Hi Bud,
Another gem of a post that will be copied into my burgeoning "Bud's
Tips" folder.
I agree with nearly all of what you say but as is always the case; a
good explanation prompts another question.
You have discussed the aerodynamic pros and cons, but of equal
importance is the mechanical consequence of an offset engine.
The apparent pitch difference of blade rotating around an offset
engine will be double the engine offset. A blade will experience a
change from -1.5 to +1.5 so a delta of 3 degrees. Now if a customer
of yours came to you with a fixed pitch Warpdrive, and you discovered
he had set one blade 3 degrees coarser than the rest Im pretty sure
you would send him away with a flee in his ear and tell him to correct
it. But we seem quite happy to accept the same degree of blade
variance if we offset our engine without question.
The constantly fluctuating angle of attack introduces cyclic loading
and unloading of the blades which can produce torsional vibration.
This is then transmitted to the swash plate and drive pins of the
constant speed unit which is not a good thing.
A good buddy of mine (and one of your fellow countrymen) has a
successful business designing and building custom propellers and wings
for the Formula 1 Reno boys. He showed me a picture of one of his
custom two-blade fixed pitch props that had failed catastrophically
during a race. It was unusual in that it had split from tip to root.
All calculations indicated that it should have been more than strong
enough to absorb the power; however the prop had failed due to extreme
torsional oscillation along the blade length. By working closely with
the owner, he was able to establish that the plane builder had offset
the engine. A new (identical) prop was made, the engine offset zeroed
out and the racer went on to win gold with no further problems. It
was during one of our sitting by the pool drinking beer and talking
planes sessions that he first asked what I was going to do regarding
the offset of my then new Europa. it got me thinking.
Now with regard to the suggestion of fitting a rudder trimmer (purely
hypothetical as I have no plans to take a saw to my fin), I was a
little surprised by the strength of reaction to this suggestion on the
grounds of increased drag. I am also curious as to why our attitude to
controlling yaw trim is so different to the way we control pitch trim.
The pitch control system uses an all flying tailplane with a (very
good) trim system. According to Don Dykins book, during the cruise,
the tailplane is constantly flying the rear of the aircraft
downwards to maintain level flight. This will be generating drag too
but we accept this without question!
When considered against the cumulative drag created by the nose wheel
, main gear, outriggers tailwheel, flap hinges, GPS antenna fuel
vents, strobes and door handles, surely, the extra drag of a rudder
trimmer would pail into insignificance but surely make life so much
easier in the climb out or cruise.
I have re-read my original response to Jonathan and I stand by
everything I said. I think the explanation of the aerodynamics is
accurate (certainly nobody has challenged it) and my advice that if
it ain broke, dont fix it also stands.
For every pilot happily flying a Classic with no engine offset and
reporting no problems, there seems to be another flying an XS with
offset experiencing problems. I suspect that the real explanation is
muscle memory. Experienced feet will move by themselves without the
owner even being aware. Those with slower feet will find themselves
behind the curve.
Keep the thoughts coming.
Nigel
-----------------------------------------------------------
Quoting Bud Yerly <budyerly@msn.com>:
> To All,
> An airplane is 20,000 compromises flying in close formation. Each
> change we make affects others, and some are necessary to fine tune
> the aircraft for normal operations.
>
> Off the very empty top of my head:
> From an aero standpoint, fuselage squareness, tail squareness, wing
> incidence differences in construction, etc. cause most of our trim
> problems at cruise in the Europa.
> As for rudder/fin offset vs. engine offset to correct for P factor,
> drag due to tail area for an offset to counteract the slipstream and
> P factor affect is small at cruise for each. However, in a full
> power 914 with Airmaster on takeoff and climb out, the rudder
> required to hold yaw in check is reduced with offset. Interesting
> to note is in propeller testing, if you trim a plane for perfect
> ball with a fixed pitch, then put on a constant speed, a touch more
> right rudder is required to hold the wings level and the ball
> solidly in the center. Reduce power on the Constant Speed prop and
> suddenly left rudder is needed. I prefer engine offset myself as
> the offset is really small (a degree or so) to accommodate P factor
> and slipstream yaw. Offsetting the rudder/fin on a short fuselage
> and induced drag becomes a factor. A one degree offset of the
> rudder is .01 Cd which is not much drag or about 5 pounds per degree
> on a 15 sq ft fin/rudder, but it is drag, whereas prop offset drag
> is nill and the loss of thrust is nill also at such low angles. I
> agree with the Europa designers, as does David and others, drag is
> drag. On Classics in my shop, the offset on an 80HP fat nosed
> Classic was very little. Lets face it, the instructions said align
> the engine with the cowl. If you cut the cowl, who knows what you
> have without measuring. But on the Classic, that was not mentioned.
> Oh well, progress was made with the XS.
>
> The light propeller aircraft is trimmed for one airspeed and power
> setting. Change speed, weight (angle of attack), propeller length
> or pitch, or power setting and the trim in pitch and roll due to yaw
> changes. Since the fuselage longitudinal axis and engine are at the
> same pitch setting, the small yaw offset of the engine is not going
> to be much, nor is a yaw due to fin trim. Normally, the 1 and 1/16
> inch offset is fine and if not, I decrease the length of the
> right/left rudder cable about inch to spring the rudder over a bit
> on some aircraft to trim the ball. However, for any roll and yaw
> combined problem, I would look at a droopy flap. Somehow flaps
> sometimes get leaned on and just a 1/16 of an inch flap droop is
> quite a bit of roll and yaw. Whereas changing the engine offset
> will be very little yaw (and small roll).
>
> Fly the plane and center the ball. If it is still rolling it is
> most likely your wing trim. Either a flap is drooping, or somehow
> spring has been built into your ailerons. (Spring in the ailerons
> is seen when the stick does not stay exactly where it is put on the
> ground. Some builders build in problems that cause the aileron to
> spring one direction or the other.) Also look at your wing drag
> covers, and pants on a Trigear for alignment. A droopy wheel pant
> due to rough field operations is quite a bit of drag as are the wing
> flap bracket covers if misaligned.
>
> When the ball can be centered and all roll stops (pat yourself on
> the back for well trimmed wings) your problem is most likely P
> factor. Shorten the cable on the side of the rudder is depressed
> will normally fix it. If that is not enough, then a tab (or
> stronger spring) is necessary on that side. Keeping a plane well
> trimmed out is a constant problem. Every 5 years, look hard at the
> plane as due to wear and tear, things get out of alignment. Any
> change from your original fly off trim settings must be
> investigated, as something has changed.
>
> Just my thoughts.
>
> Bud Yerly
> Im pulling in 12AY soon for Sun n Fun prep. Join us this year in
> Lakeland and meet up with Club members and Europafiles alike at site
> N-55 again this year and get away from the winter doldrums. Our
> display emphasis this year will be on the Airmaster propeller which
> was a game changer for transforming the Europa and many other
> aircraft into efficient cruising airplanes.
>
> From: owner-europa-list-server@matronics.com
> [mailto:owner-europa-list-server@matronics.com] On Behalf Of
> davidjoyce@doctors.org.uk
> Sent: Monday, January 16, 2017 5:42 AM
> To: europa-list@matronics.com
> Subject: Re: Europa-List: Re: 1.5 degrees right?
>
>
> Nigel, following on from this, I am most reluctant to believe that
> Ivan Shaw and Don Dykins (one of the outstanding aerodynamicists of
> our time!) got it wrong, or for that matter generations of designers
> of Spitfire/Hurricane era aircraft - which I believe al had engine
> offset and fin offset incorporated, but even so were close to
> unmanageable if full power was used on take off. Performance then
> was very much a matter of life and death and an immense amount of
> research went into optimising performance - I certainly don't buy
> the notion that folk have always done it because someone did it
> back in the dark ages and no-one has thought rationally about it
> since!
>
> Offsetting the engine 1.5 degrees makes negligible difference to
> forward thrust - actually reduces it by just 0.03%, but using
> permanent right rudder induces extra drag which must be an
> appreciably greater amount.
>
> Regards, David Joyce, GXSDJ
>
>
> On 2017-01-16 09:58, JonSmith wrote:
>
> <jonsmitheuropa@tiscali.co.uk<mailto:jonsmitheuropa@tiscali.co.uk>>
>
>
> Hi Graham, I found your posting most interesting and thought
> provoking. You go beyond my old "pilot book"! We are all familiar
> to a certain extent with the old classic theories of propeller
> effects; torque, asymmetric blade, gyroscopic etc but I'd never
> considered detrimental effects caused by an engine offset.
>
>
> I remember quite well the famous diagram of the propeller slipstream
> helix spiralling around the fuselage and whacking the fin on one
> side or the other depending on direction of engine rotation - in our
> Rotax-Europa case the left side of the fin causing the nose to yaw
> more and more to the left with increasing power. Certainly in my
> aircraft on the rare perfectly calm and smooth day with no other
> influencing factors I notice on take off that at the point of lift
> off a considerable amount of right rudder input is required at that
> point, nearly half I'd say.
>
>
> I suppose that aircraft design is always a compromise and there are
> various solutions to counter this problem, engine offsets, rudder
> trims (fixed and inflight adjustable), offset fins etc, all designed
> to help make life easier for the poor pilot who has to cope with the
> cacophony of forces his machine is constantly bombarded with! Our
> Europa is as basic as you can get in it's standard form with the
> options of an offset engine mounting and/ or fixed rudder trim tab
> or nothing!
>
>
> I note and accept what you say about an engine offset causing
> inefficiencies and undesirable handling tendencies and that from a
> perfect performance point of view it would be absolutely the best
> for the engine to be mounted square on to the airflow but wouldn't
> the overall effect of the thrust vector being offset completely
> outweigh these minor undesirable tendencies and make life easier for
> the pilot? My instinct tells me that the unwanted effects would be
> relatively insignificant but I genuinely don't know....!
>
>
> I've always considered that aircraft compromised by simplicity would
> in the ideal world be set up to fly perfectly straight and balanced
> with hands and feet off in the cruise as that's what we spend most
> of the time doing. Thus in a perfect aircraft with the engine
> correctly offset this should be achieved without any extra trim
> tabs, assuming the designer got his sums correct with the offset!
> (I'm lucky I think because my aircraft seems to achieve this quite
> nicely!). I also note that you believe a rudder trim tab to be a
> better solution than an engine offset. Do you consider that having
> the rudder permanently offset into the airflow to keep the aircraft
> balanced to be more efficient than the minor unwanted propeller
> blade effects caused by having an engine offset? Again I'm only
> asking the question because I genuinely don't know..!
>
>
> I believe that a correctly offset engine will assist the pilot
> during take off by reducing the amount of right rudder deflection
> required throughout. Without any offset to help, surely more right
> rudder deflection would be needed to keep straight thus effectively
> reducing the maximum crosswind component from the left that the
> aircraft itself could cope with? A rudder trim would not help this
> situation of course - it might make reduce the load on the pilot's
> leg but the actual rudder deflection is still required.
>
>
> As I say, an interesting post, I'm very open minded but am yet to be
> convinced that I have made a mistake by following the manual and
> building mine WITH the quoted 1.5 degree offset....!
>
>
> --------
>
> G-TERN
>
> Classic Mono
>
>
> Read this topic online here:
>
>
> http://forums.matronics.com/viewtopic.php?p=465129#465129
>
>
> ttp://www.matronics.com/Navigator?Europa-List
>
> ics.com
>
> .com
>
> .matronics.com/contribution
|