Frans,
I will probably regret this, but I will make some guesses as to why climbing and
then diving causes the speed to increase.
It sounds to me as though you are seeing the airflow boundary layer detaching
early
somewhere on the airframe at lower speed. Then when you accelerate after
climbing then diving the flow is staying attached. I cannot think of anything
else that would explain what you are describing. Early airflow separation can
cause a considerable increase in drag.
There may be another clue when you say that when you move the CG back, the speed
increases to a point. By doing this, you are decreasing longitudinal static
stability, but also reducing the download on the tailplane (or horizontal stab),
causing a reduction in drag. Once the tailplane is aligned with the airflow,
you have minimum drag from it. Move the CG back further the effect will taper
off.
So it just could be that the airflow is detaching early on the lower surface of
the tailplane in level flight at your initial cruising speed, increasing drag.
By climbing, then accelerating in a dive, you may be allowing the airflow to
stay attached later on the tailplane, reducing the drag. Then the reduced drag
would allow you to maintain the increased speed. That is only a guess, and
I have not flown a Europa so am free to speak through ignorance! You can check
if I am right by carefully measuring the fore and aft position of the stick
before you start the climb, then again once it is back in level flight after
the dive. If it is slightly further forward after the dive, then this would
confirm my guess.
If this does turn out to be the case, you might be able to delay the airflow
separating by using small vortex generators on the bottom of the tailplane. An
example for a different reason can be seen on the Zenith STOL aircraft (to
allow earlier rotation on take off). You can see this at
http://www.zenithair.com/stolch701/pic09/vg701elevator.jpg .
Now you can all tell me that I am wrong, and why!! Not many GA aircraft use an
all-moving tail, and this may just be a side effect of that.
Jerry
On Oct 5, 2012, at 3:57 PM, Frans Veldman wrote:
>
> On 10/05/2012 07:56 PM, Jeremy Fisher wrote:
>
>> Thanks for posting the photos. That is an impressive scheme that you
>> created, especially as it obviously works. As a matter of interest,
>> what difference do you see in aircraft performance? Your cooling
>> drag must be well down on the standard scheme.
>
> I think it is, but of course it is hard to measure and judge. We also
> have an efficient prop and Fred Klein's wing root fairings. On the other
> hand the aircraft is a tri-gear, hi-top, and quite heavy. We never flew
> with the stock cooling configuration, or without the wing root fairings,
> etc.
> We usually cruise with 27" to 28" of manifold pressure, and see an IAS
> of about 130 knots at MTOW. At 100% we see a speed of 157 knots.
>
> Usually at FL95 or so we see cruise speeds of 140-160 knots.
>
> Measuring speeds drive me nuts. The aircraft seems to stick to a certain
> speed, but pull up, dive down, and after the dive the aircraft maintains
> a much higher speed than before the dive. I have heard this from more
> people. Anyone who knows the cause? And then it looks like more weight
> in the back is beneficial, but only to a certain point. I'm still at a
> loss about how to get the best speeds.
>
> Frans
>
>
>
>
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