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JonathanMilbank
Joined: 14 Apr 2012
Posts: 395
Location: Aberdeen area
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 Posted: Wed Jan 18, 2017 12:53 am Post subject: Re: 1.5 degrees right? |
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After consulting another aviation guru and asking him to read this interesting discussion, he emailed this reply which I'll paraphrase:
"...the point about differential thrust around the rotation of the blade being sufficiently problematic to be worth disregarding the recommendation to have an offset.
He is concerned about a 1.5 degree offset. But in reality, if the slip ball is not kept well centred by the pilot there will be a yaw angle between the fuselage and the relative airflow and 1.5 deg seems minimal. I have never noticed any issues with strange vibrations etc from being slightly out of balance. And even more relevant, the effect he mentions isn't just about a lateral angle between prop axis and direction of airflow, the exact same situation arises when there is a vertical angle between the prop axis and the relative airflow. Obviously this angle varies as the angle of attack varies, due to flying at differing weights, speeds, and g loadings. The angle of attack can probably vary by at least 10 degrees between low and high speed flight and this makes 1.5 degrees pale into insignificance. Also if you consider the prop tip speed vs the forward speed, the effect is diluted.
Of course the aircraft will be quite flyable with or without the offset, but I think there will be less yaw with power effect with the offset. It's a bit like having differential ailerons - not essential but it makes the aircraft easier and more pleasant to fly (unless you like flying upside down!) and it will require less right rudder or right brake on a take-off roll which is surely a good thing from a performance point of view."
Therefore I'm going to accept that my engine and cowlings will from now on have a 1.5 degree offset to the right. Thanks again to everyone and particularly to Nigel for a challenging and different perspective. It made me think harder before deciding.
Remember Ivan Shaw's sage advice? "Think thrice. Measure twice. Cut once."
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kaarsberg(at)terra.com.br
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| Posted: Thu Jan 19, 2017 2:31 am Post subject: 1.5 degrees right? |
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Very interesting stuff, only one question to this: don't forget to take the gyroscopic effect into account of the increased thrust on the bottom half, bringing it to.... the left side, increasing the effect to the right further??
Alex Kaarsberg
On 16 Jan 2017, at 01:07, Robert Borger <rlborger(at)mac.com (rlborger(at)mac.com)> wrote:
| Quote: | Nigel,
Thanks for the detailed description. I have often wondered about the effectiveness of the offset. It just didn’t seem right to me. At some point in the future I’ll have to unbolt the engine for something. When I do, I’ll remove the offset and see what difference it makes.
Blue skies & tailwinds,Bob BorgerEuropa XS Tri, Rotax 914, Airmaster C/S Prop (75 hrs).Little Toot Sport Biplane, Lycoming Thunderbolt AEIO-320 EXP3705 Lynchburg Dr.Corinth, TX 76208-5331Cel: 817-992-1117rlborger(at)mac.com (rlborger(at)mac.com)
On Jan 15, 2017, at 4:08 PM, nigel_graham(at)m-tecque.co.uk (nigel_graham(at)m-tecque.co.uk) wrote:
--> Europa-List message posted by: nigel_graham(at)m-tecque.co.uk (nigel_graham(at)m-tecque.co.uk)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 it’s done because “that’s how it’s 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 you’re 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 that’s true, why has nobody noticed this pitch up attitude?” – a good question (even though I asked it myself).All Europa’s 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-draggersThe 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 wasn’t too long winded!NigelPS the roll issue has nothing to do with engine position.
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budyerly(at)msn.com
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| Posted: Sat Jan 21, 2017 1:38 pm Post subject: 1.5 degrees right? |
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Nigel my comments are in quotation marks as replies on this website are difficult to use insertions.
Your Comment:
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 – I’m 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 change in pitch of an airplane from takeoff, climb, cruise at differing speeds and descent speeds is more effective change of the pitch than torque offset. We know that propellers are designed to take these dynamic loads. That is why I am not a fan of propeller blades with limited lifetimes of only a few years now being sold. I was able to actually break one of these blades by lying flat on the table and pulling up on the tip and breaking it with my scrawny hands. Good props have very long life, shrug off the elements, bugs, rocks, and are not subject to any concern about the variances in angle of attack, yaw or normal dynamic loads. As an example, think about the downward moving blade and the upward moving blade at a small angle of attack say during climb out. The effective difference between the upward and downward blade is about 5 degrees and the prop just shrugs off this minor gnat of a force. Props are designed for this loading."
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.
"A bit of apples and oranges here: Identical prop with only a change in offset on a racing plane is not a good example. One off custom means just that, custom and one must look at the variances in build tolerance. A good manufacturer will have very few defects, but there are still defects so most blades are overbuilt except for extreme needs. Too many if’s and questions on that example as he changed more than one thing, blade and engine angle, which means mounts and cowl and many other little things we don’t know about…. Blades on an O-200 turning some 3500+ RPM and pulling 4-5 Gs goes through much more than offsetting a prop a degree or so on a small engine. The gyroscopic, pitch change on a racing prop limits their life. More than likely, the culprit may have been bad trim from the start and a worn out prop. Who knows for sure."
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.
"I think rudder trim is a glorious idea and feel it is worth the weight and drag from a pure convenience point. Go for it. I too don’t want to spend the time. I’ve got everything for it for 12AY but I just don’t have the time. I even like using springs rather than tabs to just tug a bit one way or the other using a trim motor setup. "
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!
"Again, the lifting stabilator on the Europa has a drag bucket associated with the small angle of attack near neutral on the series six type laminar airfoil used. The increase of drag for a tail force down is exceedingly small on a Europa. However the change in main wing angle to offset this due to an excessively forward CG will slow the plane down considerable. The aircraft longitudinal stability is more important than the drag as is a more aft CG for speed as it decreases tail down force a bit."
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 completely agree with that, plus a good pilot is a lazy pilot and a well trimmed airplane, flown smoothly is more efficient in its operation and a delight to fly in."
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, don’t 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”.
"I am working with Creighton Smith on 96EG, a classic with 80 HP and only ¾ inch of offset. It flies fine at 5000/24 inches. Anywhere else on the power curve and he must add rudder. If he had a 914 he would be seeing a different story on the takeoff rudder needed to keep straight and would tire without rudder trim. By the way, he too wishes he had rudder trim. Just not enough hours to allocate to justify modification. So like most of us he just leans on the right rudder if flying at 5500, and left on a 4600RPM saunter. Every experimental is slightly different so one knows apples and oranges are different but are still fruit and a Europa built and equipped by one builder will be slightly different than another. There is little homogeneity in home-made airplanes.
Trim it up and enjoy the flying.
Bud"
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JonathanMilbank
Joined: 14 Apr 2012
Posts: 395
Location: Aberdeen area
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| Posted: Thu Mar 23, 2017 6:25 am Post subject: 1.5 degrees right? Follow-up report |
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After 4 months of working in a freezing hangar, I finally flew my aircraft again a few days ago. The engine is now mounted pointing 1.5 degrees right as per the XS build instructions, whereas hitherto it was mounted pointing straight ahead on the Classic frame.
Using the same power settings in the cruise there is no discernible difference in performance and instead of the ball sitting half out to the right, it now sits very slightly left. During take-off or go-around there is noticeably less tendency to swing left when full power gets applied.
Nev Eyre's cowls do a superb job of cooling, to the extent that I've had to blank off part of the radiator core with Gorilla tape to make both oil and coolant temperatures run at about 90C in the cruise. Perhaps I'll fit a coolant thermostat some day. The oil temperature seems just about right without blanking off the core, because I took Nev's good advice and used the smaller oil cooler from the Classic installation.
Altogether a good result.
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rogersheridan(at)mac.com
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| Posted: Thu Mar 23, 2017 7:00 am Post subject: 1.5 degrees right? |
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Nice one Jonathan, would like to see a picture sometime.
Cheers,
Roger
| Quote: | On 23 Mar 2017, at 14:25, jonathanmilbank <jdmilbank(at)yahoo.co.uk> wrote:
After 4 months of working in a freezing hangar, I finally flew my aircraft again a few days ago. The engine is now mounted pointing 1.5 degrees right as per the XS build instructions, whereas hitherto it was mounted pointing straight ahead on the Classic frame.
Using the same power settings in the cruise there is no discernible difference in performance and instead of the ball sitting half out to the right, it now sits very slightly left. During take-off or go-around there is noticeably less tendency to swing left when full power gets applied.
Nev Eyre's cowls do a superb job of cooling, to the extent that I've had to blank off part of the radiator core with Gorilla tape to make both oil and coolant temperatures run at about 90C in the cruise. Perhaps I'll fit a coolant thermostat some day. The oil temperature seems just about right without blanking off the core, because I took Nev's good advice and used the smaller oil cooler from the Classic installation.
Altogether a good result.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=467541#467541
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JonathanMilbank
Joined: 14 Apr 2012
Posts: 395
Location: Aberdeen area
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| Posted: Sun Mar 26, 2017 10:57 am Post subject: Re: 1.5 degrees right? |
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Picture as requested
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