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Old Mar 06, 2012, 04:57 PM
Electric Airplane Junkie
bhchan's Avatar
United States, CA, San Mateo
Joined Jun 2000
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I flew it at 90mmCG and two experts(LSF V's) flew it told me it was tail heavy. I moved to 85mm and they agreed that is better.

Elevator compensation should be reduced slowly. Sudden reduction in elevator compensation will cause the plane to pitch up violently.

Brian, an EAJ
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Old Mar 06, 2012, 05:19 PM
Arjan
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Deventer, the Netherlands
Joined Mar 2007
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CG is a very personal matter. I like it at 90mm. It is stable, indicates lift very well and is very responsive (as a competition ship should be). What makes your experts say it is tail heavy?
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Old Mar 06, 2012, 10:00 PM
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San Bruno CA (near San Francisco)
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Before this goes too far, can we agree that the CG is measured as the distance from the very front vertex at the center where the wings meet (not from where the wings exit the pod)?

I measured mine very carefully from the front vertex, placing two pieces of tape on the underside of each wing with a narrow space between them at 90 mm. Then I balanced the glider using a home-made balance stand with sharp (~ 1 mm) thick wooden points/edges - like screw drivers - to support the glider.

Balanced this way at 90 mm I thought the glider flew quite nicely, indicating lift (and sink) very clearly. I did a shallow dive test with ailerons and flaps in line with the center tabs and it gradually came out of the dive, which all seemed to indicate that 90 mm is good. I figured that at some point I would experiment with pushing the CG further back.

Mitch
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Old Mar 07, 2012, 01:40 AM
Arjan
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Deventer, the Netherlands
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yup. Same here.
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Old Mar 07, 2012, 06:28 AM
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Quote:
Originally Posted by PH_AJH View Post
Mitch,
Please keep in mind that the folks at Reichard do not agree with me - they state that the Cumulus should be flown without flightmodes.
Interesting. What do you know about the airfoil, which of course would dictate how flight modes should be used for optimal performance.

Louis
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Old Mar 07, 2012, 10:04 AM
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San Bruno CA (near San Francisco)
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Airfoil is AG25 - AG27 according to an e-mail I received from Matej at Reichard Modelsports.

Mitch
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Old Mar 07, 2012, 10:27 AM
Electric Airplane Junkie
bhchan's Avatar
United States, CA, San Mateo
Joined Jun 2000
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I measure the CG at the LE right next to the fuselage. So there might be a couple mm off. At what I call 90mm, the plane stays at the dive angle(about 40-45) or even wants to tug under. When I finished the plane, I have to put about .6 oz(18g)of lead between the elevator and rudder to get the 90mm. At the field, after the first flight, it was recommended to remove the lead and try again. The dive test without the lead resulted with the plane coming out the dive slowly. I flew it at that CG the next day and it is very easy to fly and read lift. Granted, the plane is visually challenged, it has a small profile, and it is hard to see when it is over 1000'(300m).

Brian, an EAJ
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Old Mar 07, 2012, 12:06 PM
Arjan
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Deventer, the Netherlands
Joined Mar 2007
120 Posts
okay, if it tucks under it is not right - but as stated before, mine does not. One question though - did you trim it for minimal sink before you tried to dive it? It is a very slow plane so if you make it fly faster you will have to dial in some down-trim, which could make it unstable.

If you feel comfortable with it, that is all that matters.
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Old Mar 07, 2012, 12:24 PM
Electric Airplane Junkie
bhchan's Avatar
United States, CA, San Mateo
Joined Jun 2000
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Also I might have over speed the plane at one time and noticed the elevator is not responding. I think at speed the boom can be bent by the force of the up-elevator.Instead of pulling the plane up, the boom bent down.

Brian, an EAJ
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Old Mar 08, 2012, 11:45 AM
Arjan
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Deventer, the Netherlands
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Quote:
Originally Posted by Mathelo View Post
Interesting. What do you know about the airfoil, which of course would dictate how flight modes should be used for optimal performance.

Louis
Not much, as there is very little info on the web. AG25 - AG27 does not seem to be used a lot on flapped gliders; I am pretty sure my settings should not do much for minimal sink. It probably reduces the minimum speed a bit, which makes it possible to circle tighter.
On the other hand, my logs seem to indicate that minimum sinkrate in thermal flightmode is lower than in normal mode. Oh well....

Quote:
Originally Posted by bhchan View Post
I think at speed the boom can be bent by the force of the up-elevator.Instead of pulling the plane up, the boom bent down.
I can not confirm this from the onboard footage I have. The boom feels very resistant to bending forces, although it is pretty easy to twist (which under normal circumstances is not a problem at all).
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Old Mar 08, 2012, 01:40 PM
Electric Airplane Junkie
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United States, CA, San Mateo
Joined Jun 2000
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Quote:
Originally Posted by PH_AJH View Post
I can not confirm this from the onboard footage I have. The boom feels very resistant to bending forces, although it is pretty easy to twist (which under normal circumstances is not a problem at all).
It could be just the servo not able to push the elevator. My observation was I pull up and the plane did not until a few seconds later. The elevator is pretty big on the Cumulus.

Brian, an EAJ
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Old Mar 08, 2012, 01:45 PM
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Just curious which servo's are you guys using for the elevator?
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Old Mar 08, 2012, 01:50 PM
Arjan
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Deventer, the Netherlands
Joined Mar 2007
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I have 6x Dymond D60-SHT in mine. They are 1 mm to thick for the ailerons; d47 might be better for the ailerons.
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Old Mar 08, 2012, 01:52 PM
Electric Airplane Junkie
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United States, CA, San Mateo
Joined Jun 2000
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Airtronics 94802 on Rud, Ele, Flap.

Brian, an EAJ
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Old Mar 10, 2012, 09:44 PM
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San Bruno CA (near San Francisco)
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Cumulus 2.5e Build Thread at BayRC

I'm coming to the end of posting a build thread on my Cumulus 2.5e that will show a pull-spring setup, as well as boom stiffening for the Cumulus here: http://www.bayrc.com/boards/viewtopic.php?f=9&t=9736

Stop by the BayRC Store while you're at it. There are a few unique items that might be of interest: http://shop.bayrc.com/

Mitch
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