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  #1  
Old 03-16-2013, 03:46 PM
brad walton brad walton is offline
 
Join Date: Jul 2007
Location: Cypress, TX
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Default RV-8 stick force gradient when landing

In a recent thread on landing the RV-8, Ironflight observed that "it has been shown that with the CG well after, the stick force gradient goes negative as you slow down below about 65 knots (+/-). A pilot needs to be ready and aware of this or they can over-flare with a passenger." I have noticed the same in my RV-8, but I am at a loss to explain aerodynamically why this occurs. This seems only to occur when near the aft CG. Trimmed for landing with a little forward presure on the stick, as you slow and approuch flare the stick presure goes to neutral and I find at touchdown I am actually pushing forward to maintain the appropriate angle of attack. Any thoughts about why are appreciaated
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  #2  
Old 03-16-2013, 04:28 PM
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Dbro172 Dbro172 is offline
 
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Default

With the reduction in speed it's pry some combination of the subsequent reduction in lift and increase in angle of attack. I suppose the aft CG is just enough to get it on the other side of the curve in which forward stick is then needed.?
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  #3  
Old 03-16-2013, 07:17 PM
scsmith scsmith is offline
 
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Default stick force gradient

The stick force gradient is dependent on a lot of complex factors. Predicting hinge moments on control surfaces is still a challenge today despite all kinds of advanced computational methods. Details of the hinge geometry and aerodynamic balance, sealing, and other factors, have effects which are difficult to anticipate.

But one significant factor is the stabilizer incidence with respect to the wing, and the downwash effects on the stabilizer, which become very strong on low aspect ratio wings at high angles of attack.

It would help the aft-c.g. slow-flight stick force gradient if the stabilizer had slightly more positive incidence (or the wing had less positive incidence). But this would compromise the cruise trim drag slightly, especially at forward c.g.
So, like everything else, its a compromise. The wide c.g. range that occurs with tandem seating, combined with the low aspect ratio wing, makes it hard to find an incidence angle that works well over the whole flight envelop.
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  #4  
Old 03-17-2013, 10:19 AM
Dogcowdaddy Dogcowdaddy is offline
 
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Location: St Leonard, Md/Tampa, FL
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Default

At the face of it it sounds like what one would classically expect to happen in an airplane with the CG slightly behind the controls free neutral point. From what I've read the factory cg limits are based on fairly low static margins and given the manufacturing differences in a homebuilt aircraft it is quite possible to see how some aircraft could have lower/slightly negative margins when flown to the factory limits.

I find it interesting that in most of these discussions the tail/wing incidence angles keep working there way in. The control gradients are driven by the change in moments with angle of attack. These characteristics are unaffected by the incidence angle of the wing and tail (ie they don't affect the stability) as near as I can tell from looking at the equations, unless there is some non-linear effect that is escaping me.

Perhaps someone could enlighten me as to what I am missing.

Thanks

Jim
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  #5  
Old 03-17-2013, 11:30 AM
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hawkeyestof hawkeyestof is offline
 
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Default

Reversal of control authority with aft cg is a deficiency in the RV-8A, as detailed in this CAFE report - p. 7, I believe, discusses longitudinal stability. The report is written from test results in the "A", but I'd expect similar results from the tail dragger.

http://cafefoundation.org/v2/pdf_caf...V-8A%20APR.pdf

Every RV-8/A driver should be familiar with this report.
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  #6  
Old 03-18-2013, 12:17 AM
gasman gasman is offline
 
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Default

That link is.......

Forbidden

You don't have permission to access /v2/pdf_cafe_apr/RV-8A APR.pdf on this server.
Apache/2.2.22 Server at cafefoundation.org Port 80
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  #7  
Old 03-18-2013, 06:48 AM
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hawkeyestof hawkeyestof is offline
 
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Default CAFE

Go to cafefoundation.org research Cafe APR's RV-8A
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  #8  
Old 03-18-2013, 08:27 AM
NavyS3BNFO NavyS3BNFO is offline
 
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Default CAFE Report

You can click here then click on the RV-8A report.
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  #9  
Old 03-18-2013, 09:48 AM
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selhardt selhardt is offline
 
Join Date: Dec 2008
Location: Minneapolis, MN
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Default I flew my first aft CG test yesterday

I'm at this point in my Phase 1 testing.

Yesterdays test conditions:
Start of flight CG = 86.2 Gross weight = 1763
End of Flight CG = 86.6

The lateral stability tests were neutral to negative(Negative in the spiral stability test). As expected, the plane goes where you point it, stays where you point it and requires it to be flown at all times. = Fun to fly.

I also did a power off stall series, accelerated stalls and incipient spins.
In all cases - the plane provided substantial feedback from the tail substantially before the stall by 5-8Kts. It reminded me of a sailplane that I had where the tail stalled before the wing giving lots of feedback through the stick before a stall.

The stalls and spins were non-events - similar performance to normal CG ranges. EXCEPT, easier to approach a secondary stall in recovery - again the tail feedback was providing the evidence of this and may be a skill thing as I'm used to higher stick forces. By the end of the tests, I had the process nailed.

The tail feedback is not subtle - it is a strong buffet felt through the stick and seat of the pants. The plane is bucking and snorting well before the stall. My particular plane has warning for all stalls in the configurations that I have tested.

The plane wheel lands wonderfully at aft CG.

I just reviewed the Cafe report and I think I will do another flight in this condition to further explore this force reversal further.

Note that this condition exists for trim at 140 and then flying slower speeds. I don't know what happens if you re-trim as you would for landing.

I expected neutral Lateral stabilities and light stick forces and that is what I found.
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  #10  
Old 03-18-2013, 03:17 PM
scsmith scsmith is offline
 
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Default stick-fixed vs stick-free stability

Jim,
You have to be careful about mixing stick-fixed stability and stick-free stability. Very different.

For stick fixed stability, the incidence angles don't come into play, except for some non-linear effects that are small. The RV-8 has plenty good stick-fixed stability at aft c.g. plenty good "static margin". The only effect of incidence is on the trim position of the elevator.

Stick free stability is effected by where the control surface naturally floats, i.e. zero hinge moment, and how that natural floating position changes with angle of attack. Where ever it floats to, it still exerts a force at the hinge (but no moment) that still influences the airplane moment balance. At the same time, the stabilizer is contributing to the moment balance. With large gradient of downwash on stabilizer with respect to angle of attack (d_epsilon/d_alpha), the stabilizer is not able to create a nose-down trimming moment if angle of attack increases. If the elevator float angle doesn't allow it to make any lift, then you will have no stick-free stability.

As a pilot, what you sense is that as the airplane slows down, you have to push forward more on the stick to keep it from continuing to slow down -- that is, the normal change in trim with speed is reversed.

In addition to stabilizer incidence, the other features that have a big effect on this are elevator camber, and elevator trim tab position. This is because the camber changes the float position as angle of attack and speed changes. As the airplane slows down, there is less dynamic pressure, so the moment due to camber gets smaller. With positive camber, or nose-up trim-tab setting, the float position of the elevator becomes less negative deflection as the plane slows down, and contributes less nose-up moment from it. So the stick force gradient will tend to be more positive (even though the stick force to trim may be forward pressure, it would take less forward pressure as you slow down). Conversely, if you have nose-down trim tab, and you need some back pressure to trim, the nose-down moment contribution from the elevator gets weaker as the plane slows down, so you have to push more.

This may seem counter-intuitive -- you are accustomed to having the trim tab trim the airplane, and the natural change in trim with speed that is caused by the stabilizer is dominant. This apparent reversal of trim change with speed only happens when the wing downwash greatly diminishes the trim contribution from the stabilizer. It is a 'feature' of very low aspect ratio wings.


Quote:
Originally Posted by Dogcowdaddy View Post
At the face of it it sounds like what one would classically expect to happen in an airplane with the CG slightly behind the controls free neutral point. From what I've read the factory cg limits are based on fairly low static margins and given the manufacturing differences in a homebuilt aircraft it is quite possible to see how some aircraft could have lower/slightly negative margins when flown to the factory limits.

I find it interesting that in most of these discussions the tail/wing incidence angles keep working there way in. The control gradients are driven by the change in moments with angle of attack. These characteristics are unaffected by the incidence angle of the wing and tail (ie they don't affect the stability) as near as I can tell from looking at the equations, unless there is some non-linear effect that is escaping me.

Perhaps someone could enlighten me as to what I am missing.

Thanks

Jim
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Steve Smith
Aeronautical Engineer
RV-8 N825RV
IO-360 A1A
WW 200RV
"The Magic Carpet" Flying since Sept. 2009
Hobbs 700
also
1/4 share in 1959 C-182B (tow plane)
LS6-15/18W sailplane SOLD
bought my old LS6-A back!!
VAF donation Dec 2020

Last edited by scsmith : 03-18-2013 at 03:25 PM.
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