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40 sized phoenix models sukhoi sensitive elevator at low speed.

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Old 12-09-2011, 12:27 AM
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RS2K
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Default 40 sized phoenix models sukhoi sensitive elevator at low speed.

I built this plane a bit heavy. It has an old OS surpass 70 in the nose, so I put the elevator and rudder servos in the tail to compensate. I don't have a scale, but it certainly feels heavy for a 40. It's currently a bit nose heavy, but I have plenty of room to move the battery and receiver back as they are both currently in the nose.

The oddest issue I'm getting right now is the elevator gets really sensitive right above stall speed. This is a bit of a problem when landing. During a light flare, the model will want to start pitching (and climbing) at an increasing rate with a constant amount of elevator. Coming in with extra speed and making a 2 point landing instead of a 3 point landing avoids the problem, but the grass strip I fly from is pretty rough and high speed landings cause bouncy problems of their own.
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Old 12-09-2011, 12:43 PM
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Default RE: 40 sized phoenix models sukhoi sensitive elevator at low speed.

Are you sure that it is nose heavy?
If the elevator has much authority it is not.

Is the hinge line of the elevator sealed with tape?
That seal helps keeping control at low speeds.
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Old 12-09-2011, 12:48 PM
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Default RE: 40 sized phoenix models sukhoi sensitive elevator at low speed.

Well, the flat spins aren't very flat, and it takes more elevator than I think it should while inverted. Elevator authority is just fine at low speeds, the problem is the elevator seems to get more authority right above stall speed.

The plane also has a tendency to snap a bit with too much up elevator, but while inverted I can give it full down elevator without getting a snap.
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Old 12-09-2011, 01:27 PM
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Default RE: 40 sized phoenix models sukhoi sensitive elevator at low speed.

The odd reaction may be caused by turbulence and downwash generated by the wing at high AOA, which affects the airflow over the elevator.

I would re-check the location of the CG as per the manual:

"THE CENTER OF GRAVITY IS LOCATED 110mm
BACK FROM THE LEADING EDGE OF THE
WING, AT THE FUSELAGE. This location is
recommended for initial test flying and trimming.
There is a 12mm margin forward and aft.
BALANCE A PLANE UPSIDE DOWN WITH THE
FUEL TANK EMPTY."

Also the throws:

Ailerons : 10mm up 10mm down
Elevator : 11mm up 11mm down
Rudder : 20mm right 20mm left

Followed by a lateral balance, and sealing of all the hinges lines.
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Old 12-09-2011, 05:59 PM
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Default RE: 40 sized phoenix models sukhoi sensitive elevator at low speed.

I've been thinking about this off and on all day. I have not ever heard of such a thing due to CG location. I would be interested to know what servo is being used and the linkage geometry. The only thing I can think of is that the servo is not up to the task and is being stalled at the higher speed. We see this sometimes on the giant scale aerobatic airplanes but usually on the rudder. Anyways, if it is a standard torque servo it may get stalled at high speeds but as the speed is lowered it is able to go full travel which at low load conditions is too much.
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Old 12-09-2011, 10:51 PM
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Default RE: 40 sized phoenix models sukhoi sensitive elevator at low speed.

The CG is at 4.5 inches or about 114 mm. It can definitely use a bit more weight towards the back. The spins aren't flat and I need quite a bit of down elevator while inverted. Elevator trim stays the same between moderately low speeds and full throttle. There is a slight nose drop as the speed gets lower than moderately slow though. It's just above stall while landing I notice the elevator starts to get about twice as sensitive as normal. It gets downright squirrely.

I scavenged the radio gear off of an old 1/4 scale cap. I had these same servos installed in the 1/4 scale cap with 3D throws and they performed excellently. The servo is definitely up to the task as I'm using digital 9252s on the rudder and elevator. There is no slop in the control because I'm using the same slopless ball joint control rods off the 1/4 scale cap as well.
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Old 12-10-2011, 09:45 AM
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Default RE: 40 sized phoenix models sukhoi sensitive elevator at low speed.

Quote:
ORIGINAL: RS2K

During a light flare, the model will want to start pitching (and climbing) at an increasing rate with a constant amount of elevator.

...........

There is a slight nose drop as the speed gets lower than moderately slow though.
As CG location, hinges sealing and linkages are OK, then up or downthrust and wing-stab incidence would be the next things to check.

See these diagrams:

http://www.rcuniverse.com/forum/fb.asp?m=10814674
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Old 12-10-2011, 10:12 AM
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Default RE: 40 sized phoenix models sukhoi sensitive elevator at low speed.

What I am most curious about is your linkage setup. What is your servo arm length? What is the control horn length and what voltage battery? what gauge are the wires feeding the RXfrom your switch? These things all have the ability to reduce servo torque and/or force applied to the surface.
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Old 12-10-2011, 10:31 AM
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Default RE: 40 sized phoenix models sukhoi sensitive elevator at low speed.

Good point!
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Old 12-12-2011, 05:16 AM
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Default RE: 40 sized phoenix models sukhoi sensitive elevator at low speed.

Just a simple suggestion.

Many local post offices now have self serve package mailing kiosks in the lobby.
The lobby is usually open 24 hours a day.

Select a time when the place won't be busy with regular commerce...

Take all the bits of a model down to the post office and weigh it all (use the menu on the scales as if you're mailing a package).

The scales in a post office are calibrated often and have decent precision (measuring to the 1/10th ounce).

Also, WalMart offers several and many different scales. The Mustad digital is 18 bucks and is easy to use. There are other scales available...

Knowing what your gross weight and wing loading is goes a long way to building and preparing models for more successful flights.
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