Data or experience on aerodynamic penalties of seams and rivets?

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yankeeclipper

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Jun 1, 2009
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Hi folks.

I'm after what impact seams and rivets (as with wood or aluminum skins) have on a 140-150 kt design cruise, all else being equal of course. Is this speed so low as to be negligible? I'm guessing not, since modern sailplane designers seem to pursue ultra-smooth lines for 60-70 knots (and finishes, but that's a different story, sorta). But does it become negligible again, say, as compared to the the drag of a windmilling (or stationary) prop? Or a soft-chined fuselage?

Thanks.
 

Jan Carlsson

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Jan 11, 2009
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Rivets and gaps ... on a wing will add CD 0,003 - 0,0035, that on a wing with a CDo of 0,00785 or so.
That's 40%
But the same common aircraft will have a fo of 0.5558 m2, or a CDo to the wing area of 0,0434
That's 7,5% extra drag from rivets on the wing to the total fo (flat plate drag) not calculating the induced drag.
Room for improvments there is!

Jan
www.jcpropellerdesign.com
 

BBerson

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Depends on the location.
The first third of the wing is more important than the aft part near the trailing edge.
I often see large drops of water near the aft third of the wing in flight where the boundary layer is thicker. The drops are motionless, so I doubt rivets near the trailing edge matter.
BB
 

Aircar

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Hoerner has stacks of data on rivet heads and skin laps -- I'll see if I can drag it out .

The observation of stationary water drops on the wing rear end indivates separated flow not just turbulent transition --in fact the velocity profile in laminar flow goes to ZERO at the surface but is HIGH in transitioned turbulent flow (which is basically why skin friction is so much higher ) --was the question about rivet heads on a hull perhaps (water drag) ? The 'chine' reference is possibly to a faceted 'prismatic' fuselage though . Every bit of drag adds up and costs power but conversely if you halve the drag you only go the cube root of 2 as fast (drag builds as the cube of speed) --ergo less drag is cheaper than more engine -- another Yoda like homily. One horsepower = 550 ft lbs per second so that at 550 ft/s every pound of drag consumes one horsepower (that's 375MPH ) --150 knots is plenty fast to worry about drag --as is 20knots if you are in a man powered aircraft or 50knots in a soaring glider it is always a case of 'waste not want not' or as Yoda might put it 'waste not if want Knot"

Some mental arithmetic -- if 50Knot = 60mph (approx) 375 mph = 312 knot so at 156 knots 8 lbs (2 cubed) of drag will need the same power as one pound at 312 k or conversley what costs one horsepower at half the speed will chew up eight horsepower at twice the speed (1/8 th HP per pound of drag at ca. 150 k ) how many rivets was it...? and if you lose laminar flow as well then double the penalty.

There was an article titled "drag analysis of a skyhopper" in an old EAA publication where the writer identified every single thing in the airstream and added them up in answer to a similar question about 'why is my aircraft so slow surely a few bolts and rivet heads can't matter that much at such low speed ..? --except that they are why it was so slow.
 

autoreply

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There was an article titled "drag analysis of a skyhopper" in an old EAA publication where the writer identified every single thing in the airstream and added them up in answer to a similar question about 'why is my aircraft so slow surely a few bolts and rivet heads can't matter that much at such low speed ..? --except that they are why it was so slow.
Anderson has an interesting one too:



"Wie het kleine niet eert is het grote niet weert" Or, in English, who doesn't honor the small (gains) isn't worth the big ones.
 

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