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Hi-Performance and Sailplanes RC hotliners, electric pylon racers, F5B, F5D, sailplanes and gliders

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Old 07-20-2008, 03:59 PM   #1
jswigart
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Default Conversioin of Aspire Sailplane to brushless

I am currently converting an Aspire sailplane to brushless.
I am using a Himax motor with gearbox. It has plenty of power for a steep climb, and will be able to power it with either an 1800mah or 2200 mah lipo.
The situation I am dealing with is that all of the new stuff I'm adding is much lighter than the stock. Therefore the bird is tail heavy.

I have been cutting out balsa and recovering the holes, as well as removing nuts and bolt hardware from the tail and gluing it on.

Still a little tail heavy by my standards, however after downloading the original manual, I discovered that it calls for the cg to be 3 to 3 3/4" back from the leading edge of the wing. That is nearly 40 to 50 % of the wing cord. Is this normal for a sailplane?

Any advise or comments are welcome.
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Old 07-20-2008, 08:23 PM   #2
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Default C.G. Placement?

Hello Jswigart,
No, that's not a normal C.G. placement for sailplanes, but there are a couple of other factors that could change things.
First, I downloaded the Aspire EP ARF manual, to see if there were any changes on the electric version, compared to the "TD" (Thermal Duration or glider) model. http://www.hangar-9.com/Products/Def...ProdID=HAN1950
Click on "Tech + Spec", it's a big download, but it specifies the C.G. should be 3-3/4" from the wing leading edge.
I also found this article, a review of the glider version. The pilot doesn't mention the exact C.G. measurement, but does say he set it according to the plans and it flew fine there, needed no adjustment.
http://www.hangar-9.com/Articles/Art...ArticleID=1034 (Page 2)
So, why is it so far back? My guess is (primarily) because it has a swept wing. The C.G. measurement is taken (usually) at the wing center. But the outboard panels, about 1/3 of the total wing area, are swept back at the leading edge. No forward sweep (taper) of the trailing edge.
This effectively moves the C.G. rearward.
Here's a few wing shape examples of the change of C.G. when the wing is not a simple rectangle;
http://www.bakehead.com/rc_cg.htm (Scroll down about 2/3 of the page)
Unfortunately, none of the wing shapes shown have constant-chord centers with swept tips. But this is a common wing shape on sailplanes, particularly when they have polyhedral.
Let's say 35% of each 1/2 span has a constant chord, and we're looking for a 33% C.G.
The tips comprise the other 15% of each 1/2 span, and they are swept at the front only. We measure 33% of both the dihedral break chord and the tip chord. Carry the tip (33%) point inward to the center section.
Now we have two points at the center. The 33% line of the center chord, and behind it, the 33% point of the tip chord.
The front mark is only correct for 70% of the wing. The rest of the wing (the tips) have sweep.
There must be a formula for calculating the area vs C.G., but I believe my sketch will be correct. Measure 30% back from the forward mark or 70% forward of the rear mark (the center section is larger in area than the tips) and you will have a 33% C.G. mark for the entire wing, measured at the center section. My guess is it will fall at about 40% of the center chord.
The model also appears to have a larger-than-normal horizontal stabilizer. This is an old Free Flight "trick" to allow a more rearward C.G., since a larger stab reduces the stalling tendency.
So, based on the info I've been able to find and my (rough) calculations, the 3-3/4" mark measured at the wing center section, could be right.
Good Luck!
Ron


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Old 07-20-2008, 10:16 PM   #3
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Thanks for your detailed analysis.
I agree it is probably because of the swept back wing formation.
It is actually to my advantage, since I am having trouble getting rid of tail weight.
I really don't want to have to balance a sailplane by adding weight to the nose.

Jay
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Old 07-21-2008, 03:28 AM   #4
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Originally Posted by jswigart View Post
I am currently converting an Aspire sailplane to brushless.
I am using a Himax motor with gearbox. It has plenty of power for a steep climb, and will be able to power it with either an 1800mah or 2200 mah lipo.
The situation I am dealing with is that all of the new stuff I'm adding is much lighter than the stock. Therefore the bird is tail heavy.

I have been cutting out balsa and recovering the holes, as well as removing nuts and bolt hardware from the tail and gluing it on.

Still a little tail heavy by my standards, however after downloading the original manual, I discovered that it calls for the cg to be 3 to 3 3/4" back from the leading edge of the wing. That is nearly 40 to 50 % of the wing cord. Is this normal for a sailplane?

Any advise or comments are welcome.
For your first flights I would fly it just a little nose heavy. Use the recommended CG. Hopefully you can get the CG adjusted by moving the power components around. If not add some weight to the nose.

The Aspire is a great sailplane. I have had 2 of the EP versions. Both converted to brushless.

Todd
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Old 08-23-2008, 06:26 PM   #5
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How is your aspire doing?

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Old 08-24-2008, 01:14 PM   #6
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after converting it to brushless, it perfoms wonderfully!
I have thermaled for an hour at a time, bringing it down because I became tired.
With the original brushless motor, it would not have been so good.
It's flight characteristics are very good.
Very stable.
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Old 08-24-2008, 02:00 PM   #7
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Originally Posted by jswigart View Post
after converting it to brushless, it perfoms wonderfully!
I have thermaled for an hour at a time, bringing it down because I became tired.
With the original brushless motor, it would not have been so good.
It's flight characteristics are very good.
Very stable.
Can you tell us what motor, prop, ESC, and battery that you are using?

So glad you are having fun with the Aspire.

Todd
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Old 08-24-2008, 03:54 PM   #8
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I am not at home right now, but i believe I can remember the details:

the motor is the eflite 480 parkflyer
Cant remmeber the exact esc but used what eflite recommended.
prop is 9x4 folding apc
battery is 2200 mah 3 cell 20c lipo.

With this configuration the sailplane climbs out pretty steep. I like to get up high quick, and then sail.
Battery will allow quite a few ascents.
Hope this helps.

Jay
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Old 09-12-2008, 02:27 AM   #9
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I just finished up a conversion on my Aspire and found it rather easy to get the aircraft to fly well. Installed a Towerpro 3015-7 1000 kv/470 watt motor I had in the shop. Hooked up a on a 40 watt ESC, bolted on a Aeronaut 11 x 7 Cam folder all powered with a MaxAmp 2100 maH LiPo.
I too like the soaring part of sailplanes and like to get up in thermal territory and get on with the job at hand.
Well, this motor/prop combo doesn't mess about at all. With a fresh battery she climbs out at about a 60* angle like a gas free flight and just keeps going until you say whoa. If launched into a bit of a breeze it'll take a nudge on down elevator to keep her from standing on her tail. All up flying weight came to 52 oz which gives a wing loading just a tick over 11 oz/sq.ft.
As far a balancing the aircraft I started at 3.5" aft of the leading edge, test glided on deep grassy area*and moved battery around until I was happy then made a power launch and tweaked the CG from there to get the glide I wanted. Next up building the 140" jawn.

*Does anybody still do this??

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