Anyone know of the best price and best replacement for the hershey bar wingtips on a 67 cherokee 140, what mods out there increase the cruise speed the best?

Met-Co-Aire tips are supposed to be the best constructed, and give the best handling improvement at low airspeeds and are rumored to add possibly one or two knots cruise speed. They are not as sexy-looking as the other brands with integrated landing/taxi lights or the ones with the upswept trailing edges, but they cost significantly less money. I've flown Cherokees with the Metcos and with stock rounded Pipers and I can really tell a difference in the handling. The stock tips make the plane handle wishy-washy at low airspeeds and the Metcos give much more positive aileron authority right down to the stall speed. Once you've flown a 140 with Metco tips, you'll not like flying with the stock rounded tips any more.

Your best speed mods will be wheel pants, of course, but if you already have those, then gap seals & flap hinge fairings, and possibly wing root fairings too. The wingtips really don't change the top speed much on the Cherokee 140s I've flown.

Assuming most of your flying is going to be during daylight hours, the Metco tips (IMHO) are probably the best bang for the buck. They're inexpensive (relatively speaking) at around $650 for a set. They're construction is also very good.

Having just removed Metco's from my Cherokee and replaced them with LoPresti's, I can say that the fiberglass on the Metco's seemed to be of a heavier and sturdier construction. Also, as previous posters have indicated, the handling imparted to the aircraft in slow flight is very, very good. Once you've flown with them, it's difficult to do without them.

Relative to the speed increase, I don't think you'll see much with a wingtip change...maybe 1 or 2 knots...at least, that's been my experience.

Garnett

The advantage of replacement wingtips is better shedding of the wingtip vortexes - the primary cause of Induced drag in aircraft.

You have to understand, however, that when you look at an aircraft's drag curve, induced drag is only dominant up until Vg (best glide speed), after that point, parasitic drag quickly overtakes induced drag as your main stumbling block.

Fortunately, upgraded wingtips arn't likely to cause more parasitic drag than the standard wingtips, but you have to understand once you get up into the cruise portion of the aircraft's envelope, the parasitic drag component dwarfs the induced drag component, so no wingtip, no matter how expensive, is likely to add much on to your top end.

Wing and Flap gap seals are also along the vein of wingtip modifications - more effective at higher AOA's and lower speeds.

Now, To be completely fair, a induced drag control WILL improve low speed handling, reduce sink, and net you of climb, because in all of those phases of flight, induced drag is a fairly major component. This generally means that while all of these modifications arn't technically "speed mods" in that they don't make the plane go faster, they do substantially increase an aircraft's "net Point a to Point B" speed by allowing it to complete the climb portion of the flight faster and more efficiently. The sooner you can get to altitude, the sooner you can get into cruise configuration, the sooner you reach your destination - even if you're not cruising any faster.

This is where the popular "One 1000 dollars per knot" refrain from the anti-mod-crowd shows its fallacy... net speed gain is _NOT_ the goal of most so-called speed mods. Time to climb is. The sooner you get to altitude, the sooner you get to cruise speed, the sooner you get to your destination, period.

If your net goal is straight-out speed, reducing your parasitic drag is the name of the game. The obvious candidate is modern wheel fairings. Wing smoothing, antenna cleanup and such are also a good bet... Beyond that, there really arn't a great deal of STC's mods for parasitic drag reduction... Really, the best thing you can do is load-aft (reduce stabilator downforce) and make sure your bird stays well rigged.

I've actually been playing around with some drag models, and I'm wondering if there aren't some other gains to be had in the Piper airframe... unfortunately, I don't think I'll ever have the time to actually experiment, but its fun to think about.

-Scott

Leigh,

> Scott, your explanation of how some mods will increase
> effective average speed is one of the few that makes
> sense.

Thank you:)

Let me preface anything I say further by a disclaimer - I do _NOT_ have a degree in aerospace engineering. I am, however, an engineer with whose had an obsession with aerodynamics for several years now, and I've been reading my fiance's fluid dynamics texbooks, as well as every single article I could find on the web on the subject).

So take anything I say as the ramblings of a passionate sophomore;)

> You did leave us hanging a bit though with your comments
> regarding parasitic drag vs wing smoothing (exactly what
> is that?).

By wing smoothing, I, in general mean a new paint job, specifically, I mean the tank fairings offered by LFS.

> I'm no expert in aerodynamics but was of the impression
> that the boundary layer would be thick enough that the
> surface smoothness might not be a factor in drag.

As I understand it, the thickness of the boundary layer depends directly on the friction provided by the skin... More specifically, a rougher surface imparts more turbulent energy into the stream over the wing, providing for a thicker boundary layer, even if the rough surface itself falls well short of piercing the boundary layer. Now, on most airfoil designs, this isn't as much an issue because the thickest portion of the wing is past the point at which the flow is "expected" to go turbulent. In a laminar-airfoil, however (such as the Hershey wing), the wing is designed to keep the airflow laminar well past the thickest portion of the wing, meaning that a rough surface tends to make the wing "look" thicker to the passing air stream... Because of this, a laminar flow wing is more susceptible to the effects of a thick boundary layer - literally, it re-shapes the critical laminar flow around the wing, and provides for a less-than-optimal flow shape.

The question then becomes a matter of scales... is the rough surface of the wing enough to "thicken" it to the point that it begins to substantially effect the efficiency of the airfoil... Obviously based on the wildly varying gains of wing smoothing (that even LFS admits too) there is a pretty sharp threshold at which this is the case... beneath this threshold, wing smoothing will only have a minor effect on drag.

> Would also be interested in reading about the other
> possible ways to reduce drag on the Piper airframe that
> you speculated about.

Wow... I have a 3 page text file of my own notes on this subject... not sure if I can get into them all here;)

My main focus of attention is on the shape of the rear fuselage. Note how the fuselage presents a very sharp edge between the bottom and the side of the aircraft immediately behind the wing. Now, visualize - while the aircraft is in flight, the wing is producing a substantial downdraft (by definition it needs too to make the vehicle fly). Then this downdraft passes by the sharp angle of the fuselage at a 15-20 degree angle. During high AOA operations, the net effect isn't much, because the airflow underneath the aircraft is at reasonably high pressure anyways as the belly is exposed to the free stream. As the aircraft approaches cruise, however - it hunches its nose down ever higher- and right around Vg, the lower tail becomes shielded from the freestream and a low pressure builds underneath the rear fuselage. The downdraft of the wing (higher pressure air) gets thrown by the sharp edge of the lower fuselage into the low pressure created by the raised tail, and the net effect is a substantial, energy-bleeding vortex that extends the entire rear fuselage, pulling energy out of the system.

My solution to this would be to add a fairing underneath the tail, starting at the rear wing root, extending to the plane created by the main landing gear tires and the rear tiedown loop, and then moving up the rear tail to the tie-down loop. The faring would be blended smoothly into the sides of the tail. The idea would be to create a laminar flow path for the downwash off the wing to remain attached to the fuselage until it can be cleanly shed along a ridge which runs down the center of the fairing - preventing the vortex from forming, and recapturing the stream energy as thrust.

Now - everything above is conjecture... The simple fact is I don't know for sure that this process is indeed occurring, although I would like to do a good old-fashioned yarn-test with a video camera one of these days to see for sure. The one benefit of this modification is that it is one of the few that would actually _increase_ its effectiveness with speed, that is - as a PA-28 flies faster, the wing AOA decreases, and the low pressure created by the tail increases, increasing the tail vortex...

That is, if the tail vortex is occurring at all as I imagine it, its possible that the free stream from underneath the aircraft is sufficient to suppress it... the other potential problem is that at higher speeds, the low pressure produced by the tail could very well be a substantial contributer to the tail-down force, and by lessoning it, I could just be making the stabilator work harder, which would just rob peter to pay paul.

Either way, its definitely fun stuff to think about... who knows, maybe one of these days I'll commission an aircraft fiberglass shop to build me a prototype and test it out...

While i'm at it, I want a pony;)

-Scott