RUDDER STALL    Gallery
View related threads: (in this forum | in all forums)

Logged in as Guest



Users viewing this topic: none
    Search This Thread  
 
Printable Version


Scale Mustang Exhaust - Kit
Seller:  tony-howard
Details:   $35.00   |  5/1/2013   |  Classified Ad
We will rotate YOUR AD in this spot if you select "Forum Featured" when placing or editing your ad!

All Forums >> RC Airplanes >> Aerodynamics >> RUDDER STALL
Page: [1]

Tower Hobbies Get Coupon Codes Brands  
Login
Message << Older Topic   Newer Topic >>
RUDDER STALL - 9/7/2011 2:42 AM   
turboromy


 

Posts: 36
Score: 100
Joined: 10/21/2007
Last Login: 5/16/2013
From: Chino Hills, CA, USA
Status: offline 		I have pretty good understanding of stall itself.  But now I'm wondering if rudder can stall by itself ????

I'm currently setting up a 3D airplane.  It's a 30cc size from 3DHS.
Rudder movement initally was to the physical max sligthly shy of touching elevator.  I guess it was at least more than 50 degrees.
Compared to patternships which I usually fly, it didn't do any smooth manuevers right, but was able to do many wild stuff in the hands of best pilot in my field.
I understand 3D can't be as smooth as F3A ships... but this was quite extreme.

Some of the problems include not enough rudder authority on vertical up lines and during rolling circles, and 4 point and slow rolls were really ugly.

Then... suddenly I thought too much deflection could stall the rudder itself, and experimented with much smaller rudder deflections... to about 20 degress.
Then it does everything right. 

So, I get to wonder if rudders can stall simply due to too much deflection of the moving part thus not turning the aircraft.

Thanks in advance.


Hide Signatures
       Post #: 1

RE: RUDDER STALL - 9/7/2011 5:26 AM   
jester_s1


 

Posts: 3796
Score: 233
Joined: 12/17/2006
Last Login: 5/18/2013
From: Fort Worth, TX, USA
Status: offline 		Anything moving through air can stall. But are you sure you were getting all of the deflection? I would think a 3D plane would spin around just from the prop wash with that much rudder.

_____________________________

For me, an ARF is an airplane that someone else just crashed.

Hide Signatures

(in reply to turboromy)
       Post #: 2

RE: RUDDER STALL - 9/7/2011 6:38 AM   
BMatthews



Posts: 11655
Score: 247
Joined: 10/4/2002
Last Login: 5/14/2013
From: Chilliwack, BC, CANADA
Status: offline 		Just because a surface is stalled does not mean it stops making lift. It's just that there is an excess of drag created by sucking along the big separation bubble at the same time the surface is still producing lift. With the wing such a stall is enough to slow the plane down much like it hit a brick wall. It's the reduction in speed that causes a related reduction in lift that makes it fall out of the sky, not the stall itself.

But most planes can suffer with the smaller drag of the tail surfaces stalling when deflected to a high angle and still fly just fine. And it's a fact that by the time a model elevator or rudder is deflected by maybe as little as 8 to 10 degrees or more it's operating in a stalled condition. And certainly by the time you reach the 20 degrees that you found worked well the rudder is already stalled if you are using the idea that pulling a big separation bubble on the "high" side is the same as being stalled.

I'll bet that the model suffered some issues due to the high drag that was created. Or some other cause related to what is happening around such a strongly deflected control surface.

_____________________________

Witty saying to be plagarized shortly.....

Hide Signatures

(in reply to jester_s1)
       Post #: 3

RE: RUDDER STALL - 9/9/2011 4:20 AM   
turboromy


 

Posts: 36
Score: 100
Joined: 10/21/2007
Last Login: 5/16/2013
From: Chino Hills, CA, USA
Status: offline 		What you're saying makes sense and starts another question.
Loosing lift but creating drags explains why it wouldn't turn while I'm going vertical.
It kinda starts yawing but won't turn any further and I was using more and more rudder with frustration... until I cut it down.

Anyhow the new arising question is.... is there a different or separating region of airspeed in 3D flying between high speed and very low airspeed ?
Reason asking is because when I pull hard on elevator while having some (which I previously thought was low enough) airspeed, this Slick and previous 3D airplanes showed some jerky movement as if it was quite nose-heavy.  Previous ones never got to hovering attitude then with 3DHS Slick I had better luck allowing me to learn how to do it right..... and to me, it seems the key was entering at much lower airspeed to avoid this jerky motion. 

So...I get to wonder if that jerky motion was combination of simple stall with appreciable airspeed making it nose dive (due to normal stall) briefly followed by prop wash with extreme amount of up-elevator causing pitch up motiion.    Am I on the right track of understanding this phenomenon ?

BTW, the high rate of rudder wasn't changed being still very high and it helps very high angle of KE of about 60-70deg with not much trouble.  I think this "prop wash" theory explains why Slick did low speed KE nicely, but poor on higher speed.

Thanks for your input.
More insights will be appreciated.



Hide Signatures

(in reply to BMatthews)
       Post #: 4

RE: RUDDER STALL - 9/9/2011 5:41 AM   
otrcman


 

Posts: 482
Score: 118
Joined: 10/11/2004
Last Login: 5/17/2013
From: Arroyo Grande, CA, USA
Status: offline 		You are making some very good observations. Here are a couple of general comments to help you frame your thinking:

1. Always remember that stalling is an angle-of-attack phenomenon, not a speed phenomenon. Wings stall at low speeds not because they are going slow but because you are driving the wing to a high AOA in order to go slow.

2. Not every part of the plane is stalled just because the wing is stalled. It is possible to stall the horizontal stab before stalling the wing, for instance, if your CG is too far forward. And your vertical stab may not be stalled at all when the wing is stalled. You need to think of the angles of attack and surface deflections of each surface individually.

A good example of staling the vertical stab when the rest of the airplane is not stalled would be when operating a twin engine airplane on one engine at high power and low airspeed. The offset engine puts a strong yawing torque on the airplane. As you slow down, it takes more and more rudder deflection to keep the plane from yawing. At some point, the vertical stab may stall from too much rudder deflection. When the vertical loses left, the engine thrust suddenly takes over and the plane yaws violently. Nearly all modern lightplanes are designed so that rudder stall never occurs. But you will see some planes with vortex generators on the fin or a very blunt fin leading edge to avoid such stalls and subsequent departures. If the fin does not stall, then what you see is that at some speed you will have full rudder deflection and any further decrease in speed allows the airplane to begin turning due to unsymmetrical engine thrust.

3. The operating envelope of a surface needs to be thought of as having three distinct segments. First, there is the low angle of attack range, which is characterized by attached airflow and generally linear changes with angle. Then there is an intermediate range, in which airflow is a mixture of attached and unattached. Changes in lift and drag are highly nonlinear and usually a bit unpredictable. Beyond the intermediate range is a region of fully un-attached flow, where things become fairly linear again.

When you make a gross elevator movement to achieve very high angles of attack, you are going through the intermediate range in order to get to the fully stalled region. If you are going too fast, the unstable forces on the plane will make for rapid motions. By going slower during the pull-up you will decrease the forces on the airplane and thereby reduce unwanted motions. One way or the other, if you pull slowly and linger in the intermediate range, you will see all sorts of unstable motions with most planes. Note that some airplanes have larger or smaller intermediate regions and may or may not exhibit gross motions in the intermediate region.

Flat wing (no airfoil) foamies are an interesting case. They have almost no attached flow or intermediate region. They are flying in the fully detached flow region almost all of the time. Thus they have little instability regardless of angle of attack.

Here is a link to a NASA report in which researchers experimented with a manned glider to fly at angles of attack up to 70 degrees. Some of the report gets pretty technical, but you can easily get the idea by reading the summary.

http://www.nasa.gov/centers/dryden/pdf/87982main_H-1242.pdf


Dick

Hide Signatures

(in reply to turboromy)
       Post #: 5

RE: RUDDER STALL - 9/9/2011 12:35 PM   
rmh



Posts: 12307
Score: 252
Joined: 12/12/2001
Last Login: 5/17/2013
From: , UT, USA
Status: offline 		Good stuff- The vertical fin and rudder on a conventional aerobatic model can easily be blanked which will result in loss of directional control
easily proven by doing deep angle flight upright - then inverted
Inverted , the rudder continues to work at low speeds
The horizontal tailplane (little wing thing on the back) will continue to work at very low speeds IF it can be rotated such that it more closely follows the actual line of flight.
No wind tunnel data - just actual flight testing over n over - over .

_____________________________

Libby is still watching you

Hide Signatures

(in reply to otrcman)
       Post #: 6

RE: RUDDER STALL - 9/9/2011 11:19 PM   
otrcman


 

Posts: 482
Score: 118
Joined: 10/11/2004
Last Login: 5/17/2013
From: Arroyo Grande, CA, USA
Status: offline
quote:

ORIGINAL: rmh

Good stuff- The vertical fin and rudder on a conventional aerobatic model can easily be blanked which will result in loss of directional control
easily proven by doing deep angle flight upright - then inverted
Inverted , the rudder continues to work at low speeds
The horizontal tailplane (little wing thing on the back) will continue to work at very low speeds IF it can be rotated such that it more closely follows the actual line of flight.
No wind tunnel data - just actual flight testing over n over - over .




Agreed, RMH.

I deliberately left out the "blanking" discussion because my post was getting way too long and I didn't want to further complicate the point. But since you've brought the subject up ............

What we call blanking is loss or reduction of some control function due to gross disturbance of the airflow somewhere upstream. In the case that RMH describes, the wing and forward fuselage are probably disturbing the air forward of the vertical stab. If you could place an airspeed sensor right ahead of the vertical you would no doubt see an airspeed that is far less than the free stream airspeed of the airplane.

So we could pitch the airplane up to a very high angle of attack and see not just a stalled wing but also a big reduction in rudder control.

The difference between blanking and stalling is that blanking is the result of some upstream effect. Stalling is the result of operating the surface itself at too large an angle of attack. A blanked surface may be aligned with local flow very nicely; there's just not enough flow.

RMH, I always enjoy reading your posts. It's clear that you have spent a lot of time studying the behavior of your models.

Dick

Hide Signatures

(in reply to rmh)
       Post #: 7

RE: RUDDER STALL - 9/10/2011 9:23 AM   
charlie111


 

Posts: 275
Score: 100
Joined: 8/26/2010
Last Login: 5/19/2013
From: Lynn, MA, USA
Status: offline 		You could try extending the leading edge of the tail.Both Rudder and airleron up the fueselage.It may help stabilize the airflow some before it gets to the tail?

_____________________________

Charlie111 Looking for two h.p. motors with single channel speed control.High Performance maybe equal to o.20 or 0.49

Hide Signatures

(in reply to otrcman)
       Post #: 8

RE: RUDDER STALL - 9/11/2011 2:59 AM   
MetallicaJunkie



Posts: 5330
Score: 243
Joined: 9/17/2006
Last Login: 5/17/2013
From: Donna, TX, USA
Status: offline 		you might be over analyzingl the situation...... f3a ships fly a whole lot smoother than imac planes hand down..... 20 degrees rudder is ok for doing smooth a maneuvers, but much more is needed for 3d flight

_____________________________

"Propellers are notorious for inflicting serious bodily harm while vigorously defending their space" George Aldrich

Hide Signatures

(in reply to charlie111)
       Post #: 9

RE: RUDDER STALL - 9/11/2011 7:45 AM   
turboromy


 

Posts: 36
Score: 100
Joined: 10/21/2007
Last Login: 5/16/2013
From: Chino Hills, CA, USA
Status: offline

Thanks everyone for all the information.  Some information was quite overwhelming and seems there were much more than I initially expected.
I'll have to do my own further research to comprehend all your suggestions.
metallicajunkie, I am much more of F3A pilot myself.  More rudder was always good.  3D planes are acting quite differently.
I flew IMAC contest once and I felt it was much harder.

Thanks otrcman for the analysis for the flat wings.  I knew foamies are easier in some aspects and what you mentioned explains why.
If there's a simple way of explaining "linear" and "non linear", it will be appreciated.  I only have vague idea.



Hide Signatures

(in reply to MetallicaJunkie)
       Post #: 10

RE: RUDDER STALL - 9/11/2011 4:25 PM   
Lnewqban



Posts: 3908
Score: 163
Joined: 4/9/2007
Last Login: 5/6/2013
From: South Florida, USA
Status: offline 		The response to all your questions:

http://www.youtube.com/watch?v=6UlsArvbTeo

http://www.youtube.com/watch?v=WvwwpXmA78M

http://www.av8n.com/how/htm/aoa.html#sec-aoa

When you deflect the rudder, you are transforming the symmetrical airfoil of the vertical tail into a cambered airfoil and increasing the AOA at the same time (since the chord is measured between LE and TE).
That creates a sideways lift or imbalance of forces.
That force applied onto the vertical tail generates a moment around the CG (all rotations in flight occur around the CG), which induces a yaw.

There are three factors that limit the yaw movement:
1) The inertia of the plane to rotation (more mass at nose, tail and wingtips means more inertia or resistance to the yaw).
2) The resisting forces that the new attitude (position respect to the airflow) of the fuselage generate (more fuse's AOA = more resisting to yaw force).
3) The reduction of the AOA of the vertical tail-deflected rudder (although the angle of incidence remains the same).

Knife edge is just a balance between both forces or moments.

Any rapid deflection of a control surface, which makes the AOA of the whole surface (vertical fin-rudder in your case) bigger than the critical AOA (around 10 degrees), intermediately stalls the whole surface due to detachment of the stream over the low pressure side, dramatically reducing the lift or generated force and increasing the drag (due to turbulence).

Since, at reduced yaw angles the extra drag force generated by the stalled tail has little arm against the CG to generate a big torque or moment, the reduced lateral lift is the main cause of achieving a smaller yaw moment and yaw angle.

< Message edited by Lnewqban -- 9/11/2011 5:34 PM >

_____________________________

Lnewqban - "The recipe for perpetual ignorance is: be satisfied with your opinions and content with your knowledge." - E. H.

Hide Signatures

(in reply to turboromy)
       Post #: 11

Page:   [1]
All Forums >> RC Airplanes >> Aerodynamics >> RUDDER STALL
Page: [1]





Jump to:


 
Google 



Search | Marketplace | Event Calendar | Local Clubs | Magazine | Product Ratings | New Products | Discussion Forums

Photo Gallery | Instructor Search | Field|Track|Marina Search

Advertisers | Hobby Vendor Resources | Rate Manufacturers | Sign In/Sign Up

SITE MAP!   : :   FORUM RULES

RC Universe is a service of Internet Brands, Inc. Copyright © 2001-2013.

Charities we support that also need your help
Yorkie Rescue | Humane Society | ASPCA | Crohn's-Colitis America


0.629RCU1