Third Scale Piper cherokee Archer - Page 12 - RC Groups
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May 21, 2012, 09:57 AM
gluemaster's Avatar
Simply stunning.
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Jun 02, 2012, 03:35 PM
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Keith Pyrah's Avatar
We abandoned our proposed trip to a scale weekend due to the very bad weather forcast for Sunday. We decided to take the model to our own field for the maiden. the weaher was very murky with drizzle and rather miserable and the runway grass was wet and longer than we would like. We decided to go for it anyway.
First take off was a bit sluggish and at one point we experienced some elevator flutter on full power straight and level which stopped when Doug reduced power Doug had to hold in up elevator to keep her in the air so the first landing was not perfect but quite acceptable.
Doug retrimmed the all moving tailplane and off he went again, take off was still a bit sluggish we thought due to the drag from the wet grass but the flight was very smooth and she flew straight and level hands off. After two more take offs and landings we decided to call it a day because the weather was deteriating and the model was becoming difficult to see in the air.
Doug felt the model flew "heavy" so we carried out some detailed checks back at home and made some adjustments to the CogG. We also massbalanced the all moving tailplane by the insertion of 1/4" sleel bar just behind the leading edge which we hope will cure the flutter at full power straight and level.
A few pictures of the maiden below.
Jun 02, 2012, 03:40 PM
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tailskid2's Avatar
Except for the first picture are you sure this wasn't the full scale plane?
Jun 02, 2012, 03:55 PM
AMA 910957
EJWash1's Avatar

Thank you for sharing your build.

Jun 02, 2012, 04:47 PM
Have Fun! Land, Sea or Air
jetcaptain's Avatar
Congratulations! What an astounding job you both have done.. It looks very real in the air.
Jun 02, 2012, 05:06 PM
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Sperling234's Avatar


Looks utterly convincing.

Great, great work!


Latest blog entry: Raiders Wing from Scratch
Jun 02, 2012, 06:47 PM
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supercubman's Avatar
Keith, as usual a magnificent job!!!

Jun 02, 2012, 07:13 PM
boat butcher
the goon's Avatar
Think it's time for a wider taxi way . A truly fantastic model.

Jun 11, 2012, 12:13 PM
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Keith Pyrah's Avatar
We took the model down to our own field and carried out further trials.The change of CofG and the mass balancing of the all moving tailplane improved the handling considerably but Doug was still not quite happy with the horizontal stability. We found that the all moving tailplane did not reliably centre after up or down movements so trimming was inconsistent.
One of our club members who is very much into large 3D flying advised us that the Hitec 645 MG servo that we had on the tilplane would never reliably centre hence the unstable horizontal trim.
We are taking his advice and will fit a 15kg digital servo which should reliably centre every time.
We hope then to be able to set up a mix so that when flap is used some down elevator is applied to keep the model horizontal and not nose up when landing.
We found such a mix very helpfull with our third scale Cessna 182
Jun 11, 2012, 02:44 PM
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Won2Race's Avatar
keith, just some friendly advice.

Generally, low wing aircraft require UP trim to compensate for flap deployment. High wing aircraft generally pitch up (requiring down elevator to compensate) because the center of drag moves upwards with flap deployment which in turn creates a nose up moment.

In contrast, low wing aircraft generally experience a net nose down moment because the center of drag moves downwards with flap deployment which necessitates up trim to compensate.

Of course, this is a rule of thumb and not applicable to every design.

Here is a good synopsis I found elsewhere:

Low wing, low tail aircraft (eg. B737)
When extending flap on these aircraft, two main things happen.
1. Center of lift moves back, creating a negative pitching moment
2. Drag increases BELOW the CoG creating a negative pitching moment
RESULT: Aircraft pitches DOWN

Low wing, high tail aircraft (eg. MD80)
1. Center of lift moves back, creating a negative pitching moment
2. Drag increases BELOW the CoG creating a negative pitching moment
RESULT: Aircraft pitches DOWN

High wing, low tail aircraft (eg. C172)
1. Center of lift moves back, creating a negative pitching moment
2. Downwash from the wing increases in angle over the stabilizer creating
a positive pitching moment.
3. Drag increases ABOVE the CoG creating a positive pitching moment
RESULT: Aircraft pitches UP

High wing, high tail aircraft (eg. Dash8)
1. Center of lift moves back, creating a negative pitching moment
2. Drag increases ABOVE the CoG creating a positive pitching moment
RESULT: Aircraft pitches UP
Jun 12, 2012, 01:28 AM
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Keith Pyrah's Avatar
Very interesting and logical comment re flap deployment effects. We shall persue this and also ask James what the effect is on the full size.
Jun 12, 2012, 01:58 AM
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tailskid2's Avatar
If I remember while flying the Piper Cruser that when the flaps were deployed, the plane would balloon a bit and the nose would drop....while the Arrow the nose would raise. But that was many years ago.....
Jun 13, 2012, 12:10 AM
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Every full scale Cherokee I have flown, the nose pitches up when flaps are deployed.
Jun 13, 2012, 08:13 PM
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Won2Race's Avatar
That's interesting because i found several discussions online regarding cherokees and their pitching down with flap deployment. Its quite possible that the set trim of the plane has an effect. The nose down effect on low wing low tail aircraft with flap deployment is an aerodynamic fact due to the changes in drag and lift centers.
Jun 14, 2012, 07:02 AM
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Keith Pyrah's Avatar
James who pilots the full size G-BEYL has confirmed that when flaps are deployed the nose pitches up.
The model still seems to fly "heavy" so we are going to do some very detailed checks re CofG.
The full size manual shows a very wide range for the CofG so we are going to calculate the model using the same system of weighing and calculating then we can move CofG towards the forward limit which will make the model more responsive.

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