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Author Topic: Specific fuselage design for cw rotating props?  (Read 2297 times)

Online Peter Germann

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Specific fuselage design for cw rotating props?
« on: May 31, 2017, 05:16:11 AM »
Why do I fly pusher props?

1.) I do all of my practice flying on a hard surface circle, w/o an assistant or stooge retaining the aircraft on take-off. With electrics and using tractor props, the airplane noses in when the motor spools up at the beginning of the take-off run. Depending from motor spool-up/propeller/RPM setup, the effect is anything from unpleasant to aggressive and dangerous. Reversing rotation and using pusher propellers solved the problem as it made the nose go out during acceleration, thus keeping line tension time safe during acceleration to lift-off speed. As a welcome side effect, the CW (pusher) rotation allows slow RPM spool-up which in turn makes initial ground roll and lift-of control quite easy.

3.) Due to the added mass of the electric motors rotor, gyroscopic forces being generated by electrics are higher than those of IC drive trains. From this and with tractor props, the resulting force when doing outside loopings/corners yaws the nose of the airplane in. This reduces line tension in the upper looping of the vertical eight and in corners two and three of the hourglass manoeuvre. Reversing rotation and using pusher propellers inverts the effect and consequently increases line tension in the critical same segments of those two manoeuvres.

From the above, I’ve built and flown quite a number of (electric) F2B airplanes being equipped with cw rotating pusher propellers over the past 8 years.  All of their fuselages were laid out conventional, i.e. with the motor something like ¾ to 1 ¼ in above the wing centerline and the empennage up to 1 ¾ in up, too. Some had their motor thrust axis approx. 1 ½° down and/or a stab incidence angle of 1° up, too. All flew well in manoeuvres but suffered, some more and others less, from the same weakness:
Altitude holding in level, and more so inverted flight was tricky and required constant pilot attention. From what I have read and observed, other flyers have occasionally experienced the same and I believe to understand that this phenomenon was then called “e-hunting”.

In 2016 I have then built an in-line airplane, and, which in order to get in-line drag, was equipped with a retractable landing gear. See “List your setup”,”Symmetria” post 171, Feb. 25 2016. It flies very well and altitude holding is much easier the ever before.

Encouraged by ”Symmetria” and in order to quantify the effect of CW gyro force and define compensation fuselage design parameters, I’ve teamed with Wolfgang Nieukamp. From his much appreciated calculations the resulting measures are:

Motor thrust axis below wing centerline: 10 mm (3/8”)
Motor thrust axis up: 1.2°
Empennage above wing centerline 20 mm (3/4”)
Empennage LE radius 0.5 mm (0.018")
Stabilizer incidence down: 1°
Opposite direction operating (out with elevator up) Rabe rudder
Fixed landing gear

A new airplane (Aerodynamics as per Igor’s Max Bee) was then built and I have just done the last of the first couple (20) flights today.
It seems to perform very good, doing “Shark” like corners with clean exits. Altitude holding in level and inverted flight is effortless. See “List your setup”,”C.29” post 179, May. 25 2017.

It is of course much too early to draw further reaching conclusions but I thought it might be interesting to share this with the community.

Edited July 13 2017: 
Reasons for pusher added
C.29  now regularly flies  in competition and proves to fly fully symmetric, w/o "e-hunting" in level and inverted flight
Peter Hofacker's (SUI) new airplane built around the same numbers as C.29 functions as expected.
« Last Edit: July 13, 2017, 07:11:45 AM by Peter Germann »
Peter Germann


Offline Wolfgang Nieuwkamp

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Re: Specific fuselage design for cw rotating props?
« Reply #1 on: May 31, 2017, 12:49:03 PM »
Peter, could you show a picture taken from the front?
It shows better your radical approach.

Regards,
Wolfgang

Offline jfv

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Re: Specific fuselage design for cw rotating props?
« Reply #2 on: May 31, 2017, 01:31:56 PM »
Peter:

Do you have right thrust on the motor?
Jim Vigani

Offline Dennis Adamisin

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Re: Specific fuselage design for cw rotating props?
« Reply #3 on: June 01, 2017, 04:15:58 AM »
CONGRATS Peter & Wolfgang on developing and now proving out the new design parameters.  In addition to the promising performance, I think Peter's design also resulted in an attractive contemporary appearance - it LOOKS right!

Denny Adamisin
Fort Wayne, IN

"Sacred cows make the best burgers."
                             - Unknown

Online Peter Germann

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Re: Specific fuselage design for cw rotating props?
« Reply #4 on: June 01, 2017, 05:30:02 AM »
Do you have right thrust on the motor?

Yes, 1° out.

Started flight testing the APC 13 x 5.5 WEP prop today at 9'720 RPM set (9'650 logged) and 5.2 sec/lap. Compared to the APC 12 x 6 EP at the same RPM, here are the first findungs:

Nice, not overly sharp corners
Altitude hold ok
Solid look & feel and sufficient agility
Much better overhead line tension
12% higher energy consumption  (2'130 mAh re-charge)
Insufficient headroom of the AXI 2826/13 710 RPM motor on 5S constant speed, reaching 100% power output near end of flight.

I will now switch to the AXI 2826/12 760 RPM and report.

June 2nd 17: With the 2826/12 760 now driving the APC 13 x 5.5 EP at  9'650 RPM logged for better overhead line tension, the ESC reaches 91% power output near end of flight. As this was logged when using a 5S battery with 60 cycles on it,  chances are that there is sufficient headroom now.


« Last Edit: June 02, 2017, 05:00:18 AM by Peter Germann »
Peter Germann

Offline Avaiojet

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Re: Specific fuselage design for cw rotating props?
« Reply #5 on: June 02, 2017, 11:25:38 AM »
Fuselage offset?

CB
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Online Peter Germann

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Re: Specific fuselage design for cw rotating props?
« Reply #6 on: June 03, 2017, 08:44:18 AM »
Fuselage offset?
CB

Inner wing panel is 755 mm (29.72") and outer 745 mm or 29.33 in
Peter Germann

Offline Brett Buck

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Re: Specific fuselage design for cw rotating props?
« Reply #7 on: June 03, 2017, 09:41:00 AM »

It is of course much too early to draw further reaching conclusions but I thought it might be interesting to share this with the community.


     It's an interesting observation and other people (with the capability of evaluating it) are working on similar ideas.

    Brett

Online Peter Germann

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Re: Specific fuselage design for cw rotating props?
« Reply #8 on: June 04, 2017, 03:10:40 AM »
Its interesting to learn that others are thinking in the same direction. Wolfgang Nieukamp and me, we will be happy to share whatever we have so far...


Corrigenda:
Motor is AXI 2826/12 760 (not 2826/13 710 which does not work well when run on 5S in constant RPM mode)
Battery is 5 mm more up  (vertical c.g)
Peter Germann

Offline Brett Buck

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Re: Specific fuselage design for cw rotating props?
« Reply #9 on: June 04, 2017, 05:35:56 AM »
Its interesting to learn that others are thinking in the same direction. Wolfgang Nieukamp and me, we will be happy to share whatever we have so far...


Corrigenda:
Motor is AXI 2826/12 760 (not 2826/13 710 which does not work well when run on 5S in constant RPM mode)
Battery is 5 mm more up  (vertical c.g)

   Do you have a proposed mechanism? Right off the bat, it's not clear why the thrust line or wing/stab arrangement would need to be different for clockwise rotation. I can easily see why it has to be trimmed differently in other respects, but not why changing the thrust line would be important.

  I am being intentionally vague because I have been included in several independent efforts and I am not sure that everyone involved is prepared to share their results.

      Brett

Online Peter Germann

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Re: Specific fuselage design for cw rotating props?
« Reply #10 on: June 05, 2017, 02:28:57 AM »
We thought that the amount of compensation required to cope with the nose down gyro moment of the cw rotating prop, spinner and rotor assembly, done by thrust axis up and stabilizer incidence down, could be reduced by making use of wing and empennage drag, too. Furthermore, with wing and empennage above the thrust axis, the resulting nose-up moment helps compensating the nose-down moment of the landing gear. Due to the added inertia of the rotating magnets the results up to now suggest the following mechanism:

  • For tractor props, the motor should be above the wing centerline, with 0 to 1,5° downthrust. Stabilizer should be BELOW the wing centerline.

  • For pusher props, the motor should be below the wing centerline, with 0 to 1,5° upthrust. Stabilizer should be ABOVE the wing centerline.

Wolfgang Nieukamp & Peter Germann
« Last Edit: June 06, 2017, 02:26:53 AM by Peter Germann »
Peter Germann

Offline Brett Buck

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Re: Specific fuselage design for cw rotating props?
« Reply #11 on: June 05, 2017, 09:33:34 AM »
We thought that the amount of compensation required to cope with the nose down gyro moment of the cw rotating prop, spinner and rotor assembly, done by thrust axis up and stabilizer incidence down, could be reduced by making use of wing and empennage drag, too. Furthermore, with wing and empennage above the thrust axis, the resulting nose-up moment helps compensating the nose-down moment of the landing gear. Due to the added inertia of the rotating magnets the results up to know suggest the following mechanism:

  • For tractor props, the motor should be above the wing centerline, with 0 to 1,5° downthrust. Stabilizer should be BELOW the wing centerline.

  • For pusher props, the motor should be below the wing centerline, with 0 to 1,5° upthrust. Stabilizer should be ABOVE the wing centerline.

      I see your reasoning and it's same I used in some post here a few years ago. I am not entirely convinced about it, however, since I have seen various people  adjust their systems using the same reasoning, without obvious results. And also, people do careful controlled experiments where they changed the prop rotation, and have good results with their "thrust line" and other associated changes with the prop rotating in either direction!

     Brett

Online Peter Germann

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Re: Specific fuselage design for cw rotating props?
« Reply #12 on: June 06, 2017, 02:33:37 AM »
Swiss flyer Peter Hofacker will have his new airplane, with the same pusher prop layout as C.29, ready for flight within just a couple of weeks from now. We will report findings...
Peter Germann

Offline Brett Buck

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Re: Specific fuselage design for cw rotating props?
« Reply #13 on: June 06, 2017, 08:35:49 AM »
Swiss flyer Peter Hofacker will have his new airplane, with the same pusher prop layout as C.29, ready for flight within just a couple of weeks from now. We will report findings...

   I would also suggest that once you get it working well - try swapping the direction and retrimming, then compare the two.

      Brett

Online Peter Germann

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Re: Specific fuselage design for cw rotating props?
« Reply #14 on: June 06, 2017, 09:50:51 AM »
   I would also suggest that once you get it working well - try swapping the direction and retrimming, then compare the two.
Brett
Excellent suggestion, thank you. Willdo. However, with contests coming up pretty soon, it might take a while before I can report
Peter Germann

Offline Wolfgang Nieuwkamp

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Re: Specific fuselage design for cw rotating props?
« Reply #15 on: June 06, 2017, 11:35:33 AM »
In addition to the prop rotation swapping, also the Rabe rudder direction would have to be swapped. Not so easy!

Offline Brett Buck

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Re: Specific fuselage design for cw rotating props?
« Reply #16 on: June 06, 2017, 01:19:19 PM »
In addition to the prop rotation swapping, also the Rabe rudder direction would have to be swapped. Not so easy!

  You shouldn't be using a Rabe rudder in the first place, but the overall contribution is negligible if you have everything else correct. Just use a fixed rudder and I guarantee it can be made to work just as well.

    Brett

Online Peter Germann

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Re: Specific fuselage design for cw rotating props?
« Reply #17 on: June 09, 2017, 03:03:42 AM »
The in-manoeuvre precession moments & directions calculated for the Fiat C.29 setup (E-Pusher, including rotating magnets mass) are:
Inside looping, dia. 16 m: 0.4 Nm Nose-In
Inside corner, radius 3 m: 2 Nm  Nose-In
So far my small size rudder, deflecting 15° out with full up elevators, seems to do a good job compensating precession induced yaw as line tension in in- and outsides feels to be equal and no yaw in turn/corners has been observed.
Peter Germann

Offline Dennis Toth

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Re: Specific fuselage design for cw rotating props?
« Reply #18 on: July 04, 2017, 04:54:19 PM »
Peter,
I have noticed the thrust line nose down effect on models that I have converted to electric with CW props. The first one I did was my Stuka. With the CW prop it was quite noticeable that it would hug the ground waiting for you to apply up elevator to come off. On my Tutor II which was my first electric the same tendency was noticed changing from CCW to CW props of the same size and pitch. Once in the air both ships were fine and the outside maneuvers were significantly more solid. I have both ships set with the motor thrust line at 0 deg, no rudder offset and no stab incident.

On thing I noticed on the plan layout was the leadout position was quit forward looks like just about in line with the CG. This is similar to the  Yatsenko Shark layout. We have had a discussion on the line rake in another thread and using the LineIII program it wants to get the leadouts back from the CG a significant amount for the line diameters and length commonly used in the US. Can you elaborate on your thinking about the Leadout position.

Best,    DennisT

« Last Edit: July 06, 2017, 11:14:03 AM by Dennis Toth »

Online Peter Germann

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Re: Specific fuselage design for cw rotating props?
« Reply #19 on: July 06, 2017, 04:56:36 AM »
Hi, Dennis

The following is a rather simplified answer, not taking into account the significant effects of yaw forces generated by the rudder and by the motor thrust axis out. It is an attempt to explain why rake angles calculated are typically too small for C/L stunt airplanes:

Based on centrifugal force and lines drag, software tools such as Line III and Line Rake Analysis (http://nclra.org/Programs/LineRake.php) are returning line rake angles required to get drag minimizing fuselage tangency in level flight of racing airplanes.

In case of a stunt airplane flying on top through the centre of the hemisphere, centrifugal force is momentarily reduced by the weight of the airplane while lines drag remains constant. At this critical moment, the calculated level flight rake angle (typical 1.6° or 0.8in rear of c.g.) is no longer sufficient to maintain fuselage tangency and the nose will yaw in.

Attempting to simulate the overhead passage situation by playing around with the calculators and by entering a weight of 1/3 less returns values around 2.4° rake angle or leadouts at 1.3 in rear of c.g.  Adding a bit of safety margin, such as to see “1/2 of the outside wheel”, then leads to leadouts position well behind the c.g. Such as 60 mm or 2.3 in for my C.29.
Peter Germann

Offline Dennis Toth

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Re: Specific fuselage design for cw rotating props?
« Reply #20 on: July 06, 2017, 11:10:14 AM »
Peter,
Thanks for addressing the question. You give a line of reason that makes sense and is inline with what Paul W and Igor are doing on their latest ships. It seems that not only do you have gravity up top but also just the slowing of the airplane as it goes to the plus 45 deg maneuvers will also reduce the centrifugal force. It seems the only thing that could counteract this would be for the flying speed to increase in the +45 maneuvers, not easy to get a smooth transition. Even the 4-2-4 doesn't increase speed enough but would help a little.

In the other line rake thread (in the open forum) Brett B questions the high rake approach (Paul & Igor) and the low rake of the Yatsenko Shark. He poses the question what does it do to the rest of the pattern and suggests either will cause unwanted consequences.

Is the approach to move the leadouts rearward to get just enough tension for the up top (above +45) then adjust any low level trim issues? Or, is it better to position them for max tension up top and just deal with the low level trim issues as they would be minimal?

Best,   DennisT

Offline Brett Buck

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Re: Specific fuselage design for cw rotating props?
« Reply #21 on: July 06, 2017, 12:42:07 PM »
In the other line rake thread (in the open forum) Brett B questions the high rake approach (Paul & Igor) and the low rake of the Yatsenko Shark. He poses the question what does it do to the rest of the pattern and suggests either will cause unwanted consequences.


   I will freely admit that I don't know what is going on in some of these cases, but it's very fundamentally different from what I have been doing, and it doesn't work in a general case for IC engines - plenty of people have tried, including those who can tell the difference, but not succeeded. But I cannot deny Paul's observational skills and being willing to try things and having the requisite knowledge to fairly assess the results is beyond question.

   the "fuselage design" question (which is being looked at by multiple competent experimenter)  and the leadout sweep question are really two different things. I am pretty well certain that most of the attempted explanations for either one are easily dismissable, but I am equally sure that there is a real explanation that we just haven't figured out yet. 

   It's hardly the only example - for instance, we still haven't figured out why one engine seems more powerful than another of identical design/manufacture/prop/rpm lap time, but it is clearly a real thing - even when it's the same engine with different oil on two consecutive flights.

    Brett

Online Peter Germann

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Re: Specific fuselage design for cw rotating props?
« Reply #22 on: July 10, 2017, 04:13:00 AM »
Peter Hofacker's new electric pusher prop airplane is now flying. It was built around the same numbers as my C.29:

Motor thrust axis 10 mm (0.4in) below wing c/l
Motor thrust axis 1.2° up
Empennage 25 mm (1in) above wing c/l
Stabilizer incidence 1° down
Sharp stabilizer LE
Rabe Rudder deflecting out with elevators up

Initial findings are equal to what I have experienced with the C.29:

Good tracking ("altitude hold") in level and inverted flight
Very little if any handle trim input required to hold altitude in inverted flight
No "e-hunt"
Equal turn rates for in- and outside loops, with very little if any elevator trim adjustment needed.


Until the end of the 2017 flying season we will now evaluate both airplanes further, trying to find out whether we possibly have a "proof of concept"



Peter Germann

Online Peter Germann

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Re: Specific fuselage design for cw rotating props?
« Reply #23 on: November 10, 2017, 05:11:34 AM »
Pusher or Tractor?

I would also suggest that once you get it working well - try swapping the direction and retrimming, then compare the two.

November 9th 2017 Pusher/Tractor Back-to-Back Flight Test
In its July 2016 configuration my Crossfire (list your setup 157  18.12.16), while being a competitive flyer, kept making it very difficult for me to accurately maintain 5 ft altitude in inverted flight. Aerodynamic modifications, such as sanding the stabilizer leading edge sharp and/or installing turbulators at various positions, did not help. From meanwhile made experiences with my Fiat C.29 (list your setup, reply 179, update from Nov. 9 2017) I suspected that the inverted flight instability might have something to do with the combination of the Crossfire's conventional  (motor & empennage above wing centerline and 0° thrust axis) fuselage layout and (pusher) propeller sense of rotation.
Without any further change to the trimmed airplane I yesterday changed the APC 12 x 6 EP prop to its tractor version 12 x 6 E, running at the same RPM. While slightly reducing line tension at unassisted take-off from hard surface and in outside loops/corners, the change has eliminated the altitude holding problem in inverted flight.
Peter Germann


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