|Dec 02, 2012, 10:58 AM|
I fly with a group of hard core and excellent world class pattern guys .
(Not including me- -I no longer compete thanks to a degenerative eye thing )
But anyway the theories about what needs streamlining and what does not continues .
Most of the results are simply obvious-
Shifting drag aft has alwas been a good idea - since the invention of the bow and arrow - no math /tunnel results needed here .
Years back we played with the location of canopies on simple pattern planes
depending on where the canopy was placed (front to rear) the model would change from a smooth stable flier to a tail wagger -
Nothing to do with "spirialing prop wash (hogwash)" just simple disturbances which cancelled the vertical fin's stabilizing .
Along came bigger better F3A and IMAC stuff and the old tail wag showed up on some designs
the quick fix was a strip of something on the rudder TE- anything change a low stabilizing force of the fin
It's all compromise - just like airfoils on wings -
change something - something else is usually improved OR screwed up.
I guess I could calculate it -- but obvious is obvious and I can't count past 15.
|Dec 02, 2012, 01:22 PM|
Joined Apr 2009
I apologize for not having completely read this thread, and I also apologize because I'm way too busy to hang out in this subforum.
But, there seem to be some severe misconceptions bounced around on this thread.
Let's look at that drag on the back end thing, with respect to tail wag.
If the plane is perfectly in line with the airflow, then that drag simply provides a retarding force on the plane. It does not provide any yaw component (minor caveats that the plane actually looks like a conventional plane).
Now, let's say the plane is yawed. The drag from the vertical tail is now offset to the side of the center of mass. That drag is in the direction of the freestream... So that drag provides a restoring force, roughly proportional to the yaw angle for sufficiently small angles.
What happens if one puts a long rod of some mass on a frictionless pivot at its center of mass, attaches a long rope to one end, stands back, and tries to pull the rod into alignment with the rope? The rod oscillates back and forth. That's essentially the contribution of drag. It doesn't dampen the oscillation but instead contributes slightly to its continuance. For those into the physics of it, it is just another variant of the harmonic oscillator problem with little in the way of damping.
There is another force that is roughly proportional to the yaw angle for sufficiently small angles. That is the lateral lift of the vertical tail. The vertical tail is a low aspect ratio wing. As is typical for wings, its 'lift' is generally an order of magnitude or so higher than its 'drag' for small angles. So that lateral lift generates a lot more force than the drag. The drag is a red-herring.
A plane turned on its side uses the fuselage as the (very low aspect ratio, inefficient) primary wing. The vertical tail acts as a horizontal tail. If one wants the plane to be stable in this orientation, the same stability rules apply as for any other orientation. One doesn't want to move the CG to affect stability, as both the stability when in normal flight orientation and the stability in knive edge are tied to the same CG. What one can do instead is things like move the canopy (moving some of the main "wing" backwards or forwards) or adding/removing area on the vertical tail (increasing/decreasing its size and moving its aero center).
BTW, the drag on the back for arrows analogy is wrong as well. Arrows are just like any other missile. If one wants the front to stay in front of the back, then there has to be positive stability and/or spin. Arrows typically use both. An arrow with a lot of drag added to the back is a flu-flu; its purpose is to prevent the arrow from going a long way, not solely to stabilize it. FWIW, arrows are stable with field or target points, with no fletching at all. That is because the CM is still in front of the neutral point, due to the point having a higher density than the shaft. It is just a lot harder to shoot such an arrow accurately.
|Dec 02, 2012, 01:30 PM|
GT you really should do some hands on testing
The turbulance from fuselage canopies does, in easily demonstrable insances - setup a condition where the fin will not hold the craft "on line" - what happens is a very nice oscillation. simply changing the flow by repositioning the canopy will stop or change the oscillation-
OR adding some drag at the rudder TE will fix the problem
Done his test/ fix over years .
Call my interpretation wrong if you will bu tI do know how to fix basic problems.
|Dec 02, 2012, 01:33 PM|
|Dec 02, 2012, 03:19 PM|
Look for some old codger who flew the Phil Kraft Kwik Fly models and experienced the "Quick Fly waggle".
The problem occurs simply because an absence of dihedral/sweep etc., creates a setup which is easily upset and when the craft yaws very slightly as it will-- the momentum carries it till the greater pressure DIFFERENCE starts it moving back the other way. and so on-- IF conditions are right - the waggle is a nice slight side to side movement.
Nothing requiring windtunnel testing - or textbook searching.
Oscillation is a subject which is seen in lots of applications
Old cars with poor steering damping were notorious for developing a "shimmy" or in some cases an extreme lock to lock wheel excursion.
On n on -- just an other common problem .
|Dec 04, 2012, 01:07 PM|
The problem was simply about selecting a good fin
I just included examples about stability.
Your opinions notwithstanding --
|Dec 04, 2012, 02:51 PM|
Aerodynamics is aerodynamics, It doesn't matter if it's a plank or a pattern model.
Adding drag does make a model more stable. There's simply no doubt about that. Anyone that has flown a control line combat model that is a bit twitchy on its own knows that it'll settle down when there's a full leader and streamer attached to it. The effect is directly noticable by the pilot and it is VERY noticable at that.
GT, unlike your pulled string analogy the effect of drag through aerodynamic means isn't the low loss spring like pull that you'd get from your described setup with a string and direct pull. Instead the drag from a blunt tail trailing edge would tend to have a more viscously damped like force. So I suspect that it would not result in a resonance situation such as you described. And certainly the pull on a combat model's tail by the streamer would be a very strongly damped sort of pull that aids in promoting stability rather than resulting in a resonant effect. THis is again borne out by the doggy nature that is given to the combat models with streamers attached and how much they perk up and become quicker reacting as the streamer is cut away.
Still, stability through drag isn't the optimum way to do things if high speeds are the desired goal. I can appreciate it being used as a trimming option where speed doesn't really matter but I tend to not fly designs of that sort too often.
|Dec 04, 2012, 03:17 PM|
Drag on a tail fin certainly adds stability. Displace the tail laterally and the drag will create a torque that will want to restore the tail to its original position.
Bruce, I've got to side with GT on the question of damping. Although intuition might suggest that drag (something that tends to dissipate energy) would have a damping effect, I think in this case it will not significantly damp small yaw excursions (if at all). The restoring torque due to the drag on the trailing edge of a fin is equal to the drag times the lateral displacement. Unless the drag acting on the trailing edge drops as the tail picks up a yaw rate back toward the center, it will not provide any damping.
So unlike the stabilizing side force acting on a tail fin (which WILL drop due to yaw rate back toward the center) the drag force acting on a tail fin will not provide significant damping. It would not be difficult to set up a simple experiment to demonstrate the difference.
|Dec 04, 2012, 03:30 PM|
Having seen and fixed this problem is stuff ranging from the old models -like the Kwik Fly and other models such as the little Quicky types and and on current models - Ithought the idea of increasing drag at center -especially applicable to a design such as the plank - with it's straight wing and extremely short moments .
Wouldn't have mentioned it if I had not tried it in various apps .
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