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Thrust centerline vs CG, CD, P-factor, with the SLOW STICK?!?!?!?!

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Thrust centerline vs CG, CD, P-factor, with the SLOW STICK?!?!?!?!

Old 12-13-2007, 04:59 AM
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CFIT
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Default Thrust centerline vs CG, CD, P-factor, with the SLOW STICK?!?!?!?!

First off, I’m a newb to R/C , (just purchased a GWS Slow Stick and a GWS E-starter and have yet to fly either of them), yet not so new to the concepts of aircraft flight dynamics and design as it applies to full scale aircraft, dabbled a little with C/L back in A&P school, also hold a my Private Pilot SEL certificate. (I’m NO expert and have much to learn, but I feel comfortable with the “basics” as I understand them.) With that said, if I am missing anything here or barking up the wrong tree, feel free to correct any misinformation.

In a recent CG, CL, discussion with another member here, the topic of thrust centerline and pitch caused by rapid application of power, came up and this got me thinking.

Abrupt application of throttle generally will cause most planes to pitch upward. This has to due with where the thrust is being induced from, at what angle that thrust is being induced, and against where the CG is located in the air frame. The fix to this issue seems simple enough, just draw an imaginary line from the prop centerline through the CG and that should be the ideal thrust centerline to offset induced pitch from abrupt throttle application, right? Wrong! There is another variable to think about, "aerodynamic drag!". Aerodynamic drag induced from the flying surfaces, fuselage, landing gear, exposed battery, servos, ESC, etc combined will have a location on the air frame from where those forces are concentrated, much like the Center of Gravity. With CG, that is a constant no matter what air speed the aircraft is flying at, so long as the aircraft is flying straight and level with no other acceleration forcers being applied. Drag on the other hand is not so constant as it is influenced by air speed and that influence is not linear, but on a exponential scale and also affects how much or little the aircraft will pitch from abrupt application of thrust. In an ideal world, our CD would be where the CG is located, which for us slow flyers, would make it much easier to compensate for thrust induced pitch. There are many other variables to consider as well in designing the “ideal” slow flyer, which is beyond the scope of this thread.

With our Aircraft, we are constantly making compromises and it how efficiently we deal with those compromises for the type of flying we do, that can turn any aircraft into a sweetheart of flyer or a nightmare worthy of a life as a post hole digger.

With my particular Slow Stick project, I’m hell bent on making mild adjustments to the design of this slow flyer in an effort to soften its bad habits and hopefully make it a "sweetheart". One of those mods is compensating for P-factor and thrust induced pitch. In my searches, it seems that most hard core builders will add a little right tilt to the motor mount to compensate for P-factor and also some down tilt to compensate for thrust induced pitch. With the current model Slow Stick, that is not an easy task so I set out to build a motor mount with a built in amount of down thrust and right thrust. In this case I have nothing to go by other than a genuine uneducated guess of 5 degrees “down” and 2 ˝ degrees “right”.

After a couple minutes of head scratching, a quick pencil sketch, I walked out to my shop and put my fab skills to work on my first of MANY custom RC parts. A custom Slow Stick motor mount with a 5 degree down tilt and a 2.5 degree right tilt.

This mount started out as a piece of ˝” x ˝” 6061 T-6 Aluminum bar stock. It fits snuggly inside the slow stick “stick” and the portion that the engine mounts to has the same outside dimension as the Slow Stick “stick", but with a 5 degree down tilt and a 2.5 degree right tilt, (+/-1/2 degree accuracy)

I am looking for input, ideas, is this worthy of possibly building more of or am I wasting my time, etc?

I did not perform any calculations or actual testing to find this SS thrust centerline. What I’m “hoping” for is that this mount will be a “decent” compromise…

Here are some pics during, and after machining. (quality and finish are “rough” as this is merely a “proof of concept” part,)


The machining process...







The finished "one off"... The “T” stamp indicates “top





As installed with 2˝ degree right tilt...





As installed with 5 degree down tilt...

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Old 12-13-2007, 07:04 AM
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pace_likethesalsa
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Not bad, my "play-thing" background includes a lot of paintball, where all sorts of crap is custom milled on home work bench and becomes a massive revolution in the sport, so I fully believe this can go somewhere. Depending on how far you want to get into this, you can go all out. A small mount onto the stick fuselage with a universal joint and a mount to the motor, then give it some set screws to make it adjustable. Then regardless of your power setup or aerodynamics, which change on so many SSs, you'll have a system that doesn't weigh much but is fully tunable to your plane.

I would look into seeing if you can find some simplified equations on this, even working with the equations in an ideal sense, you'll be able to get a benchmark of where to start with your offsets.

edit: My aerodynamics isn't the best, but wikipedia (who I don't really trust) says the offset of thrust due to P-factor will be parallel to the motor, so wouldn't you just want the motor offset, and not angled?

Personally, since these factors will change throughout the flight, and you can only optimize for one specific set of conditions, I think it would be easier to subconsciously learn how your plane flys at top speed, at different climbs, etc, through experience, rather than try to fix them for one set. I used to play lacrosse, you can read all you want about how lacrosse sticks throw and catch differently, and the difference between a traditional leather hand strung basket and a stretched nylon one, and all that, or you can just go out and play for a few hours with each stick and be wayyy better off.

But, I am all for tinkering. Completely. Hack, adjust, fix, etc.
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Old 12-13-2007, 07:47 AM
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Biplane Murphy
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The real test will be flying it....Most flying is done at medium throttle, and most people trim their aircraft for that throttle setting...Ideally though, setting it up for perfect flight at all speeds is nice...

I think the most commonly used offsets are 2 degrees right and 2 degrees down...Just enough to overcome the torque of the motor while on the ground (right thrust) and the lift generated by the wing (down thrust)

However...Pattern aerobatic planes with symmetrical airfoils frequently have no motor offset at all....in order to fly straight lines in all attitudes....

I have assembled a few Slow sticks for other people...I used motor mounts with no offset at all.... they flew fine....but a couple degrees down thrust might have improved performance a little...

P.S. looking at the photo....it looks like a whole lot of down thrust to me.....
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Old 12-13-2007, 11:03 PM
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BRAAP, Nice job on the motor mount.

As Murphy said most common is around 2 degrees to start. But 5 degrees may work.

The down thrust is necessary for planes with under cambered or flat bottom airfoils more than with semi symmetrical or full symmetrical airfoils.

I have some planes with with semi symmetrical airfoils that do not need down or right thrust.

The right thrust is to counter torque and rotating prop wash not P-factor.

While the motor is trying to turn the prop one direction, a force is applied to the opposite direction referred as torque.
An airplane with a lot of mass and large surfaces is harder to rotate opposite the propeller.

P-factor only has an effect when the airplane's pitch is positive.

The descending blade gets an increase in angle of attack while the ascending blade's angle of attack is reduced.

Looks like your on the right track and your design looks good as well.

Just my two cents.

Paul
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Old 12-14-2007, 02:01 AM
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CFIT
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Originally Posted by pd1 View Post
BRAAP, Nice job on the motor mount.

As Murphy said most common is around 2 degrees to start. But 5 degrees may work.

The down thrust is necessary for planes with under cambered or flat bottom airfoils more than with semi symmetrical or full symmetrical airfoils.

I have some planes with with semi symmetrical airfoils that do not need down or right thrust.

The right thrust is to counter torque and rotating prop wash not P-factor.

While the motor is trying to turn the prop one direction, a force is applied to the opposite direction referred as torque.
An airplane with a lot of mass and large surfaces is harder to rotate opposite the propeller.

P-factor only has an effect when the airplane's pitch is positive.

The descending blade gets an increase in angle of attack while the ascending blade's angle of attack is reduced.

Looks like your on the right track and your design looks good as well.

Just my two cents.

Paul



Yes! Exactly and very well stated. I agree 100% with your definitions of P-factor, and why we want to induce a right angle thrust. Thank you for correcting that. I apologize for the misleading terminology in my original post.

At this time, based on another established members input, I will fly my SS with NO angle or offset. I’m new to the hobby and really should have some experience flying RC before I go about altering the design of my first RC aircraft.

From everyone’s input thus far, when I do go about building another altered thrust angle motor mount for my SS, it will have more subdued thrust angles.

Thank you guys for your input.

Paul Ruschman
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Old 12-14-2007, 02:08 AM
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Murocflyer
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Paul R,

I think you are going to find the SS a little more difficult to fly without adding some down and right thrust correction.

For the life if me, I don't know why anyone hasn't marketed a thrust correction mount!

Frank
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Old 12-14-2007, 02:10 AM
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BRAAP,
It's all good.

I was thinking maybe you could make a few different adaptors with different angles and change them in the field to fine tune your plane?

It's all a little guesswork anyway.

Paul
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Old 12-14-2007, 03:02 AM
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Originally Posted by Murocflyer View Post
Paul R,

I think you are going to find the SS a little more difficult to fly without adding some down and right thrust correction.

For the life if me, I don't know why anyone hasn't marketed a thrust correction mount!

Frank


Originally Posted by pd1 View Post
BRAAP,
It's all good.

I was thinking maybe you could make a few different adaptors with different angles and change them in the field to fine tune your plane?

It's all a little guesswork anyway.

Paul


Ok. How about this guys.

Being as I wouldn’t know a straight true flying Slow Stick if it flew up and bit my butt, (read; I have never flown one yet so how could I know?), would any of you experienced, GWS Slow Stick flyers want to test this mount in their Slow stick and report their full, thorough “unbiased” results? I would even machine at least one other mount for side by side testing as well with a different down and right thrust angle.

I realize what would work for one Slow Stick and that pilots flight regime wont necessarily work for another aircraft/pilot due to engine, battery, and other component selections as well as type of flying involved though I would like to think the results would at least point to a particular attribute.

It would seem that this should be tested on a bone stock assembled Slow Stick, (built to the instructions verbatim including power plant and recommended battery?), and also a mildly modded slow stick such as Brushless, Li-po upgrade, and for giggles, possibly even a more radical slow stick with Ailerons, altered dihedral, etc.

I’m more than willing to produce a couple/few prototype motor mounts for testing but the testing must be done by a competent experienced flyer using the GWS Slow Stick. If this testing proves successful, possibly a limited run of these motor mounts could be produced?

What do you guys/gals think?

Last edited by CFIT; 12-14-2007 at 03:04 AM. Reason: Typo...
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Old 12-24-2007, 01:33 AM
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Send me one, I'll test it. But, frankly, the down and right thrust angle is often corrected with a couple of washers on the top and left screws between the motor and mount. Want more down? Add a washer ...
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Old 12-26-2007, 10:27 AM
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Paul,

How goes the flight testing?

Frank
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Old 12-26-2007, 11:27 PM
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Originally Posted by wmac View Post
Send me one, I'll test it. But, frankly, the down and right thrust angle is often corrected with a couple of washers on the top and left screws between the motor and mount. Want more down? Add a washer ...

WMAC,
You’ve got PM!


Originally Posted by Murocflyer View Post
Paul,

How goes the flight testing?

Frank




Murco,
Maiden and subsequent flights with the Slow Stick are INCREDIBLE! This thing is ADDICTIVE! I’m trying to get this posted up to go fly some more before it gets dark.
I built another mount like this one, though to much lesser degree. The one I am using has a 1.5 degree down thrust angle and as 1.5 degree right thrust angle. No idea if it is helping, don’t think it is hindering anything as the plane flies and maneuvers in a fluid graceful manner, landings are smooth with no bounce. Loops are tight in less than a 15 foot circle, rudder has equal authority.
Here is the build and flight testing results thus far… Oh, and that darn Chellie is teasing me with another plane in my build thread! .. uh.. shhhh... I’m ordering one this evening,

Click ME for the Slow Stick build and flight report...
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Old 12-27-2007, 01:12 AM
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Biplane Murphy
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Originally Posted by BRAAP View Post
Oh, and that darn Chellie is teasing me with another plane in my build thread! .. uh.. shhhh... I’m ordering one this evening,
She's good at doing that ...
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