Very interesting stuff, only one question to this: don't forget to take the g
yroscopic effect into account of the increased thrust on the bottom half, br
inging it to.... the left side, increasing the effect to the right further??
Alex Kaarsberg
> On 16 Jan 2017, at 01:07, Robert Borger <rlborger@mac.com> wrote:
>
> Nigel,
>
> Thanks for the detailed description. I have often wondered about the effe
ctiveness of the offset. It just didn=99t seem right to me. At some p
oint in the future I=99ll have to unbolt the engine for something. Wh
en I do, I=99ll remove the offset and see what difference it makes.
>
> Blue skies & tailwinds,
> Bob Borger
> Europa XS Tri, Rotax 914, Airmaster C/S Prop (75 hrs).
> Little Toot Sport Biplane, Lycoming Thunderbolt AEIO-320 EXP
> 3705 Lynchburg Dr.
> Corinth, TX 76208-5331
> Cel: 817-992-1117
> rlborger@mac.com
>
> On Jan 15, 2017, at 4:08 PM, nigel_graham@m-tecque.co.uk wrote:
>
>
> 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 n
o asymmetric flying characteristics speaks volumes.
> The whole idea of canting an engine sideways and forcing the propeller dis
c 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 hap
pening and it=99s done because =9Cthat=99s how it=99
s always been done=9D!
>
> On your Classic, you sensibly mounted your engine head on to the wind and y
ou set you propeller blades pitch to the recommended angle. Each time the bl
ades rotate their angles of attack remain equal to each other and constant t
o 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 ca
nt the engine 1.5 degrees to the right.
>
> If you=99re flying straight-and-level behind a right-hand tractor (R
otax 912, 914), each time a blade passes over the top of the ark, its pitch i
s effectively reduced by 1.5 degrees and as it swings through the bottom of t
he ark, its effective pitch is increased by 1.5 degrees. This means that yo
ur propeller is producing significantly more thrust from the bottom half of t
he propeller disc than the top half =93 and that produces a pitch up c
hange in attitude =93 and not the sideways thrust you had hoped to ach
ieve by mounting the engine sideways.
>
> =9CSo if that=99s true, why has nobody noticed this pitch up a
ttitude?=9D =93 a good question (even though I asked it myself).
>
> All Europa=99s are fitted with a pitch trimmer =93 so these ef
fects are unconsciously trimmed out by the pilot during different phases of f
light.
>
> =9CAh, but what about the propensity to swing to the left on take-of
f?=9D - Same thing, different plane.
>
> The Monowheel sits on the ground at a deck angle of (is it about 12 degre
es? 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 mo
re thrust on the right hand side of the disc than the left, resulting in a t
urning 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 non
e of this asymmetric thrust =93 so is less prone to dive off to the le
ft on take off; another reason why the Tri-Gear is perceived to be more beni
gn than the Mono.
>
> Canting an engine is a very crude way of addressing a relatively transient
problem =93 Fitting a rudder trimmer would be a far more elegant solu
tion =93 should it be necessary.
>
> Hope that wasn=99t too long winded!
>
> Nigel
>
> PS the roll issue has nothing to do with engine position.
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