. I recall getting about 20:1 in
landing configuration, but I can't find my notes. Who did the 17:1 measurement?

Another data point: In 2008, Dr Jack had the belt break on his 26E, stopped the
engine, but was unable to stop the freely spinning propeller. He wrote "From my
GPS trace the glide ratio during the straight portion of my descent, at around 55
kts in flap 3 with prop spinning, as 18.5"

So, gear up, but prop spinning, he was able to glide about 7 NM to an airport,
then encountered some lift and thermalled up, and briefly considered thermalling
the 11 NM to his home airfield, but decided it was smarter to land. Hardly sounds
like a glider in "plummet mode"?


--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

So the moral of all this is that motor equiped sailolane fliers should get some 1-26 xc time?
LOL

On Monday, September 23, 2019 at 1:57:40 PM UTC-4, Eric Greenwell wrote:
2G wrote on 9/22/2019 9:21 PM:
On Sunday, September 22, 2019 at 8:15:41 PM UTC-7, Eric Greenwell wrote:
2G wrote on 9/22/2019 1:43 PM:

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..

Don't forget the massive landing gear that puts the wheel and tire almost entirely
outside the fuselage (and it has doors with a frontal section nearly as big as the
engine doors)!

I'll have to redo my measurements of years ago. I recall getting about 20:1 in
landing configuration, but I can't find my notes. Who did the 17:1 measurement?

Another data point: In 2008, Dr Jack had the belt break on his 26E, stopped the
engine, but was unable to stop the freely spinning propeller. He wrote "From my
GPS trace the glide ratio during the straight portion of my descent, at around 55
kts in flap 3 with prop spinning, as 18.5"

So, gear up, but prop spinning, he was able to glide about 7 NM to an airport,
then encountered some lift and thermalled up, and briefly considered thermalling
the 11 NM to his home airfield, but decided it was smarter to land. Hardly sounds
like a glider in "plummet mode"?


--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

I had a non start(pilot induced) the other day in my ASG-29E. I was positioned on down wind for my selected field. Rather than mess with the engine(and associated distraction)I put the gear down and landed normally. Since I usually fly a fairly steep approach, I did not sense any meaningful affect on my landing.
FWIW
UH

wrote on 9/23/2019 12:24 PM:
I had a non start(pilot induced) the other day in my ASG-29E. I was positioned on down wind for my selected field. Rather than mess with the engine(and associated distraction)I put the gear down and landed normally. Since I usually fly a fairly steep approach, I did not sense any meaningful affect on my landing.
FWIW

In my Guide (and here!), I recommend you put the gear down when you are in the
pattern planning a restart. One less thing to forget if the engine doesn't start.


--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

On Monday, September 23, 2019 at 10:57:40 AM UTC-7, Eric Greenwell wrote:
I'll have to redo my measurements of years ago. I recall getting about 20:1 in
landing configuration, but I can't find my notes. Who did the 17:1 measurement?

When I had my '26E, I once flew several self launches to pattern altitude, shut down the engine and landed with the prop extended. It was a complete non-event and felt the same as a pattern and landing in an ASK-21. I like to fly a high and steep final so maybe that's why. I probably used less spoiler in the '26E to maintain the same approach angle. I also applied landing flaps sometime between base and final.

When making a "straight in" type final glide with just a couple hundred feet arrival when some distance out, I'd pick a (or already have one in mind) a few miles out if it still looked marginal. I'd then start the engine, make a few orbits over the safety field to gain altitude, then motor home. Once home, I'd retract the engine part way to cool it and make a landing.

I once joined a ASW-20B in an extremely weak thermal, shut down the motor and again lowered it to the slightly extended cooling position. We were pretty much even in climb and even while gliding to another thermal a mile or two away. So my takeaway there is that at speeds around 60 knots, the open engine doors and partly extended engine produce negligible drag.

IMHO, the dire warnings about poor LD on the POH are there to avoid any liability if someone decides to sue after an engine extended mishap. Or to be a bit kinder, like what we tell new glider pilots about thermalling low - stop trying to stay up and just land once at XXXX above ground.

5Z
Now flying ASW-27b

5Z wrote on 9/26/2019 12:21 PM:
lowered it to the slightly extended cooling position.

The "slightly extended cooling position" is about 30 degrees from the fuselage.
There is still all the stuff hanging in the breeze: the propeller, the radiator,
almost the entire mast, and the doors are fully open. It looks dreadfully draggy
to a pilot circling in the same thermal with you, but it's just like Tom says: the
drag is so small, you don't know it's there.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

On Thursday, September 26, 2019 at 1:13:07 PM UTC-7, Eric Greenwell wrote:
5Z wrote on 9/26/2019 12:21 PM:
lowered it to the slightly extended cooling position.

The "slightly extended cooling position" is about 30 degrees from the fuselage.
There is still all the stuff hanging in the breeze: the propeller, the radiator,
almost the entire mast, and the doors are fully open. It looks dreadfully draggy
to a pilot circling in the same thermal with you, but it's just like Tom says: the
drag is so small, you don't know it's there.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1


Got so concerned with staying under the LAS Class B once, didn't completely put away the ASH26E mast until after a cruise to the second thermal. A bit noisy but no big deal.
Once climbed in the 26 together with a Discus 2. I still had the mast in cooling position, the D2 was "P7". Impressed!

Enjoying Fred Drift this time, Seems like Kawa was being condemned for nothing.
Jim