RE: What causes pitch with rudder?  
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All Forums >> RC Airplanes >> Aerodynamics >> RE: What causes pitch with rudder?
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RE: What causes pitch with rudder? - 2/5/2004 8:27:32 PM   
Tall Paul



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quote:

ORIGINAL: Strat2003

I wish you guys were at my kitchen table so we could wave our hands around and draw diagrams and stuff.......

If I understand correctly, the sum of the forces, or at least lift, for the airplane as a whole, acts through the neutral point?

And those forces are the lift, drag, and any pitching moments gernerated by th wing, the tail, and the fuselage?

Paul, I noticean "empirical pitching moment factor" and a "downwash factor" in that equation. Where do they come from and how are the arrived at?

Thanks again

.
You noticed!
Way back when in my senior aero class the neutral point equation came up, with those same 'fiddle factors".
The way I undertand it, these are "derived" by the somewhat circular reasoning that a 10% "static margin" gives a stable airplane.
This airplane is stable, therefore it has a 10% static margin.
With this answer then, those comparaitve unknowns can be solved for, adjusting their values to get the established answer.
Cheaper and easier than running a wind tunnel test, particularly if the new plane isn't that different than the previous.
The previoius one flew, so the new one should fly also.
.
Many aero books will go more into detemining downwash etc, to a getter precision, but you'd be surprised how many full-scales have used the "the old one worked, the new one will too" approach.
I've heard the c-119 was done using used this approach.. It wasn't -that- much different from the C-82, ergo it should fly. And it did.
With today's overwhelming compuational power of the analytical computer, this TLAR approach is seldom used, except for models, where it still works.

(in reply to Strat2003)
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RE: What causes pitch with rudder? - 2/6/2004 7:40:15 AM   
mithrandir



Posts: 683
Joined: 7/16/2003
From: adelanto, CA, USA
Status: offline 		I am tired... but here is something fun!!

back_it_up.zip is the file name

You can retrieve this file at

http://141.248.186.227/asisite/upload/2932

The userid is: asi2932

The password is: hangtime

Right click on the file in your browser and select -Save Link As- to download.

I did this in the high desert of california.... the next day it snowed!!! LOL... I can only leave stuff here for 3 days, so friday will be last day to download. (My work web based FTP site) I love that OS 160!!!!

And a snap for further discussion

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RE: What causes pitch with rudder? - 2/6/2004 6:22:09 PM   
Tall Paul



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Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		20 M in the -zipped- format?
Why is it important?
Whenever I see a diagram with "center of pressure"... I wonder.
Here's one such...
Pressure is something pressing against something else.
Out in front of this wing, there's nothing to be pressed against.
Pitching moment is a much more coherent description relying as it does on measureable forces.
C.p. is a mathematcal concept which as Martin Simons points out, can move out of the universe to infinity under normal
operating conditions. Few things at infinity affect our reality.

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< Message edited by Tall Paul -- 2/6/2004 1:48:41 PM >

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RE: What causes pitch with rudder? - 2/12/2004 6:06:50 PM   
95tequesta


 

Posts: 76
Joined: 11/18/2003
From: Troy, OH, USA
Status: offline 		Lifting horizontal stabilizer? NOOOOO!! If you look at the forces of a wing, you have to take into account the cg(center of gravity) and cL(center of lift). Center of lift is aft of cg. Therefore if a standard configuration airplane were to lose the horizontal stab it would pitch the nose down. Therefore, to counter this the horizontal stabilizer acts against the CL allowing level flight.

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RE: What causes pitch with rudder? - 2/12/2004 7:01:03 PM   
tommy s


 

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From: houston, TX, USA
Status: offline 		Man, you guys get too analytical for me! All I know is I agree with Dick.
I've flown pattern ships for years and you can trim it out with CG changes.

tommy s

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RE: What causes pitch with rudder? - 2/12/2004 7:20:57 PM   
siclick33


 

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Status: offline 		I haven't read all the posts here so forgive me if I repeat something.

Conventional aircraft such as pattern planes have negative lifting stabs. If you were to fly a plane with zero incidence wing and zero incidence tail you would have to hold up elevator to fly level. The counteracting couple for this is caused by the distance between centre of pressure and centre of gravity.

If you don't believe this then ask yourself 'why were canards invented?' one of the reasons for this is by putting the surface at the front you can have a lifting wing and stabiliser.

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RE: What causes pitch with rudder? - 2/13/2004 6:45:19 AM   
Ben Lanterman



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Joined: 10/27/2002
From: St. Charles, MO, USA
Status: offline 		siclick33, no pattern airplanes have lifting stabs. You have misinterpreted what you know. Read the posts in this thread and I explain it. There is a common error made that makes people think the stab is lifting down. Read over the material and see what you think.

The short version is moments are zero about the cg for level flight.
The wing force is in front of the CG
The tail force is behind the CG

The tail force is up.

Again look over the material and diagrams, draw up your own. Remember the wing lift is at 25 percent of its chord and the tail is at 25% of its chord. Remember the wing and tail are at the same angle of attack (assume the tail is out of the downwash, a valid assumption for this discussion).

If the wing is at 10 degrees angle of attack the tail must be at 10 degrees angle of attack. The elevator deflection at any trim condition is not enough to reverse the tail lift from up to down unless you are trying to do an inside loop. But in steady flight the tail lifts up.

Interesting thought isn't it?


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Ben Lanterman

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RE: What causes pitch with rudder? - 2/13/2004 11:19:58 AM   
siclick33


 

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Status: offline 		I don't think I have misunderstood. I have read the posts and will just give one more thought!

quote:

So... we run the CG as close to the AC as possible.


Assuming that you are exactly correct with your CG placement then the condition above will result in no couple at all. The stab is providing damping and control (no lift at all). If you want to pitch up, up elevator is fed in which results in a momentary negative llifting stab to climb. If you want to pitch down, you apply down elevator which will cause a positive lifting stab.

You say that neutral stability is the optimum but in practise it is hard to obtain. A slightly forward CG, however tiny, will require a negative lift from the stab to counteract the couple.

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RE: What causes pitch with rudder? - 2/13/2004 5:03:27 PM   
Ben Lanterman



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From: St. Charles, MO, USA
Status: offline 		The first question answer is yes. The Neutral Point of the airplane is the location of the CG at which there is no moment created when the angle of attack changes (neutrally stable, hard to fly). At that time indeed the tail and wing moments balance at all angles of attack. But consider our neutrally stable airplane flying at 1 degree again. Both wing and tail are flying at 1 degree and the moments are equal but both still have a positive lift. Bounce it to 5 degrees suddenly and both wing and tail go to 5 degrees angle of attack and still the moments are balanced but at 5 degrees. The airplane starts climbing and probably looping. It makes it really hard to fly.

The last statement is wrong though.

You can look at the lift as two components (at wing AC and tail AC) or as a single vector at the AC (or the Neutral Point) of the airplane. That lift vector already has the tail contribution included in it which is up. The moment produced by the total lift at the AC times the distance between the NP and the CG is balanced by the up elevator trim. However it is very small and only lessens the up load of the tail by a little, leaving the tail lifting up.

Look at the pictures again. For the tail to have negative lift in steady state flight requires that the CG must be ahead of the AC of the wing. Since all moments are about the CG the wing lifts up and the tail lifts down to balance the airplane.


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RE: What causes pitch with rudder? - 2/13/2004 5:45:32 PM   
Tall Paul



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From: Palmdale, CA, USA
Status: offline 		There's some terms here being equated with each other when they shouldn't be.
AC (aerodynamic center) is not CG (center of gravity) is not NP (neutral point).
All three are seperate quantities which exist independently.
Let's keep them organized.
This is the way they relate to each other..:

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Agreed, I was trying to sneak them in. - 2/13/2004 11:28:44 PM   
Ben Lanterman



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From: St. Charles, MO, USA
Status: offline 		I don't like the one diagram that shows a down load on the tail with camber. It implies that any camber will cause a down load when it reality it depends on the amount of camber. Maybe he says that in the text?

Most folks find it hard to believe the tail can have an upload even when faced with the diagrams. Perhaps a diagram like it put at the home page of the forum would be a good thing to have.


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RE: Agreed, I was trying to sneak them in. - 2/13/2004 11:53:20 PM   
FHHuber



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Status: offline 		A question about the diagrams....

Isn't the NP normally 30% to 40% MAC (offset based on how much force we are putting n the tailplane...) and between CG and center of lift?

If CG was on center of lift... wouldn't neutral point be the same spot? (and thus we'd be at... 33 to 35% MAC {for a typical RC aircraft airfoil..} and have a neutral force at the tailplane)

We typically balance in the 20 to 30% MAC range, to keep the CG ahead of the wing's center of lift, forcing use of downward force being used at the tailplane to keep the plane level.

In the 1930's FF modelers typically used a 60% MAC CG to keep the CG aft of the wing's center of lift... and then used a lifting tailplane. (which is VERY pitch sensitive to small incidence changes, and gives almost no margin of error for the CG)

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RE: Agreed, I was trying to sneak them in. - 2/14/2004 1:33:54 AM   
Tall Paul



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From: Palmdale, CA, USA
Status: offline 		Ben, Martin Simons goes into detail in a series of articles on the "dive test" in QFI and R/CSD... in essence he claims any camber with its resulting nose-down pitch creates a requirement for a down-load from the horizontal, for most flight conditions. It's of course possible to control the static margin for a zero tail-load.
I believe I've posted this before...
All the horizontal failures I've seen when examining the fibers of the broken parts show the surface failed downwards.
Airplanes with symmetrical airfoils are almost a special case, especially on pattern planes, where neutral longitudinal stability is desired; the plane goes where pointed until the direction is changed by the pilot.
There's always exceptions to any general conclusion, but these (Lennon and Simons) apply for most commonly encountered configuations.
.
I have heard of fun-flies with negative static margins..
And others which develop such as a result of a "flight condition:.
From personal experience I know these can be flown, it's not something to be wished for, though.
When Tony F's electric pattern plane threw the motor batteries, it developed an instant 'negative static margin", but Tony recovered the plane with no problem.
Generally it requires holding a lot of down elevator to keep the plane from the zoom-stall-dive-zoom.... crash situation. But it can be done.

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