Sig Four-Star 20EP Maiden Flights - RC Groups
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This thread is privately moderated by WAGliderGuy, who may elect to delete unwanted replies.
Dec 05, 2010, 12:07 AM
Airliner Builder
WAGliderGuy's Avatar

Sig Four-Star 20EP Maiden Flights

I will say the maiden went good but not great. Here is the video:

Sig Four-Star 20 EP Kit Maiden Flights! (2 min 2 sec)

As you can see the model seemed very tail heavy even though I balanced it out right on the main wing spar where the manual states to. I landed it, took one lead weight out of the back and took it up again. Still tail heavy! This time It wasn't as bad so I decided to just stick alot of down trim in and land a little early (down trim and a tail heavy plane causes drag).

Even though the model was tail heavy it flew great! Rolls, loops, immelmans, spilt-s's you name it! This model is a floater and does not fly very fast due to its semi-symetrical airfoil. The motor setup I use gives it unlimited vertical but I only need 1/2 throttle or less to cruise around. Landings are a breeze and as you can expect this light Four-Star hanges on ground effect for awhile! Stalls? Well it just slowly pitches downward! All in all, this model flies as you would expect any famous sport plane to

Now more about the 'cg' issue I am having. I took a 3rd flight up with a bigger and heavier 3000 mah 3S and set the trim to neutral. Still tail heavy! So I landed and took the weights out of the tail and then what up again. Tail heavy but not as bad as the last time. But this is ridculous, the model is way nose heavy on the CG machine. I thought it may be a aerodynamic issue so I checked the angle of attack on the horizontal stab and the wing....all are perfect as per the plans. I expected it to be nose heavy with the bigger motor and battery I was using!

Any thoughts?

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Dec 05, 2010, 01:59 AM
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machinate's Avatar
May I ask how the plane behaves which lets you determine so quickly that it's tail-heavy?

I apologize in advance if you're already aware of what I'm about to say, but I've seen a lot of people lately confusing this and want to make sure it's not your issue. Just because the plane needs down-trim, does not mean it's tail-heavy. Longitudinal balance (front-to-back) is purely a stability concern and should not be used to trim the airplane. Because model aircraft are not perfectly designed or constructed, it's normal for them to require some trim to fly straight, even if they are perfectly balanced. Balancing should only be used to correct the way the plane recovers from deviations in normal flight.

A tail-heavy model, when it hits turbulence, will tend to go the direction the turbulence pushed it. When the plane speeds up, it will dive, gain more speed, and try to do a tight outside loop. When the plane slows, it will climb and stall. In contrast, a balanced or slightly nose-heavy plane will recover from any deviations and resume its normal flight. If it's too nose-heavy, it'll still fly fine but will maneuver slowly and require substantial trim changes at different speeds.

You look like a very capable pilot, but I don't know anybody, even the competition Pattern pilot at my field, who would be able to correctly identify a new airplane as tail-heavy within three seconds of breaking ground.
Dec 05, 2010, 02:22 PM
Airliner Builder
WAGliderGuy's Avatar
I think your right. As soon as I got off the ground the model almost shot straight up and my first reaction was to say tail heavy. I tried some inverted flight and the model pitched up. While in knife edge the model heavily leaned torward the canopy. I know most low-wing sport planes do that but this one was really pulling. Once I trimmed it, it flew ok but I could tell it still wanted to pitch up. On the Four-Star 20 thread someone suggested enlarging the tail section but I believe Sig already did that to this kit. I have added some down-right thrust (doh forgot to do that before the maiden flights) so maybe that might help a little.
Dec 05, 2010, 02:32 PM
Registered User
machinate's Avatar
Inverted flight is one of the simplest ways to test for CG, actually. Trim the plane to fly well upright, then flip it over. If you have to hold a small amount of down-elevator, you're perfect for this type of plane. If you need no corrections, the plane is totally neutral and might be challenging to fly, but should be very capable at aerobatics. If you actually need to hold up-elevator to keep the plane from climbing out, you're tail-heavy and need to correct it.

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