Horizontal Stab stalling/ ballooning?  
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All Forums >> RC Airplanes >> Aerodynamics >> Horizontal Stab stalling/ ballooning?
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Horizontal Stab stalling/ ballooning? - 8/15/2003 3:03:29 AM   
Paul Cataldo


 

Posts: 816
Joined: 2/9/2003
From: Fayetteville, GA, USA
Status: offline 		Hal,
I am having this same problem with my DP Edge. I too thought that the stab needed some positive incidence. However, when I added a little positive incidence, the plane dove down towards the ground at higher speeds.
I was thinking that I should add some upthrust to the engine to compensate for this. I already have 1 degree of UPTHRUST in the engine. Should I add more upthrust to compensate for the positive incidence in the tail? Thanks.

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Paul Cataldo

(in reply to Flyin Woodbutcher)
       Post #: 51

Horizontal Stab stalling/ ballooning? - 8/15/2003 9:34:10 AM   
Ben Lanterman



Posts: 1296
Joined: 10/27/2002
From: St. Charles, MO, USA
Status: offline 		Hal you are ok until you say..

Note> when tail lift is increased the neutral point moves back......

The Neutral Point (NP) is just areas for the most part. It shows up in the slope of the Cm vs Cl curve which is dCm/dCL. In an airplane with an all moving tail such as the F-4 and F-15 the slope of the curve (and NP) is the same regardless of the deflection of the horizontal tail. You do get a big moment change but also a corresponding lift change that makes the curves the same.

My very first job in the aero dept for McDonnell Aircraft Co. was reading the slopes of several hundred curves from a wind tunnel test. Little did I realize that the next 35 years would be doing the same thing off and on.

But since for moment balance about the CG (no matter where it is) if you want the same airplane to fly level after you have introduced a positive angle on the tail (which is just like moving the horizontal on the F-4) you have to change the moment arm of the tail (smaller) and wing (larger) to make the aero pitching moments about the CG the same. The easiest way to do that is to move the CG aft. It will make the airplane a little less stable but we fly with a lot of excess stability for the most part.

In the case of the first Cap indeed the tail lift seems to be not changing the same as the wing lift (moments and all included) but it seems that the tail lift angle of attack is being changed by something. Since the total angle of attack the tail sees is just the airplane angle of attack plus downwash effects with angle of attack and perhaps a burble from the cheek cowls (Tall Paul) it would seem the tail lift is being changed by one of the above.

The fact that the airplane is apparently behaving OK through the rest of the flight envelope would indicate the balance of tail and wing forces and moments are OK, mostly.

Questions, comments??


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Ben Lanterman

(in reply to Flyin Woodbutcher)
       Post #: 52

Horizontal Stab stalling/ ballooning? - 8/15/2003 2:59:15 PM   
Flyin Woodbutcher


 

Posts: 836
Joined: 12/16/2002
From: Ashland, VA, USA
Status: offline 		Ben I will agree with the mostly, even if the mostly is at 99%. The 1% is the death zone thats going to crash the plane.

Kent

(in reply to Flyin Woodbutcher)
       Post #: 53

Horizontal Stab stalling/ ballooning? - 8/17/2003 7:04:51 AM   
Tall Paul



Posts: 4725
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		I've shimmed the wing on my CAP so it aligns with the horizontal, which was at +1° (TEdown) relative to the wing.
Previous flight trim had the elevator about 1° airplane nose up.
Today the elevator is in line with the horizontal. I'll call that 0°.
I flew it and Tony did, with about the same comments.
Inverted it needs a LOT of down elevator.
Why this should true is a puzzlement!
It did this yesterday also.
Downthrust still at -6° FRL.
The airplane is somewhat less exciting to fly, but it still needs excessive care to avoid departures!

(in reply to Flyin Woodbutcher)
       Post #: 54

Horizontal Stab stalling/ ballooning? - 8/17/2003 11:23:00 PM   
Ben Lanterman



Posts: 1296
Joined: 10/27/2002
From: St. Charles, MO, USA
Status: offline 		I have never flown a Cap configuration until my present model. In level flight I give a little rudder and it seems like I must have moved the elevator stick the same amount. It dives for the ground.

Although I expected some nosedown I was still amazed. I have been pulling some pattern ships out of mothballs and (after replacing some motors and radio gear) had just flown a QuickFli III (trivial coupling) and a Curare (small coupling) and they were quite benign by comparison.

In looping and rolling maneuvers and even at low speeds the Cap is comfortable to fly, no worse than a lot of airplanes I have flown.

But considering the rudder coupling and the work required to trim it out I have no idea why someone would choose this airplane given the other designs available. Well I do, it does a lot of things right and is good at snaps, etc. but....... yeech!


_____________________________

Ben Lanterman

(in reply to Flyin Woodbutcher)
       Post #: 55

Horizontal Stab stalling/ ballooning? - 8/18/2003 4:52:24 AM   
Tall Paul



Posts: 4725
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		[QUOTE]Originally posted by Ben Lanterman
I have never flown a Cap configuration until my present model. In level flight I give a little rudder and it seems like I must have moved the elevator stick the same amount. It dives for the ground.

Although I expected some nosedown I was still amazed. I have been pulling some pattern ships out of mothballs and (after replacing some motors and radio gear) had just flown a QuickFli III (trivial coupling) and a Curare (small coupling) and they were quite benign by comparison.

In looping and rolling maneuvers and even at low speeds the Cap is comfortable to fly, no worse than a lot of airplanes I have flown.

But considering the rudder coupling and the work required to trim it out I have no idea why someone would choose this airplane given the other designs available. Well I do, it does a lot of things right and is good at snaps, etc. but....... yeech! [/QUOTE]
.
I flew my scale Ultimate today. It will do a complete snap roll solely on rudder!
Yet it's much more comfortable to fly than the CAP, which gives the impression it's just waiting/hoping to depart at any excuse!
.
There's a well-built Extra in the LHS... proper motor, all servos... good price.. awful color scheme;white,teal, blue... tempting, if I didn't have 50 other airplanes here now.

(in reply to Flyin Woodbutcher)
       Post #: 56

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All Forums >> RC Airplanes >> Aerodynamics >> Horizontal Stab stalling/ ballooning?
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