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Old Oct 14, 2012, 05:32 PM
Home of A-10 - Farmingdale NY
boiko's Avatar
United States, NH, Londonderry
Joined Dec 2006
936 Posts
Quote:
Originally Posted by pda4you View Post
Wow crickets.....
This plane is $120 and balsa...from Value Hobby (of which I'm a big fan)

But for $64-67 (US Warehouse) you can get a slightly larger EPO F3A version from HK. In fact...in the "Sport Planes" Section here on RCGroups, it is the 3rd most popular thread with 111,000 views as of this date.

http://www.rcgroups.com/forums/showthread.php?t=1239960

No wonder the sound of Crickets is all we've been hearing lately on this thread....and this is coming from the person who started this thread..me

-mike-
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Old Oct 15, 2012, 10:40 PM
scratch-built
Jim Lee's Avatar
United States, WA, Port Orchard
Joined Apr 2007
1,015 Posts
One of the guys at the club has a RainBow, But I've only seen him fly it once. He flies his balsa built pattern planes all the time.

I have some foam planes, But they are all 3D. I'm really partial to balsa built planes and If I had room for another one, I'd have the Oasis. I either need to give away a bunch of my planes, Or I need to list them and sell a few. Here's a video of him flying the RainBow

J

Rainbow.wmv (5 min 43 sec)
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Old Oct 19, 2012, 11:58 AM
ProBro, 3D
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United States, FL, The Villages
Joined Jul 2008
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Quote:
Originally Posted by boiko View Post
This plane is $120 and balsa...from Value Hobby (of which I'm a a big fan.

No wonder the sound of Crickets is all we've been hearing lately on this thread....and this is coming from the person who started this thread..me

-mike-
I can't find anything on this F3A or the 59" Yak55M. I have the Sabre47 and the SabreX and they are both built well and fly extremely well. Anyone flown either of these yet?
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Old Nov 07, 2012, 10:00 AM
Registered User
United States, AZ, Surprise
Joined Jul 2011
25 Posts
VH Oasis f3a 51 inch pattern ship--questions questions!!

I put one of these babies together -- and maidened it yday successfully (but not gracefully!). Here are my thoughts--
-stock motor (after talking with Alex) electric .25 seems to be grossly underpowered for this airframe and the specd 3.5 lbs -- 4S setup draws only 30Amps at WOT -- they had originally specd (it shows up in the manual) the motor as a .15 -- and this baby would never fly with that powerplant!
-I have sequence, vanquish,vyper in the smaller footprints similar to the oasis and they all use bigger motors swinging the same 12/6 props. They behave very differently!
-Needless to say slowing down for landing was no problem!!
-Watch out with this setup cause the wing will stall the minute you cut power over the runway-- did it to me twice --and scared the crap outa me!
-Ran the CG and MAC calculations and this may be the problem.
http://www.scaleaero.com/mac_page.htm
-Based on the wing dimensions the MAC shows up at about 13 inches, and at 25% CG /MAC that would mean the CG should be 5 inches back from the le -- the spec is for 165 mm or about 6.5 inches!!

Any suggestions or problems with my logic -- my plan of attack next will be
to adjust the CG forward (drastically) and see what that does to the stall characteristics. Any thoughts appreciated.
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Old Nov 07, 2012, 10:29 AM
agnotology
kcaldwel's Avatar
Joined Jan 2007
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Quote:
Originally Posted by cweeks0515 View Post
-Ran the CG and MAC calculations and this may be the problem.
http://www.scaleaero.com/mac_page.htm
-Based on the wing dimensions the MAC shows up at about 13 inches, and at 25% CG /MAC that would mean the CG should be 5 inches back from the le -- the spec is for 165 mm or about 6.5 inches!!

Any suggestions or problems with my logic -- my plan of attack next will be
to adjust the CG forward (drastically) and see what that does to the stall characteristics. Any thoughts appreciated.
CG location has to take into account the horizontal tail. Calculating the wing MAC is only one step in the process. The CG of a tailed pattern airplane with their typically long tail arms will be quite a distance behind MAC 1/4c, and still be more than adequately stable.

This is good CG calculation spreadsheet:

www.rcgroups.com/forums/showthread.php?t=1106300

Even this simple one will give better results than just calculating wing MAC and 1/4c. A static margin of 10% will be far enough forward to start tuning the CG, and still leave some room for the error in these simple calculations:

http://www.geistware.com/rcmodeling/cg_super_calc.htm

The bigger and further aft the tail is, the further back the aircraft neutral point. The CG must move aft to maintain the reasonably small static margin you will want for pattern flying.

If you can get it high enough with the anemic power, you can do a dive test (45 degree down line, neutralize the stick, and see if, and how fast, the nose pulls up - should perhaps maintain the 45 down line, or very slowly pull up), and/or see how much down stick it takes to fly inverted (it should take only a small amount of down elevator).

I routinely fly sailplanes and other airplanes with zero static margins, which is typically a very long way behind 1/4MAC. They fly very nicely. I find too much static stability (CG too far forward) causes far more problems than a CG near the aircraft neutral point.

Kevin

Edit: Had the wrong link for the Geist CG calculator.
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Old Nov 07, 2012, 01:18 PM
Registered User
United States, AZ, Surprise
Joined Jul 2011
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Static Margin/CG

Kevin-
I plugged in all of the values once over on the geist calc and wow -- I am back to square one I guess. At 10% static margin (CG is 1 inch in front of MAC) the CG would be 7 inches (about 178 mm) back of the le root chord. That is within half inch of the specd C. G. of 165 mm .

Thanks for helping out!

Chet
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Old Nov 07, 2012, 06:13 PM
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gregory.da's Avatar
United States, FL, The Villages
Joined Jul 2008
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I've read World Champion Andrew Jesky runs his F3A at just forward of neutral CG.....and the same for his 3D airplanes. If that's good enough for him, it's good enough for me . As kcaldwell said, when inverted you want just a tad bit of down elevator to maintain level flight.
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Old Nov 07, 2012, 08:31 PM
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neutral CG

Thanks for the comments and I understand and use the inverted trim check.
What has me stumped is the difference in the location of the CG in this Value Hobby setup. I have similar configurations -- same basic airframe form factors and the CG is no where near as far back as this model specs. The sequence is 4 3/4 inches, the vanquish is 125 mm (about 5 inches) and suggestion to move it forward for pattern precision flight. 165mm just seems way off!!
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Old Nov 07, 2012, 08:44 PM
agnotology
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Quote:
Originally Posted by cweeks0515 View Post
Thanks for the comments and I understand and use the inverted trim check.
What has me stumped is the difference in the location of the CG in this Value Hobby setup. I have similar configurations -- same basic airframe form factors and the CG is no where near as far back as this model specs. The sequence is 4 3/4 inches, the vanquish is 125 mm (about 5 inches) and suggestion to move it forward for pattern precision flight. 165mm just seems way off!!
You would need to calculate the neutral points and static margins on all the models to compare. Just going by distances back from the LE/fuse junction doesn't tell you anything.

The Vanquish doesn't have anywhere near as much LE sweep, so the wing MAC will be much more forward. The Oasis looks like it has a straight trailing edge, which will move the wing MAC back from the leading edge, and therefore the CG location also will move back for a given stability margin.

Tail size and moment arm will also move the CG for a given static margin forward and back when measuring from the wing leading edge.

Without doing the math (or in flight checking to get a neutral CG), you just cannot compare CG distances from the wing LE. I have my 47" Osiris at 165mm back, and it doesn't have a lot of LE sweep, a lot less than the Oasis.

Kevin

Edit: The Geistware CG calculator and others like it have a fixed stabilizer efficiency, don't take the fuselage into consideration, and have several other simplifications that make the results less accurate than the spreadsheet I linked above.
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Old Nov 08, 2012, 11:22 AM
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United States, AZ, Surprise
Joined Jul 2011
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I think this is a useful discussion (at least for me!) so here goes-
I did run the numbers on my Sequence f3a ship. It is manual specd at CG 4.75 inches back from le -- and it flies very nicely there!(well for me!) more on that later!
The giest calculation when I use 15% static margin caclulated a CG of 6.25 inches - with the CG at 1.5 inches forward of the neutral point--the 15 % margin.

The mac is 10 inches on both of these machines -- and if we just for argument sake suggest that 25% of MAC is a "good starting point" doesnt that move the CG forward an inch to 5.25 inches and pretty close to the manual 4.75??
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Old Nov 08, 2012, 02:20 PM
agnotology
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These pattern airplanes aren't very typical of the dimensions that the old modeller's "rule" of 25%MAC was made up around. It isn't a very good "rule" in any case. Variations in tail size, tail arms, height of stab in the wing wake, fuselage shape, etc. mean 25% MAC can be anywhere from 50% static margin to 10%. The only thing you can say for it is that a tail-aft airplane will likely be pitch stable with a 25% MAC CG location. The pitch response and stability will be all over the map.

The point of calculating the static margin is to obtain similar pitch response and stability across airplanes. If you like the feel of an airplane with 30% SM, then set-up the new airplane with 30%, and it will feel similar in pitch and stability.

I find manual suggested CG positions are best taken with a grain of salt. The difference in the static margin at the recommended CG for the 47" and 62" versions of the Osiris is quite large, for no reason that I can understand.

These pattern style airplanes, with their long tail arms and generous stab areas, are very tolerant of a wide CG range, at least as far as basic flying goes. Tuning for precision pattern flying is another thing entirely! I am used to flying sailplanes with far less pitch stability than seems to be used on pattern airplanes these days, and I hate the down stick inverted that seems to be the norm on a pattern airplane.

Bottom line is, if you like a huge static margin on one plane, you will like a huge static margin on your new one. It sounds like your Sequence is at about 30% SM, and you like that. It could be because you have far too much elevator throw - you really need to reduce the elevator throw a lot as you reduce the SM. The throw should be reduced mechanically so you use as much of the servo throw and resolution as possible. The elevator servo has to have good centring and a tight linkage. Neutral CG positions need very small elevator throws. Big static margins numb the elevator control, and the airplane wants to fly at trim speed very much. Lower static margins require the pilot to manage the attitude and airspeed more actively.

Kevin
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Old Nov 09, 2012, 10:16 AM
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United States, AZ, Surprise
Joined Jul 2011
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Static Margin/CG calculations OASIS vs Sequence

Here is my CG correlations between these two pattern airframes --
these numbers are starting to make sense now and the idea of moving the CG
forward now seems the wrong thing to do!
The run for the oasis comes out with the CG at 6.45 inches or 163mm at 15% static margin==EXACTLY where the manual specs.
The sequence comes out with a CG at 5.5 inches --at 15% margine very close to the manual starting point of 4.75 inches.

Thanks Kevin for your help on this! Now back to getting the airplane powered correctly with a 4S setup --
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Old Feb 17, 2013, 08:52 PM
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DonJuan's Avatar
Illinois
Joined Jul 2005
223 Posts
So has anyone ever flown one of these?
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Old Apr 15, 2013, 03:37 AM
Home of A-10 - Farmingdale NY
boiko's Avatar
United States, NH, Londonderry
Joined Dec 2006
936 Posts
..and the answer is...

So what was the decision on the correct CG for the Oasis?

-mike-
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