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Author Topic: Stab incidence  (Read 11685 times)

Jim Roselle

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Stab incidence
« on: December 03, 2016, 03:05:30 PM »
I'm building a new profile plane around my repaired Twister wing and want to make sure I understand stab incidence. The stab should be built with positive (leading edge higher) incidence to combat the gyroscopic procession of the left turning flight pattern, correct?

How much incidence for a plane with "Fancherized" Twister moments, stab area 25% of wing area, 3.5 aspect ratio?

The wing centerline will be about 1cm below the thrust line.

Thank you,

Jim

Online Brett Buck

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Re: Stab incidence
« Reply #1 on: December 03, 2016, 03:28:26 PM »
I'm building a new profile plane around my repaired Twister wing and want to make sure I understand stab incidence. The stab should be built with positive (leading edge higher) incidence to combat the gyroscopic procession of the left turning flight pattern, correct?

How much incidence for a plane with "Fancherized" Twister moments, stab area 25% of wing area, 3.5 aspect ratio?

  I would use the smallest increment you can be sure exists. On my airplanes, it's ~1/64" over a chord about 5" -  21 thousandths of an inch.  After David's article on the topic (where he added the part about incidence at my suggestion), people have been using more and more incidence to the point of absurdity using the "stunt flier's maxim - if a little is good, a lot is better". Don't use the 2 degrees that Al recommends for his designs, that has pretty consistently proven excessive on normal airplanes.

     If you need more, you can always rig in down elevator on top if it. The most important finding of the entire experiment was that the tolerance of the pitch stab misalignment was all in one direction. You can tolerate a lot more positive incidence than you can tolerate *any* negative with a conventional layout. In other words, even a tiny bit of negative incidence can be almost fatal, but a fair bit of positive seems OK and predictable. I suggest when installing the stab, and you are getting down to the last little nudge one way or another, make sure the LE is up rather than down WRT your reference.

   If you put in too much, that's OK, but you might have to rig in up elevator to get the turn rate and the turn load the same in both directions. Note also that the tolerance in the other directions - tilt or skew WRT the wing - are essentially zero. That's why people are always trying to beef up profile fuselages, because you almost cannot keep it straight in flight.

    Brett

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Re: Stab incidence
« Reply #2 on: December 03, 2016, 07:03:39 PM »
If you draw a line AT the Stab , the Full Chord , at ZERO incedance , or put a strip of masking take on and draw on that ,

you can thereby Eyeball ' a bit ' relitive to THAT . a Bias !  1 mm on the stab is about 3/32 in real money , across the FULL CHORD of the Stab / Elevator . Being twice as long , its therefore GREATER ACCURACY . Though ( tho ! ) A ' near parrallel ' line a foot ong is more accurate still .
Particularly if your trying to fudge degrees . One is plenty of them , unless youve a semi scale ' wide seperation ' of datums , ship . H^^

Offline FLOYD CARTER

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Re: Stab incidence
« Reply #3 on: December 04, 2016, 04:58:06 PM »
A 1/64" stab measurement on a centerline drawn in with a pencil, with reference to wing c/L, which is difficult to determine due to large LE radius, is very hard to nail down with any accuracy.  Since I don't build from CAD-machined parts, I'm afraid my measurement tools are simply not good enough to reliably crank my stab up by a mere 1/64".

Just how is it done?
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Offline tom brightbill

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Re: Stab incidence
« Reply #4 on: December 04, 2016, 05:18:17 PM »
A hand full of Robart incidence meters is quite helpful.  A couple on the wings (without the flaps if possible) will set zero. If your stab is flat, then the dial incidence gauges from cheap a$$ tool can be used. There's enough space between 0 and 1 degree on the meter that Brett's 1/4 degree is possible. If the stab is not flat, then another Robart meter will work. And as Brett mentioned, don't forget checking the other two axis while doing your setup.
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Re: Stab incidence
« Reply #5 on: December 04, 2016, 07:35:49 PM »
Quote
Just how is it done?

Aye , well laddie . It canna be done ; UNLES . . .

If you put your completed tailplane on the board , with a even THING under the say 1/2 Sq LE , and get out your BALLPOINT
( If you DIDNT mark the C/L of the L.E. Before doing anything ! , and scrawl a line across , or slide IT across the PEN ,
then Flip Over , and Do the Other Side . Then get the other ( colour ) pen , and SPLIT the lines .

Same at T.E. , though a ruler across over & under , or offcuts. Straight Ones .  Pined , With Equal Packers at T.E. , for Full Chord Fitting .

Good Glasses , A Eyeglass , and Flashlight should do the rest . A ball Pen lines about a sixty fourth .
A SHARP 6H on Paper is about 1/`10 th of a Millimeter ! .

 ;D ;) H^^

Offline Target

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Re: Stab incidence
« Reply #6 on: December 04, 2016, 07:50:58 PM »
I always thought a mm was close to .040", so less than a 1/16"....
2mm is about 3/32" (a little under).

Vr,
Chris
Regards,
Chris
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Online Brett Buck

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Re: Stab incidence
« Reply #7 on: December 04, 2016, 08:13:13 PM »
A 1/64" stab measurement on a centerline drawn in with a pencil, with reference to wing c/L, which is difficult to determine due to large LE radius, is very hard to nail down with any accuracy.  Since I don't build from CAD-machined parts, I'm afraid my measurement tools are simply not good enough to reliably crank my stab up by a mere 1/64".

Just how is it done?

   Trammeling.

     Brett

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Re: Stab incidence
« Reply #8 on: December 04, 2016, 08:32:18 PM »
Pecisely .  

If one placed thin straight ( new ) pins in strategic places & had a set of feeler guages , the skys the limit . S?P

Offline Steve Helmick

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Re: Stab incidence
« Reply #9 on: December 08, 2016, 09:29:53 PM »
10" Sine Plate.  VD~ Steve
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Offline Ted Fancher

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Re: Stab incidence
« Reply #10 on: December 08, 2016, 11:38:06 PM »
I'm an old stunt geezer who doubts his ability to measure accurately such small dimensions.  On the other hand, since I was a kid I've done pretty well by doing the simple thing of building the wing and stab zero/zero to the best of my ability and trimming inside/outside turn responses with an adjustable flap/elevator push rod.  99% of the time I end up with some down elevator trim.  Only exception was the unnamed original version of the 1977 Concours winning Citation that placed tie 3rd with Bob Baron.  That original airplane (which is likely still flying for someone up in Washington state somewhere) required a noticeable amount of up elevator to equalize inside outside response.  My best guess after years of discussion about the subject is that the airplane was probably built with excessive positive stab incidence and required a reverse "band-aid".

Ted

p.s.  If anybody up in the Northwest knows the whereabouts (or grave site) of that old black ship I'd be interested to hear out it's doing.

Offline Howard Rush

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Re: Stab incidence
« Reply #11 on: December 09, 2016, 01:00:54 PM »
I last saw it with Bob Emmett about 20 years ago.
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Offline Mike Haverly

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Re: Stab incidence
« Reply #12 on: December 10, 2016, 02:15:52 PM »
Don Schultz spoke of many flights he put on a nice black model left in his good hands while an airline pilot was out of town.  Oh.....  never mind.
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Offline Mike Alimov

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Re: Stab incidence
« Reply #13 on: December 12, 2016, 07:28:19 PM »
Hmmm... if we are indeed compensating for gyroscpic precession in normal flight with positive stab incidence (or down elevator trim), shouldn't the plane require opposite trim when flying inverted?

Online Brett Buck

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Re: Stab incidence
« Reply #14 on: December 12, 2016, 08:43:06 PM »
Hmmm... if we are indeed compensating for gyroscpic precession in normal flight with positive stab incidence (or down elevator trim), shouldn't the plane require opposite trim when flying inverted?

  No. The precessional torque from yawing is fixed with respect to the body of the airplane (always positive pitch), so you always want to force the stick forward.  In the circle frame nose up when upright, and nose down when inverted.

    Brett

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Re: Stab incidence
« Reply #15 on: December 12, 2016, 10:03:13 PM »
P Factor and Gyroscopic procession are two differant THINGS.



This works real well if you stall out a turn & are hanging on the prop .  ;D
If its gone Four Stroke & leans of , The Acceleration , TOURQUE Effect , Combined wit The ' P ' Factor Effect , can require imediate footwork .

http://www.flyingmag.com/technique/proficiency/p-factor-real



This is Actually INERTIA & Relitive Mass .

Short video showing a Gnome rotary powered Sopwith Camel starting and then idling, which highlights the effect that the torque of the rotary engine has on the aircraft.

Balloney , its the INIRTIA of rotation !  VD~ Read a real very WW! Camel Pilots remarks regards training . If a pilot didnt quickly assimilate
the required inputs regariny GYROSCOPIC EFFECT , they DIED .



So P Factor , Gyroscopic Procession & Tourque arnt the same , then theres inertia & acceleration , At times theyre All Combined , cumulatively.  VD~






Offline Howard Rush

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Re: Stab incidence
« Reply #16 on: December 13, 2016, 03:33:48 AM »
P Factor and Gyroscopic procession are two differant THINGS.

This works real well if you stall out a turn & are hanging on the prop .  ;D
If its gone Four Stroke & leans of , The Acceleration , TOURQUE Effect , Combined wit The ' P ' Factor Effect , can require imediate footwork .

This is Actually INERTIA & Relitive Mass .

Short video showing a Gnome rotary powered Sopwith Camel starting and then idling, which highlights the effect that the torque of the rotary engine has on the aircraft.

Balloney , its the INIRTIA of rotation !  VD~ Read a real very WW! Camel Pilots remarks regards training . If a pilot didnt quickly assimilate
the required inputs regariny GYROSCOPIC EFFECT , they DIED .

So P Factor , Gyroscopic Procession & Tourque arnt the same , then theres inertia & acceleration , At times theyre All Combined , cumulatively.  VD~

The precession discussed here gives a moment about the pitch axis.

But yes, the airplane's response is to a vector sum of all the moments acting on it.
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Offline Peter Germann

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Re: Stab incidence
« Reply #17 on: December 13, 2016, 08:29:54 AM »
Attempting to summarize the effects of precession and P-effect I've tried to do a comparison sheet as attached. Due to lack of math knowledge in my side values shown for forces are dimensionless estimates only. Critical feedbacks would be very welcome, of course

Dec 14; comparison sheet edited
« Last Edit: December 14, 2016, 05:31:58 AM by Peter Germann »
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Offline Wolfgang Nieuwkamp

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Re: Stab incidence
« Reply #18 on: December 14, 2016, 09:16:47 AM »
Peter,

the idea of your table is good, but the values should be calculated where possible.

For example, the precession torque in level flight can be calculated as follows:

The moment of inertia J of a propeller (two or more blades) can be estimated by J=1/12 * mass * D^2.
If the mass of the prop is 24 grams, and D is 12 inch, then J prop=1/12*0,024 kg *0,1 m^2=0,0002 kg m^2.

The rotating part of an outrunner motor, dia 42 mm, will have a mass of about 0,1 kg. Assuming the rotor is 4 mm thick, we can use the formula for the inertia of a cylinder, J rotor= 1/2 m (r1^2 + r2^2). This leads to  J rotor=1/2*0,1(0,021^2+0,017^2) = 0,0000 kg m^2.
Total J = J prop +J rotor =0,00024 kg m^2.

The precession torque T (horizontal level flight) is T= J * omega1 * omega 2, where the omegas are 2*pi*rev/second.
So, assuming 9000 rev/min and a lap time of 5,3 seconds, we obtain omega1=9000/60 *2*pi, and omega2=2*pi/5.3
If you then fill in the values for J, omega1 and omega 2 you obtain  T=0,27 Newton*meter

For tractor props this torque is nose up, for pusher nose down.

Please let me know if you agree to this strategy. If you agree, I can continue to calculate the horizontal and vertical forces, and the resistance moments for tractor  and pusher.
Regards,
Wolfgang

Offline Tim Wescott

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Re: Stab incidence
« Reply #19 on: December 14, 2016, 10:13:14 AM »
 I would use the smallest increment you can be sure exists.

+1

Don't use the 2 degrees that Al recommends for his designs, that has pretty consistently proven excessive on normal airplanes.

Al uses airfoils with slight camber, yes?  If so then the zero lift angle will differ from the zero incidence angle -- so of course the stab will need to be tilted to match.
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Offline Igor Burger

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Re: Stab incidence
« Reply #20 on: December 14, 2016, 02:29:57 PM »

The moment of inertia J of a propeller (two or more blades) can be estimated by J=1/12 * mass * D^2.
If the mass of the prop is 24 grams, and D is 12 inch, then J prop=1/12*0,024 kg *0,1 m^2=0,0002 kg m^2.


This value is overestimated, real value for typical 12" pipe prop (not hollowed, narrow blade) is aproximately half of that.

Offline Chuck_Smith

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Re: Stab incidence
« Reply #21 on: December 14, 2016, 04:24:05 PM »
This value is overestimated, real value for typical 12" pipe prop (not hollowed, narrow blade) is aproximately half of that.

And very easily determined experimentally.
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Online Brett Buck

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Re: Stab incidence
« Reply #22 on: December 14, 2016, 08:17:01 PM »
This value is overestimated, real value for typical 12" pipe prop (not hollowed, narrow blade) is aproximately half of that.


  Indeed, Pete Soule' tested a bunch of props and for a wide range of different types, you can calculate the approximate radius of gyration as .226*diameter. Note that this works for 2 or 3-blade props, since it's based on the mass distribution of each blade, and you get 3/2 as much mass with  3-blade. Then the moment of inertia is mr^2 as usual.

     I measured a bunch of typical stunt props and it was surprisingly close, probably good to ~10% in most of the cases I tried.

     Brett

Online Lauri Malila

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Re: Stab incidence
« Reply #23 on: December 15, 2016, 04:07:23 AM »
Sorry for a very basic question, but maths has never been my strongest skill: How do changes in rpm, prop weight and prop diameter affect to this gyroscopic precession? What is linear, what is exponential? I'm asking because I am still pondering if molded hollow props would be worth the trouble compared to my current wooden props. L

Offline Igor Burger

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Re: Stab incidence
« Reply #24 on: December 15, 2016, 05:32:32 AM »
Sorry for a very basic question, but maths has never been my strongest skill: How do changes in rpm, prop weight and prop diameter affect to this gyroscopic precession? What is linear, what is exponential? I'm asking because I am still pondering if molded hollow props would be worth the trouble compared to my current wooden props. L

You can see it in Wolfgang's equatin, so the torque is:

linear to mass of prop
linear to rpm
square to diameter

But it all also depends on gemetric mass distribution in the prop. His equation 1/12 * mass * D^2 is for constant cross section body (rod, stick, prop wirth constant width and thicknees) but props are already optimised to limit both the precession and also motor load, so they have less mass at tip amd more close to hub, so it also depends how thinn you can make prop tips. When I spoke to Sandor Havran (maker of those nice wooden props used on ST60 and which Richie K. still use also on electric) the biggest problem was making of so thin but still resistant wood tips. That is reason why they have wider blades than carbon props. So it could easy be that also heavier carbone prop can be better then lighter wood prop. (for precession). I remember Ivan Cani already many years ago used wood props with glass on tips just for this reason (smaller and thinner tips).

And there is also another point of view - since it is liear to RPM and also to mass it is very good to check what happend if you have two props with identical power use and having 2blades on 12" vs 3 blades on 12" .....  or 11" running at 11 000 vs 12" running at 9000  (power for prop comes from power of 4 to prop diameter)

... and when you are done with calculations, then compare it to hollowed carbon props and you will see why to switch to electric finally :- )))))))))))) ... I know I know ... your home machines need some work, ok ok, I understand :- )))


Offline Peter Germann

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Re: Stab incidence
« Reply #25 on: December 15, 2016, 06:06:25 AM »
Thank you, dear friends, for your comments.

Because I find the positive effects of pusher props, such as better line tension in outside turns and no nose-in tendency during unassisted take-offs from hard surfaces, being of value for me, I wish to carry on using those props. I have, however, repeatedly experienced annoying altitude holding problems in level flight, and typically more so in inverted, with electric airplanes of the conventional design type i.e. motor and empennage above the wing center-line and all thrust / drag axis at 0°.
My latest model now is of in-line design with all components at 0° and it has retracts, too. Interestingly enough, holding altitude is no longer a problem in level flight and substantially easier to achieve in inverted. This makes me assume that the combined effects of precession torque and wing/gear drag influence level and inverted flight stability different. From this, it was the purpose of my attempt to define the combined direction of disturbing forces and compensation measures. This in order to build-in those compensation measures difficult to implement into a finished airplane.

Unfortunatey my attempt lacks the actual forces and I therefore would find it really helpful if I could count on contributions of friends offering their math expertise. In particular, if their results would give me an indication on how much we should tilt the motor up or down and/or add stab incidence in order to best possibly compensate the combined effects of precession and wing/gear drag torque.
Peter Germann

Offline Igor Burger

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Re: Stab incidence
« Reply #26 on: December 15, 2016, 06:31:36 AM »
better line tension in outside turns and no nose-in tendency during unassisted take-offs

I would say that those (and especially that second) problem is simply trimm issue which must be solved before comparing tractor and pusher props. If you want see best potential of one or the other, you must trim whole model for particular prop. If you siply change and you do not solve all tim problems, you are comparing trimmed and not-so-well trimmed model, not just props. Usual experience with classic models (not inlines like your) is that it comes clearlu better with tractor prop then pusher. I GUESS that inline will be neutral (tractor or pusher) somewhere in the middle between those two.

Model cannot turn inward when accelerates, if it does, then it is happening also during climbing to some extent and no wonder that you need some asistance in 2nd and 3rd corner of that mentioned houglass.

Offline Peter Germann

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Re: Stab incidence
« Reply #27 on: December 15, 2016, 07:36:01 AM »
As my airplane starts its take-off run with a bit of AoA, could it be the immediate nose-in is caused by the P-effect of the tractor, same as with Team Racers nosing-in violently at take-off?

Clarification, as exeperinced with three different airplanes (2 conventional & 1 in-line design):
The nose-in/out effect I am referring to happens only when taking-off from a hard surface without helper or stooge: As soon as the motor spools up sufficiently to make the airplane begin its take-off run, while still being on an angle on ground, the nose immediately swings in (tractor) very noticably, lessening line pull critically. After a ground  run of perhaps 5 metres, speed is high enough to re-instate line tension. I can help keeping line pull up by making a couple of quick steps in take-off direction, just like Team Race pilots routinely do. Once at speed for lift-off, and with the fuselage then being level, no further problem exists.
Changing to a pusher prop makes the nose swing out only during spool-up and initial acceleration on gound. Consquently, line tension is gradually building up, helping to control lift-off and initial climb. As the swing-out is momentarily only and happens at low speed, tire grinding is minimal and take-off/acceleration is without excessive yaw out. This keeps me from running to save the airplane when flyinig alone on hard surface, which I do all the time...

Other than described above, I have not observed further tractor/pusher related effects at time of take-off and acceleration.
« Last Edit: December 16, 2016, 10:14:25 AM by Peter Germann »
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Offline Igor Burger

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Re: Stab incidence
« Reply #28 on: December 15, 2016, 08:07:56 AM »
As my airplane starts its take-off run with a bit of AoA, could it be the immediate nose-in is caused by the P-effect of the tractor, same as with Team Racers nosing-in violently at take-off?

I do not think so, but you can easily check, when your motor stops. If model changes the yaw after motor quits, it is not p-effect (if I assume that in flight AoA is too small to produce any).

Yaw on start is usually caused by air swirl from prop on fuselage sides (especially thos far from CG, means nose and rudder area) ... that is next effect, which was not mentioned in Matt's post, but certainly present. Simply give it little right rudder and move lines back, if it helps you got it. If you cannot find usefull combination of rudder and LO, you will probably need to play also with motor right offset. Neutral behavior needs proper combination of right thrust, rudder and LO position. Usually the more yawed model flights, the less motor offset it needs, but it really depends on fusleage shapes. My Max Bee has low rudder area (under the fuselage) to better limit this effect, but also high rudder is trimmable.

Online Brett Buck

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Re: Stab incidence
« Reply #29 on: December 15, 2016, 08:23:51 AM »
As my airplane starts its take-off run with a bit of AoA, could it be the immediate nose-in is caused by the P-effect of the tractor, same as with Team Racers nosing-in violently at take-off?

      I don't know enough about T/R models to say, but in the case of the takeoff run, P-factor and precession tend to act in the same direction. Theres's a positive angle of attack, which tends to yaw you nose-in from P-factor, and the pitch rate is negative as it pitches down to make the fuselage level, causing it to yaw nose-in. Of course, the P-factor is very small when you first release it, since there is no forward motion, but as it builds up, it tends to be in the right direction.

      The infamously dangerous takeoff characteristics of the OTS airplane the All-American Sr.  combines these two, and add a third - wildly excessive wing asymmetry. The inboard wing is 3" longer than the outboard, so the CG tends to wind up well to the left of the engine thrust line. The resulting torque is nose-in yaw, which is added to the other two effects. What happens with a normal release is that the airplane yaws in crazily immediately upon release. It usually recovers but it's an unnerving second or two. The real solution is to cut 3" off the inboard wing which makes it illegal for OTS but causes it to fly much better. n erzatz solution is to put in a huge amount of engine offset, which makes the thrust line pass through the CG. Despite Howard's probably correct suggestions that it is more complex than I make it, this seems to work a treat.

    But another technique, suggested by WAM pioneer Lee McKown, is to have the helper hold the airplane by the tail only, with the fuselage level, and to hold the elevator at neutral. That doesn't fix the thrust line but it does get rid of P-factor and precession, since the velocity builds up perpendicular to the prop, and the pitch angle is already level so no abrupt negative pitch rate. The airplane yaws in a little bit, but nowhere near as badly as it is in three-point takeoff release.

   Another interesting thing about this situation is that the inboard yaw motion from the thrust line mismatch causes a small *nose-up* pitch torque from P-factor AND GP, so it tends to pitch up whether you want it to or not, and its much wilder in the case of the 3-point takeoff position. That's a contributory factor to the wild uncontrollable takeoffs - it's also causing what amounts to uncommanded control inputs!

     A usual suggestion as to why this didn't matter to Pappy DeBolt is that he flew clockwise, so the engine torque (which tends to roll you in at takeoff, on top of the tippy landing gear that is also wildly off-center) "helped you". It sort of does, but the other effects are still present  and they still cause completely unexpected/uncommanded motion. It's slightly less likely to chase you around the field than the counter-clockwise case, but it is still unpredictable.

     Moral of the story is to build the airplane with the correct amount of asymmetry (about 3/8" would be right on an All-American SR. and similar-sized models, put in tip weight to trim the airplane properly) and forget about trying to use the fuselage at tip weight.

     Brett

Offline Wolfgang Nieuwkamp

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Re: Stab incidence
« Reply #30 on: December 15, 2016, 08:41:52 AM »
Lauri,

the effects of rpm and mass are linear. The effect of diameter is squared. So it makes sense to make an hollow three bladed prop.

Peter, a sure way to eliminate gyroscopic effects is a twin engine model with the motors inline…. Next project?

Online Lauri Malila

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Re: Stab incidence
« Reply #31 on: December 15, 2016, 10:04:08 AM »
Thank you.
Peter, having flown the same model (IC) with both tractor and pusher, I have opposite feeling about take-off problems. I find it more annoying when the model likes to yaw out in acceleration. With tractor prop, the small tendency to yaw in after release can be compensated with model position in stooge (nose out).
But it's also about wheels, for example Dave Brown lite foam wheels have a nasty stick-and-jump -tendency when dragged sideways on tarmac. Better choise ould be those light Graupner wheels, those with more like soft plastic than rubber tires.
Also, I have no tracking problems whatsoever, neither with tractor nor pusher. I feel that even an (allmost) in-line model like Shark is much happier with a tractor. L

Online Brett Buck

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Re: Stab incidence
« Reply #32 on: December 15, 2016, 01:39:18 PM »
Lauri,

the effects of rpm and mass are linear. The effect of diameter is squared. So it makes sense to make an hollow three bladed prop.

Peter, a sure way to eliminate gyroscopic effects is a twin engine model with the motors inline…. Next project?

   Which is why 3-blade props are used so extensively in stunt. You can get the blade area necessary to absorb/transmit the power without getting excessive diameter, which makes it yaw less from GP, and also has less gyroscopic stability to hinder the turn.

     Brett

Offline RandySmith

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Re: Stab incidence
« Reply #33 on: December 15, 2016, 11:18:19 PM »
   Which is why 3-blade props are used so extensively in stunt. You can get the blade area necessary to absorb/transmit the power without getting excessive diameter, which makes it yaw less from GP, and also has less gyroscopic stability to hinder the turn.

     Brett


Another interesting thing is that the 3 blade of the exact same diameter and blade shape  , will  exhibit  less  GP than the  2 blade does.  I think  it must be  something to do with the blades NOT being  180 degees  from each other, so the  GP force is lessened

Randy

Offline Igor Burger

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Re: Stab incidence
« Reply #34 on: December 16, 2016, 02:07:11 AM »
Another interesting thing is that the 3 blade of the exact same diameter and blade shape  , will  exhibit  less  GP than the  2 blade does.  I think  it must be  something to do with the blades NOT being  180 degees  from each other, so the  GP force is lessened

Randy

2 blade prop and 3 blade prop of the same blade design and of the same weight has aproximately the same GP. There is another difference, 2 blade (also one blade) prop has mass distributed in one line, so the gyroscopic torque has 2 maximums and 2 minimums during one prop revolution. It is not so clear on IC engines, but smooth and silent electric models show it clearly, those props are rattling on them in corners and also bearing does not last as long. 3 blade and more have mass distributed in 2 directions, so gyration forces from every blade are summed "inside" prop so gyroscopic moment on shaft is even and shaft sees it as constant torque from disc and not 2 impulses every revolution.

Offline Wolfgang Nieuwkamp

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Re: Stab incidence
« Reply #35 on: December 16, 2016, 09:03:27 AM »
Regarding stab incidence:

Taking into account Igor´s comment, lets assume that the precession torque is not 0,27 Nm, but only 0,15 Nm, nose up.

Assuming 300W being delivered at the prop, at 22,5 m/s the thrust would be 13 Newtons.
When the motor axis is 20 mm above the CG (Peter´s document), the nose down moment would be 0,26 Nm. This is more than the precession torque, so the excess would have to be compensated by a NEGATIVE incidence of the stabilizer.

Where is the mistake? How can we measure prop inertia?

Thanks for comments,

Wolfgang



Offline Igor Burger

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Re: Stab incidence
« Reply #36 on: December 16, 2016, 09:30:50 AM »
1/ Where you got that thrust? You will need prop efficiency in that regime, do you have it? How much? My calculation for Max Bee shows in flight drag 5 or 6 Newtons tangent, but it is little out so say it is half of those your 13N and you are in ball park.

2/ It is known fact that incidence needs more than just AoA to cancel precession, several people explored it. It was written lot in past either here or on Stuka, there are minimally 2 sources for that value: it is p-effect mentioned upper and airflow angled little down by wing. 

Offline Wolfgang Nieuwkamp

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Re: Stab incidence
« Reply #37 on: December 16, 2016, 10:54:00 AM »
1) 5,5 N times 22,5 m/s is only 124 Watts. During vertical climb you need (in the beginning) a thrust of at least the weight of the model, plus half the weight of the lines, which sums up to at least 16 N. A factor 16/5,5 = 3 between thrust during horizontal and vertical flight seems (for me) too high.

2) Agree. Thanks.

Offline Igor Burger

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Re: Stab incidence
« Reply #38 on: December 16, 2016, 11:00:52 AM »
During vertical climb you need (in the beginning) a thrust of at least the weight of the model, plus half the weight of the lines

Yes I need I also want, but I do not have (at least not with constant RPM) :- ))

Online Brett Buck

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Re: Stab incidence
« Reply #39 on: December 16, 2016, 05:20:24 PM »
2 blade prop and 3 blade prop of the same blade design and of the same weight has aproximately the same GP.

    True, for the same reason the estimate of the radius of gyration is independent on the number of blades.

     Brett

Offline RandySmith

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Re: Stab incidence
« Reply #40 on: December 16, 2016, 08:40:37 PM »
2 blade prop and 3 blade prop of the same blade design and of the same weight has aproximately the same GP. There is another difference, 2 blade (also one blade) prop has mass distributed in one line, so the gyroscopic torque has 2 maximums and 2 minimums during one prop revolution. It is not so clear on IC engines, but smooth and silent electric models show it clearly, those props are rattling on them in corners and also bearing does not last as long. 3 blade and more have mass distributed in 2 directions, so gyration forces from every blade are summed "inside" prop so gyroscopic moment on shaft is even and shaft sees it as constant torque from disc and not 2 impulses every revolution.


The  3 blades  do not  move the nose as much as the  2 blade  does,  there is somethng different going on with 3 blade props, the effect  is less on shaking the nose in and out, could it be  GP force that is manifested 90 degrees later in the direction of rotation from where the force was applied, is some how better when the blades are  120 degrees apart than  180 degrees apart?
I do have less with 3 blades  than I do with the  2 blades

Randy

Offline Allan Perret

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Re: Stab incidence
« Reply #41 on: December 18, 2016, 07:32:51 AM »
Back to original question.  There are "right triangle calculators" [Google]  like this                     http://www.csgnetwork.com/righttricalc.html.

Using that you will find that a 3/16" rise over 12" run will give you .895 or just under 1°.  That's what I use to layout reference line to mount stab.  Good trick for fuse layout is to put pins at front and back to establish wing centerline.  Put the aft pin at stab hinge line, fwd pin in front of wing cutout. Draw straight line, that's wing centerline reference. Put 3rd pin 12" fwd and 3/16" up from stab hinge pin to establish reference line to mount stab.  Maintain pinholes throughout building process so it will always be easy restore that wing centerline as a reference.  You will need that also for checking flap deflections.       
Allan Perret
AMA 302406
Slidell, Louisiana

Offline Jim Mynes

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Re: Stab incidence
« Reply #42 on: December 18, 2016, 11:37:25 AM »
I'm trying to understand this gyroscopic precession stuff. The whole '90 degree after' concept was a bit of an epiphany for me, so that should give you a clue as to my level of expertise.
Is it safe to assume that positive stab incidence as discussed here applies to tractor props, and would be a detriment if using pusher props?
Also, and this might be thread drift, but would a Rabe rudder be all wrong if using a pusher prop?
Some of my planes are tractor, some pusher. As I move forward I want to 'standardize' my setup in some of the more basic aspects, thus making my learning curve a little more shallow.
I have seen the light, and it’s powered by a lipo.

Offline Wolfgang Nieuwkamp

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Re: Stab incidence
« Reply #43 on: December 18, 2016, 12:16:28 PM »
For pusher props, stab incidence should be negative when the motor axis is still above the wing.
Also, for pusher props the Rabe rudder has to  be inverted, so the rudder would go to the outside when giving Up.


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