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Author Topic: The story of Big Red - Happy End  (Read 1214 times)

Offline Matt Piatkowski

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The story of Big Red - Happy End
« on: July 28, 2021, 05:49:02 AM »
Hello,
Please see the attached.
This is my Big Red after changes to its aerodynamics in July 2021.

Change 1: addition of 2mm. carbon composite tube to the LE of the horizontal stabilizer. LE radius before (in 2019): 2.2 mm.
Change 2: addition of "wart" to the outside flap. Location and dimensions - from Igor Burger's Max Bee II plans
Change 3: increase of the area of the elevator by 8% (yellow extensions along the TE)

Changes 1 and 2 were suggested by Igor.

Krystian Borzecki, the best F2B flier in Poland, flew Big Red in August 2019 and told me: "this plane does not want to turn".
No further explanations were provided.
I have made about fifty patterns with Big Red until September 2019 but could not fix the "no turning" problem, whatever the root cause was. 

At that time, I did not fully understand the hinging problem in sharp corners for the models having unequal wing panels and one propeller. Change 2 fixed this problem completely. Funny thing: I had Igor's plans but simply forgot to add "wart" in 2019. Consequently, for the entire 2019 flying season, I was flying the model that exhibited all sort of parasite yawing and rolling, associated with the eulerian movement of the rigid body in the 3D space when the sum of the moments is not zero.

Change 3 is my own initiative, based on my current understanding of the sharp corner dynamics.

Big Red flies very well now: there is no hinging, pivoting corners are executed with the audible "swish"and the parasite yawing and rolling disappeared. I managed to move the CG about 13 mm. aft. and this made the model very sensitive in the pitch channel. I can handle this increased sensitivity because my flying skills improved in 2020 and in the first half of 2021. Tracking and grooving are very good and I am quite pleased with everything I learned (and I am still learning) in the process.

Thank you to all who, on this forum, participated in discussion about hinging in sharp corners.

Happy Summer Flying!
Best Regards,
M

 

Offline Ted Fancher

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Re: The story of Big Red - Happy End
« Reply #1 on: July 28, 2021, 04:29:27 PM »
Good work, Matt!  Way to hang in there.

Just between me and thee (and whoever reads this, I guess) that slightly over 1/2 inch aft movement of the CG was probably the single biggest step towards competitive corners.  If you were able to move it that far back and still have a controllable glide to a nice landing you could probably experiment with even further aft movement (in very small steps now).  This sort of "tuning" should always be accompanied by an adjustable handle allowing you to narrow spacing as you move the CG aft to keep the response rate in corners at an optimum level for your preferred input technique.  IOW, you don't have to consciously "back off' on input as the response rate increases.

Here's why I feel additional effort in that area "could" prove fruitful. 

To the degree that you can get your CG in close longitudinal proximity to the center of lift you can minimize the impact of wind on the amount of extra (if any) increased flap/elevator deflection necessary to fly maneuvers of your "rule book" "good air" size.  This is, by far, the biggest advantage of today's modern adjustable handles (and, by the way, advantage of today's large area stab/elevators and more modest flap area).  They allow you to make the response rate ideal for the pilot's preference at the same time you improve the airplanes ability to be less affected by varying levels of "bad" air.

IOW, trim the airplane to be happy with the widest range of "conditions" and trim the handle to make the pilot happy with that airplane status.  With the CG and CL in close proximity the deterioration in input response rate and acceleration of airspeed in high winds due to the foreward of the CL CG location is minimized. 

Good grief!  So many words.  Did they make any sense!

Ted

p.s. Gooood lookin' airplane by the way.

Offline Matt Piatkowski

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Re: The story of Big Red - Happy End
« Reply #2 on: July 29, 2021, 01:02:26 AM »
Hi Ted,
At this moment, the CG of the RTF model cannot be moved more aft without the battery compartment surgery.
This surgery (or "dremeling") will allow positioning the ThunderPower 6S 2800 at the angle w/r to the horizontal plane (X-Y).
This angle can be even 90 degrees and this means the battery is vertical in the fuselage touching the wing LE.

If I decide to do it, the CG of the RTF model will move about 15-17 mm. aft.

Seeing and feeling Big Red flying now, I do not think this kind of modification is needed. I have to make at least 30 patterns to learn how to effectively use the model's newly found "turning agility". If I see more pivoting corners and they become consistent, I will consider the "surgery by dremeling".

Big Red has the hinge-to-hinge distance (H-T-H) = 480 mm. Brett wrote some time ago that this is not H-T-H that matters but the distance from the wing's maximum lift to the horizontal empennage maximum lift. Change #1 modified the lift distribution across the horizontal empennage, by moving the maximum lift line a bit aft. How much? I do not know but the effect can be seen and felt: the plane exits the corners a bit better and it holds the level flight exceptionally well.

I will fly a lot this summer and fall. I am currently in Poland and there is a soccer field five minutes from our home. I simply take Big Red, batteries, and other stuff and walk there in the morning. The grass is kept short, the field is well maintained and the maintenance crew is very sympathetic.

Thank you for your thoughts - they always help.
Regards,
M

 

Offline PJ Rowland

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Re: The story of Big Red - Happy End
« Reply #3 on: July 29, 2021, 02:11:39 AM »
Change 1: addition of 2mm. carbon composite tube to the LE of the horizontal stabilizer. LE radius before (in 2019): 2.2 mm.

So can I ask ? This was a radius reduction of 0.2mm ?

Then the observation was;
Change #1 modified the lift distribution across the horizontal empennage, by moving the maximum lift line a bit aft. How much? I do not know but the effect can be seen and felt: the plane exits the corners a bit better and it holds the level flight exceptionally well.

So ... if I understand this correctly, your observation is that by reducing the leading edge radius 0.2mm you improved the turn and lock ( plane exits better and sits better )

Interesting data.

I wonder if then by approximation you continued to reduce the leading edge radius... would the turn and lock continue to improve??

You COULD in theory reduce the leading edge radius down to a finitie sharp point.. almost a razor blade leading edge.

But thats only a theory, It would take a boeing engineer to even attempt such tests.

If you always put limit on everything you do, physical or anything else. It will spread into your work and into your life. There are no limits. There are only plateaus, and you must not stay there, you must go beyond them.” - Bruce Lee.

...
 I Yearn for a world where chickens can cross the road without having their motives questioned.

Offline Brett Buck

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Re: The story of Big Red - Happy End
« Reply #4 on: July 29, 2021, 12:31:37 PM »
It's amazing how a few little changes can make all the difference.

    As Ted note - moving the CG back by ** 5/8-3/4" ** is a HUGE change that swamps all the others, and also the most direct solution to the "does not turn" issue. A typical CG change I make is .050"* - 1/4 ounce of weight into or out of the nose.The wart on the flap is a result of noting what the airplane was doing and fixing it. The other changes are tiny/cosmetic.

  I would also add that the current tip weight with a wart seems to work - did you try less tip weight and no wart> Better or worse. You can always stop hinging by reducing the tip weight sufficiently, it's a matter of trading off how much tip weight VS how much wart/differntial flap area, to get the best combination of round and square corners.

    So I think this is a matter of "burying the lede", that is, the problem appears to have been solved by classic trim changes from the dawn of time.

      Brett


*And no, I don't claim to be able to measure it with my fingers at the field. That is the CG change arising from adding/subtracting 1/4 ounce from the nose of my 64 ounce airplane

Offline Air Ministry .

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Re: The story of Big Red - Happy End
« Reply #5 on: July 29, 2021, 10:42:37 PM »
 " almost a razor blade leading edge. "

L E by Gillett . Would reduce the drag . when Supersonic .

The Flap Moment with aft C G Shift is bebneficial related to the % elevator to C G moment . or C L . or something .

Can you fit a half flight , half length , battery pack ? . To see if its unflyable with the  C. G . right aft .


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