Jonathan,
I think you have provided the answer to your own question.
The fact that you have run your Classic with no engine offset and
noticed no asymmetric flying characteristics speaks volumes.
The whole idea of canting an engine sideways and forcing the propeller
disc through the air at anything other than normal to the oncoming
airstream is daft.
The theory that canting the engine sideways will counter the yaw
effect of the prop wash seems to be based on a misunderstanding of
what is really happening and its done because thats how its always
been done!
On your Classic, you sensibly mounted your engine head on to the wind
and you set you propeller blades pitch to the recommended angle. Each
time the blades rotate their angles of attack remain equal to each
other and constant to the oncoming wind and each blade generates the
same thrust throughout each revolution of the prop.
Now consider what happens when you follow the XS build instructions
and cant the engine 1.5 degrees to the right.
If youre flying straight-and-level behind a right-hand tractor (Rotax
912, 914), each time a blade passes over the top of the ark, its pitch
is effectively reduced by 1.5 degrees and as it swings through the
bottom of the ark, its effective pitch is increased by 1.5 degrees.
This means that your propeller is producing significantly more thrust
---From the bottom half of the propeller disc than the top half and
that produces a pitch up change in attitude and not the sideways
thrust you had hoped to achieve by mounting the engine sideways.
So if thats true, why has nobody noticed this pitch up attitude?
a good question (even though I asked it myself).
All Europas are fitted with a pitch trimmer so these effects are
unconsciously trimmed out by the pilot during different phases of
flight.
Ah, but what about the propensity to swing to the left on take-off?
- Same thing, different plane.
The Monowheel sits on the ground at a deck angle of (is it about 12
degrees? I forget) so the engine is now canted up at the front by this
amount. At the beginning of the take-off run, the upcoming blade on
the left hand side has 12 degrees wound off its effective pitch, while
the down going blade on the right has 12 degrees added to its pitch.
This produces significantly more thrust on the right hand side of the
disc than the left, resulting in a turning moment to the left. It's a
potential problem with all tail-draggers
The Tri-Gear variant of course sits horizontally on the ground, so has
none of this asymmetric thrust so is less prone to dive off to the
left on take off; another reason why the Tri-Gear is perceived to be
more benign than the Mono.
Canting an engine is a very crude way of addressing a relatively
transient problem Fitting a rudder trimmer would be a far more
elegant solution should it be necessary.
Hope that wasnt too long winded!
Nigel
PS the roll issue has nothing to do with engine position.
Quoting jonathanmilbank <jdmilbank@yahoo.co.uk>:
>
> I'm converting my Classic to XS firewall forward and the
> instructions for mounting the engine call for the 4 washers on each
> lord mount to be arranged so that the engine is angled 1.5 degrees
> to the right.
>
> The Classic build manual didn't give this stipulation and so mine
> was pointing straight ahead. What effect will there likely be when I
> start flying in the new configuration. Up until now there has been a
> tendency for the aircraft to roll gently to the right in the cruise.
>
> Will angling the engine to the right make the right roll tendency
> better or worse? Please give me the aerodynamic logic for this 1.5
> degree offset.
>
> Thanks.
> Jonathan
>
>
> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=465078#465078
>
>
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