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Old May 09, 2012, 10:04 AM
SSP ASP#88
labrown's Avatar
United States, NC, Durham
Joined Apr 2009
86 Posts
Quote:
Originally Posted by skyhawk newbie View Post
Does anyone fly a no decalage mod , but has there CG set aft ? or it anyone flying Decalage with stock CG ?.
I turned the fuse into packing peanuts after losing my Radian in the sun and plan to experiment with rearward CG after I complete moving everything to the new fuselage I bought. Hopefully some results this weekend. If I have to crank a lot of elevator trim in to make the CG work at 3" - 3.5" I'll think about doing the decalage mod.

One thing I noticed: the new fuse is quite a bit stiffer vertically then the old fuse. The fiberglass stiffener must be glued in better.

--[Lance]
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Old May 09, 2012, 10:19 AM
Tossing planes into the snow
Canada, BC, Smithers
Joined Nov 2011
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The last few posts have cleared up my mind considerably, and it is all about the way things are worded. It is one thing to say you change the decalage and then have to move the CG back to compensate, and another thing to say you move the CG back (just because you want to) and then change the decalage to compensate. The second one makes total sense to me.

Is it possible to get the CG back that much without putting weight on the tail?
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Old May 09, 2012, 10:21 AM
More Altitude, Less Attitude!
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United States, SC, Ninety Six
Joined Mar 2008
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No question, an aft CG is the key to better thermal performance. You can certainly dial in enough down elevator trim to compensate, but the horizontal stab still has many times more surface area than the elevator and so has a greater influence on the pitch of the fuselage as force (downward force as air flows over the angled stab) is applied. This is acceptable only so long as the forces acting on the horizontal stab do not change the pitch of the fuselage in such a way as to cause the angle of attack of the wing to be come excessive. This would explain why the stock Radian has a tendency to loop under power, balloon up in a head wind, and porpoise in flight whenever optimum gliding speed is exceeded. Moving the CG aft seems to exacerbate those tendencies, as even less force acting on the stab is required to pitch the nose up. My opinion is that the stock decalage angle is perfect for stable flight with the CG set at 2.5 from the LE, within a certain speed envelope. The modified decalage angle seems to allow better performance over a wider variety of flight conditions with the CG set aft to as much as 3.5. Nothing dogmatic here. Its not for everyone. Purely a matter of preference.
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Old May 10, 2012, 08:37 AM
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Quote:
Originally Posted by skyhawk newbie View Post
Does anyone fly a no decalage mod , but has there CG set aft ? ... or it anyone flying Decalage with stock CG ?
The stock horizontal stabilizer setting is the equivalent of having constant up-elevator trim. Flying the mod without moving the cg is like applying down elevator trim as suggested by Kenny. The plane will take a gentle dive attitude, and the offset elevator will add some drag. Neither is good. On the other hand if you just move the cg back without the mod it becomes "tail-heavy" and unstable, as with any plane.

The key is AoA. The goal is to achieve the AoA that will result in the optimum glide angle. AoA is influenced by three things; horizontal stab incidence, elevator trim and CG. Applying various combinations of the three can achieve exactly the same AoA. Thus moving the CG back after doing the mod applies a nose-up force that offsets the nose-down affect of changing the stab angle.

The difference in performance comes from the plane's dynamic response to thermals. The stock decalage acts sort of like a shock absorber in a car's suspension. It damps the pitch-up moments caused by thermals (and wind gusts). The reason for having decalage is exactly that - to make the plane more stable. Removing it makes the plane more sensitive to thermals but also more likely to stall in certain conditions. That is why some say that the mod is not for everyone, or for more experienced pilots only.
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Old May 10, 2012, 08:48 AM
turn, turn, turn.
Athol, Massachusetts
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Thing is, if you move the cg back and re-trim, you've accomplished the same thing.

The extra drag is negligible, and is not noticed.
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Old May 10, 2012, 09:06 AM
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Servos

Pulling info from the Radian thread... a recent post mentioned this servo data base site of over 2000 servos

http://www.servodatabase.com/servos/all

Great reference for those who might build their Radians from parts or need to replace a stripped PKZ1060. Unfortunately there are no Parkzone servos in this database.

So... from another post

http://www.rcgroups.com/forums/showthread.php?t=693395

The PKZ 1060 used on the Radian and Radian Pro is speculated to be 28 oz/in"

I've now used the HTX900 HK specials for a couple of weeks in high wind, loops and snap rolls and one nose snapping crash on my Radian and they work fine even though they spec out at about 80% of the PKZ1060's torque.

I'm also using used DSV130M digital metal gear Parkzone servos from a dead Stryker which I suspect is way over spec of the PKZ1060, but, as with all PKZ servos, it is hard to get a spec on them.
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Last edited by Radio.Active; May 10, 2012 at 10:15 AM.
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Old May 10, 2012, 10:18 AM
Not a newbie anymore..
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Ireland, Cork
Joined Oct 2010
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Quote:
Originally Posted by chickenwing View Post
No question, an aft CG is the key to better thermal performance. You can certainly dial in enough down elevator trim to compensate, but the horizontal stab still has many times more surface area than the elevator and so has a greater influence on the pitch of the fuselage as force (downward force as air flows over the angled stab) is applied. This is acceptable only so long as the forces acting on the horizontal stab do not change the pitch of the fuselage in such a way as to cause the angle of attack of the wing to be come excessive. This would explain why the stock Radian has a tendency to loop under power, balloon up in a head wind, and porpoise in flight whenever optimum gliding speed is exceeded. Moving the CG aft seems to exacerbate those tendencies, as even less force acting on the stab is required to pitch the nose up. My opinion is that the stock decalage angle is perfect for stable flight with the CG set at 2.5 from the LE, within a certain speed envelope. The modified decalage angle seems to allow better performance over a wider variety of flight conditions with the CG set aft to as much as 3.5. Nothing dogmatic here. Its not for everyone. Purely a matter of preference.
Quote:
Originally Posted by kaptondave View Post
The stock horizontal stabilizer setting is the equivalent of having constant up-elevator trim. Flying the mod without moving the cg is like applying down elevator trim as suggested by Kenny. The plane will take a gentle dive attitude, and the offset elevator will add some drag. Neither is good. On the other hand if you just move the cg back without the mod it becomes "tail-heavy" and unstable, as with any plane.

The key is AoA. The goal is to achieve the AoA that will result in the optimum glide angle. AoA is influenced by three things; horizontal stab incidence, elevator trim and CG. Applying various combinations of the three can achieve exactly the same AoA. Thus moving the CG back after doing the mod applies a nose-up force that offsets the nose-down affect of changing the stab angle.

The difference in performance comes from the plane's dynamic response to thermals. The stock decalage acts sort of like a shock absorber in a car's suspension. It damps the pitch-up moments caused by thermals (and wind gusts). The reason for having decalage is exactly that - to make the plane more stable. Removing it makes the plane more sensitive to thermals but also more likely to stall in certain conditions. That is why some say that the mod is not for everyone, or for more experienced pilots only.

Thanks for the reply guys...

I was just wondering

As you know one of my Rads does have the Dacalage Mod with CG @ 80mm
While the other one does not and the CG is stock..

I'm still waiting for the right day when I can get a friend to fly my stock Rad and
I'll fly my Decalage moded one at the same time

He's tryed to fly my moded one but found it hard

Yeah... It's not for the inexperienced Radian flyer..

Sean

.
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Old May 10, 2012, 10:18 AM
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Quote:
Originally Posted by Kenny Sharp View Post
Thing is, if you move the cg back and re-trim, you've accomplished the same thing.

The extra drag is negligible, and is not noticed.
The result is not the same. If so Paul Nation, who knows a lot more about the subject than I (and you perhaps?), would not have gone to so much work. However, if it is satisfactory to you that is all that matters.
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Old May 10, 2012, 11:17 AM
turn, turn, turn.
Athol, Massachusetts
Joined Oct 2005
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Quote:
Originally Posted by kaptondave View Post
The result is not the same. If so Paul Nation, who knows a lot more about the subject than I (and you perhaps?), would not have gone to so much work. However, if it is satisfactory to you that is all that matters.
I've done it to my hotliners.... And it is good to understand why and how things happen.

Paul has taught me a lot through his videos... It is why I started going to contests.
I will always defer to Paul.
He's a great guy, and a friend.
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Old May 10, 2012, 12:04 PM
Tossing planes into the snow
Canada, BC, Smithers
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Quote:
Originally Posted by Radio.Active View Post
I'm also using used DSV130M digital metal gear Parkzone servos from a dead Stryker which I suspect is way over spec of the PKZ1060, but, as with all PKZ servos, it is hard to get a spec on them.
The DSV130M is also used for the flaps on the Radian Pro. Do you know why the flaps would have digital metal-gear servos, and the ailerons have plastic analog ones? I've been wondering about that for a while.
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Old May 10, 2012, 01:09 PM
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Jovanx,

They put stronger servos on flaps because they take a pounding from long grass, or even short grass, when landing. A small plastic geared servo won't hack it.
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Old May 10, 2012, 01:32 PM
Tossing planes into the snow
Canada, BC, Smithers
Joined Nov 2011
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It makes sense that metal can take more abuse than plastic, but why digital? I thought that digital servos were supposed to be faster and more precise than analog. Wouldn't that extra precision be of more use on ailerons? It doesn't really matter how fast the flaps go down.
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Old May 10, 2012, 01:42 PM
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Last edited by Radio.Active; Jun 13, 2013 at 01:58 PM. Reason: Correction
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Old May 10, 2012, 01:49 PM
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Old May 10, 2012, 01:50 PM
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I don't know why they would go digital. Perhaps they don't make an analog with metal gers? Or, perhaps, they just don't make an analog metal geared servo that's the right size?
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