Dec 09, 2012, 09:08 PM Registered User USA, KY, Louisville Joined Oct 2002 817 Posts Here's a good description I clipped from another thread Nose heavy planes (forward cg)require up trim to fly level in straight, level flight. When you put them in a dive, the speed increase will cause the plane to gradually pull out of the dive rather than holding the line. Things are just the opposite for a tail heavy plane, which will tend to tuck under or increase the downward direction when put in a dive.In both conditions the plane can be made to fly level with trim adjustments. How many of you would have caught the catch in this explanation...and its a critical one. "Can be made to fly level" YES it can...sort of...read through it again....if it is Made to fly level with trim adjustments, then what happens when the airspeed changes either slower or faster? The tail directs the nose...and airspeed empowers tailfeathers. So as in the past....when models were set up with a "positive stability" you know so that if they got into a dive they pulled out before they blew into balsa bits in the air. Aero elasticity caused models to increase their dive as the airforces bent the incidence of the rear end downward. Crooked airplanes can be made to fly straight...as long as the airspeed is constant, as soon as it varies from the speed the crooked model needs to hold attitude, the attitude changes. Gordy Latest blog entry: Check out my YouTube Channel
Dec 11, 2012, 12:07 PM
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Quote:
 Originally Posted by MrE Im not sure which controversy your talking about and Im also not familiar with the math of dynamic stability. Could you explain what you mean?
It took me longer to find my notes than I had originally thought.

I won't be able to reproduce much because it would be difficult for me to condense a semester of math, geometry, and PDE into an intelligible post.

Secondly, after review of the notes and some thought, it would appear to me that the Drela dive test is a measure of "stick fixed" static stability and not dynamic stability. Why? Because with the dive test we are looking for the aircraft's response immediately following a nose-down "disturbance" created by a nose-down elevator input.

In order to understand the margin of stability, we need to be able to ascertain the distance between the aircraft's neutral point and CG. The problem is that we can't analytically determine the neutral point, it has to be determined experimentally ... which is where the dive test is useful.
Dec 11, 2012, 12:18 PM
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Quote:
 Originally Posted by Gordysoar AND no one is waving hands in the air about the science, in fact since most are not builders, the science doesn't mean anything -- at least in the context of this thread which is about setting up task sailplanes. The science was done for us by guys like Dr. Drela and Selig, etc.
The science means everything because the science dictates the layout of the aircraft and CG placement.

Quote:
 As to why full size rules are different for RC sailplanes? Simple...we aren't in the seat. I can not believe how many of us don't understand that full size aerodynamics have one base line Priority...the safety of the pilot.
Red herring. The bottom line is that a passenger jet requires a positive stability margin for the same reason as one of our RC birds ... so that the guy on the sticks can control the aircraft, and land the plane without making a big hole in the ground.

Quote:
 A model that pitches up with an airspeed increase and drop its nose with an airspeed decrease, is a model that sends bad information to its pilot. A sailplane that doesn't change its attitude but does change its altitude without an airspeed increase makes its pilot a happy boy :-).
I think the real issue here is one of pilot preference. Some may prefer to fly with the CG farther forward than others. Note that in Drela's dive test guideline, he clearly states to make CG changes "until the glider behaves as you like."

Quote:
 Soooo much of what Herk learned...for us, just doesn't apply because of the equipment and materials used today in our aircraft.
So what you've done here is wave your hands in the air and say that the science is irrelevant. I would argue that the modernization of the materials and building techniques are getting us closer to what the math has told us all along. Perhaps we are in violent agreement ... I don't know.
Last edited by Cap_n_Dave; Dec 11, 2012 at 12:31 PM.
 Dec 11, 2012, 12:24 PM Registered User Joined Mar 2008 644 Posts I would also add that perhaps another confound, pointed out by Gordy, is the difference between "balance" and "stability." Balance is where the aircraft can fly straight and level because the moments about the CG add to zero. Stability is where the plane tends to return to its original trajectory after a disturbance. The two concepts are related because with that knowledge we can then deduce that the dCm/da (variation in moment with variation in alpha) has to be negative, which then leads us to conclude the CG should be located ahead of the aerodynamic center. This can be plotted on a graph, and "off design" conditions can be considered which then lead us to decelage, trim, etc. (apologies ... not sure how to condense all this into something coherent that the casual layperson can digest) Last edited by Cap_n_Dave; Dec 11, 2012 at 12:29 PM.
 Dec 11, 2012, 12:30 PM Registered User United States, CA, Mountain View Joined Apr 2001 636 Posts One thing that always leaves me with a feeling of something missing with the idiom "airspeed empowers tailfeathers" is... shouldn't airspeed empower the wing and its pitching moment in exactly the same way?
 Dec 11, 2012, 01:52 PM Registered User Michigan, USA Joined Jul 2006 580 Posts Disclaimer: I may not know what I'm talking about. But here's how I think about it... The Center of Pressure changes with angle of attack, not sure that it moves with changes in airspeed. I assume if it does that it's not much. If the Center of Pressure and the Center of Gravity are in the same location, then any lift generated will have no pitching moment on the wing. If the CG is ahead of the CP, then any lift causes a nose down pitching moment. Imagine balancing the plane on the CG, and then pressing your finger up on the CP. If the CP is behind the CG, the tail will go up, and the nose down. To counteract this in flight, you can add a little up elevator (to force the tail back down). That solution is only good for one speed though. If you speed up more, the elevator becomes more effective and the nose pitches up. If you slow down, then the elevator becomes less effective and the nose drops. The goal is to have the CG as close to the CP as you can without creating stability issues.
Dec 11, 2012, 02:12 PM
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Quote:
 Originally Posted by GliderJim Disclaimer: I may not know what I'm talking about. But here's how I think about it... The Center of Pressure changes with angle of attack, not sure that it moves with changes in airspeed. I assume if it does that it's not much. If the Center of Pressure and the Center of Gravity are in the same location, then any lift generated will have no pitching moment on the wing. If the CG is ahead of the CP, then any lift causes a nose down pitching moment. Imagine balancing the plane on the CG, and then pressing your finger up on the CP. If the CP is behind the CG, the tail will go up, and the nose down. To counteract this in flight, you can add a little up elevator (to force the tail back down). That solution is only good for one speed though. If you speed up more, the elevator becomes more effective and the nose pitches up. If you slow down, then the elevator becomes less effective and the nose drops. The goal is to have the CG as close to the CP as you can without creating stability issues.
The lift of both the wing and the tail increases and decreases with airspeed with the same proportion... balance should still hold, or should not?
Last edited by Francesco; Dec 11, 2012 at 02:23 PM.
 Dec 11, 2012, 02:31 PM Registered User Joined Mar 2008 644 Posts I guess I can try and do this in installments. For the purposes of stability and control analyses, the aerodynamic center ("AC" and "ac") is defined as the point on the aircraft where the pitching moment is independent of the angle of attack (not where the moment is zero). This is a convenient place to locate all of the aerodynamic forces for the purpose of determining what is happening at the CG. The next step is to summarize the forces acting on the aircraft as a combination of forces and moments about the CG ... this is done thusly: Mcg = Mac+Lxcg-ac where: Mcg = moment about the CG Mac = moment about the AC (which is independent of angle of attack) L=lift force at AC xcg-ac = distance between CG and AC along the aircraft's nose-to-tail axis Generally speaking the AC is located in a fixed position which doesn’t change. When Mcg is zero the aircraft is “balanced.” The next step is to assess what happens when the value for xcg-ac is varied, there are 3 combinations: 1. AC is ahead of the CG 2. AC and CG are at the same point 3. AC is behind the CG to be continued ...
Dec 11, 2012, 03:17 PM
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Quote:
 Originally Posted by Francesco The lift of both the wing and the tail increases and decreases with airspeed with the same proportion... balance should still hold, or should not?
Maybe, but the tail wins in the leverage department. Any pitching moment the wing has is easily overcome by the pitching moment of the tail, because the tail is so much further away from the CG or balance point. Longer lever and all that.
Dec 11, 2012, 03:26 PM
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Quote:
 Originally Posted by GliderJim Maybe, but the tail wins in the leverage department. Any pitching moment the wing has is easily overcome by the pitching moment of the tail, because the tail is so much further away from the CG or balance point. Longer lever and all that.
But the leverage ratio does not change with airspeed... so, why should the balance be upset by a change in airspeed?
 Dec 11, 2012, 04:31 PM IT'S NOSE HEAVY!!!!!!! United States, CA, San Jose Joined Mar 2012 3,459 Posts hehehe, I may just have to unsubscribe to my own thread. All the math and theory means nothing to me...only what is actually happening to the plane, which a silly little dive test (and the info on how to interpret it) took care of. But by all means, carry on!
Dec 11, 2012, 05:17 PM
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Quote:
 Originally Posted by Francesco But the leverage ratio does not change with airspeed... so, why should the balance be upset by a change in airspeed?
The "leverage ratio" doesn't change, but it's in favor of the elevator, which is why the balance is upset. If the ratio was 1:1 then you'd be correct, but it's more like 30:1 (or more) in favor of the elevator. Add a little airspeed to the equation and whatever effect the elevator has on the pitching moment is multiplied 30 times compared to whatever effect the wing has.
Dec 11, 2012, 05:20 PM
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Quote:
 Originally Posted by GliderJim The "leverage ratio" doesn't change, but it's in favor of the elevator, which is why the balance is upset. If the ratio was 1:1 then you'd be correct, but it's more like 30:1 (or more) in favor of the elevator. Add a little airspeed to the equation and whatever effect the elevator has on the pitching moment is multiplied 30 times compared to whatever effect the wing has.
Sorry I can't agree... if two forces are in balance with a given (whatever, 1:1 or 30:1) lever ratio, they will still be in balance if you double, triple, etc. both forces, which is what happens when you change airspeed only.
Dec 11, 2012, 05:46 PM
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Quote:
 Originally Posted by cityevader hehehe, I may just have to unsubscribe to my own thread. All the math and theory means nothing to me...only what is actually happening to the plane, which a silly little dive test (and the info on how to interpret it) took care of. But by all means, carry on!
I am definitely with you = = unsubscribed