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INVERTED TAIL RIBS QUESTION

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Posted (edited)

On the Zenith designed 701 and 801 planes the tail ribs are like an upside-down wing rib, producing more tail-down forces.  I question how effective this is at low airspeeds when it is most desired, like when flaps are used.  Seems like it would have more force at higher airspeeds?  Would it produce more tail-down force in ground effect?   Any thoughts on this?  I am attaching a very rough drawing to show what it sort of looks like.   This was designed by an Engineer with an Aeronautical Degee - Guess Piper's and Taylor's planes, and lots of others, shouldn't be able to fly with their flat tails?   ;<)   EDMO

Scan0459.jpg

Edited by EDMO

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Posted (edited)

The horizontal tail usually produces a down force to balance the airplane, especially if the aircraft CG is in the middle or forward. That is why most tail airfoils are symmetrical or upside down, the tail is flying upside down most of the time. When the tail needs to most lift, in a slow approach with some flaps, it is operating at high negative angle of attack, and that upside down airfoil comes in handy for getting the most elevator control power during the touchdown.

 

BTW, the downforce from the tail makes the aircraft "heavier" and robs performance, that's why the Wright brothers used a canard, so the balancing smaller wing was also lifting. They didn't want to lose one ounce of performance.

Edited by nlappos
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Posted

^nice clear explanation, thanks

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Posted

I think I have seen something similar on the tails of helicopters - something that I know little about.  Thanks Nick.   EDMO

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Posted (edited)

Yup, and that's why an aft CG is more efficient. Less tail "down lift/force" required (reduced AOA) so less drag . . .

Edited by RobS
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Posted

There is an argument that highly cambered wing airfoils like those on the Avid STOL add to induced drag since they, like flaps, add to the wing's nose-down aerodynamic pitching moment, putting the tail in download.   Low-moment sections like NACA 23012 would require less download on the tail, which lessens load on the wing.  Moving the CG aft decreases pitch stability, so any comparisons would have to be made at the same static margin.  Dean Wilson apparently developed the Avid airfoils to use the front tube spar as leading-edge shape.  In my experience, this design compromise comes at a cost; cambered elliptical leading-edge shapes seem to do much better for max CL.  Is this why we see so many VGs on these birds?  BTW, I've seen much worse "airfoils" on other ultralights.  Almost anything reasonable will provide lift.  Even a flat plate can get to CL of 0.7!

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Posted (edited)

Big difference between CofL of 0.7 and 4.0 which I believe I have seen for some wings.  According to some writers, NACA really produced some "Dud" airfoils - Maybe not the one you mentioned?   Dean designed a very-light plane - easily built by a novice builder - which could get off of the ground quickly with the modified Eppler undercambered airfoil and he wasn't designing for speed.  Wittman designed for speed with the added cost of higher speed landings and takeoffs.  Everything is a compromise!   EDMO

Edited by EDMO
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Posted

CL of 4 is possible with slats and flaps.  For a single-element airfoil the max is around 2.2 for Reynolds numbers of interest to us.  A former colleague, Bob Liebeck, used variational methods and boundary layer theory to develop a series of airfoils that establish what is possible.  Even worse than the 5-digit NACA airfoils, they have a very nasty stall..  They lose a lot with just a few bug-splats, and are not suitable for use on fabric-covered wings.   

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Posted

So what are your thoughts on using the US35b (mod) airfoil like a cub on these planes?  Or have you come up with a better airfoil that would not have a nasty stall, give lower stall speed yet still have a higher cruise?  Something like the Sherpa uses?  That has a very impressive speed range on it.

:BC:

 

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Posted

i'm not professional engineer btw i add my idea : the 4412 would be nice on that two equal spars type wing, very sweet stall and high lift.

down side: the center of lift is moving a lot along the chord.  on avid wings, it won't be an issue

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Posted (edited)

So what are your thoughts on using the US35b (mod) airfoil like a cub on these planes?  Or have you come up with a better airfoil that would not have a nasty stall, give lower stall speed yet still have a higher cruise?  Something like the Sherpa uses?  That has a very impressive speed range on it.

:BC:

 

Leni,  I know folded width probably wont matter to you because you don't trailer - But, have you figured how much you would have to shrink the 35b, or if you do, to fit your bird?  I think those are about 10" longer chord than the 51" Avid including the flaperons/ailerons.  I've got about 14 aileron ribs for the Pipers/Maule - Maybe D&E made - ridicules how much they want for those!  EDMO

Edited by EDMO

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