A friend of mine mentioned I need bigger tires for the Bearhawk. He's right, but wait there's more -

He also suggested that going up a size to 8.50's could actually increase the cruising airspeed slightly, because of something he called the "Coke Bottle Effect". He made a couple of remarks but didn't really explain why it happens, he did say they'd seen it happen when they went up from 8.00's to 8.50's on their A185F and got a couple of knots extra cruise.

Now from a basic aerodynamic point of view, its counter-intuitive, but there are stranger things in Heaven and Earth. Can anyone confirm / refute this effect, or shed some light on why it happens (if its real).

This is new to me although, I suffered from the same effect when I had to go to bifocals.
I am certainly interested in hearing more on it.

the only "coke bottle effect" I ever heard of comes into effect at about .9 mach. whitch is why the fuselage of an F-106 was shaped like a coke bottle. I suspect he may be putting to much rum into his bottle of coke, but if larger tires can increase speed i would like to know how it works.

My Pacer definitely goes slower with a bigger tire. 8:50 X 6 is slower than 8:00 X 6. 26" Goodyears are slower than 8:50 X 6.
MontanaPacer

The only thing I can find is maybe he's getting confused with the Coke bottle area found on an F1 car, between the rear wheels. But that would not apply to aircraft.
I have never heard of anything like this before.

We went from 8.00's to 8.50's on our Bearhawk and definitly didn't gain anything in cruise. Makes no sense to me.

Might be talking about 8.50 X 10's. The wheel covers, that is to say fits around the caliper and disc. On the C180 series there is very little speed penalty for this change from 8.00's...might even go so far as to say a wash but no perceived increase in speed either. YMMV on a different airframe.

Don't count on it I lost 10 miles an hour going from 7's to 8.5's

Battson wrote:He also suggested that going up a size to 8.50's could actually increase the cruising airspeed slightly, because of something he called the "Coke Bottle Effect".

I'm pleased to verify the validity of the "Coke Bottle Effect" and its wonderful impact on speed. There are essentially two parameters that govern it.

First, it is related to the Obama Economics Plan, where increasing everyone's taxes by 100% makes everyone in the USA 50% more prosperous. It is also called the Ponzi Effect, the Barnum Effect, and the Bede Effect in various financial and aero engineering circles.

The key to making the Coke Bottle Effect work is the second parameter, which is itself a combination of three things: Loud Disco music, a three-piece white suit with matching white patent leather shoes, and a very very small spoon. The spoon is critical because the little brown Coke Bottle and its opening are very small.

Once you have combined these three parameters, within a few moments the Coke Bottle Effect begins, and things like bigger tires increasing airspeed, or bigger taxes increasing prosperity... can actually start to make sense.

After doing this several times, you will indeed start to feel 'speedy', further validating this incredible engineering phenomenon.

And don't ever ask me what I was doing in 1979 when this engineering breakthrough occurred.

He's just being a good mate trying every trick in the book (and not in the book) to convince you to put sensible shoes on the Bearhawk - you could possibly get another 5kts going up to 31" Bushwheels

I think if you want to see an example of the coke bottle effect, take a look at a radial-engined 1930's air racer: GeeBee, Laird-Turner Meteor, etc. Big round engine followed by a tapered fuselage. Seems like I read about an actual formula used to figure the most eficient taper. The wasp-waisted Lancair fuselage might be a similar deal. Don't see how fatter tires figure into it, unless you put some Maule-style wheel spats behind them to negate the extra drag.

EZFlap wrote:The key to making the Coke Bottle Effect work is the second parameter, which is itself a combination of three things: Loud Disco music, a three-piece white suit with matching white patent leather shoes, and a very very small spoon. The spoon is critical because the little brown Coke Bottle and its opening are very small.

Good one. LOL from me and the Mrs.

Given such logic, I would be foolish not to invest in some Goodyears 26s...

There are very likely a few people on here that are far more educated (and degreed) than I, in the aerodynamics realm. But I believe I'm on solid ground in saying that the Lancair's "waisted" or "pinched" look is a different application than the F-106 "Coke Bottle" fuselage, which was more properly called the "area rule".

The pinched rear fuselage was originally referred to as the "pumpkin seed" principle, and the modern aero guys now formally call it a "pressure recovery shape". This was put into practice as far back as 1935, when CG Taylor needed to have the high speed performance of his new Taylorcraft embarrass that of his previous design (the Cub). It did

The pinched pressure recovery shape has found its way into many many aircraft and aero-related designs from gliders to 300 mph Lancairs. But this is still a "low speed aerodynamics" principle. The Coke Bottle area rule had to do with keeping trans-sonic shock wave formation and supersonic pressure spikes under control, as part of "high speed aerodynamics".

Any aero guys can feel free to correct me if I have any of this incorrect.

EZ:

Absolutely right on the area rule stuff. It's the Whitcomb Area Rule that states that the total cross sectional area of the aircraft should vary smoothly. That includes the area of the wings, engine nacelles, wing-fuselage fairings, etc. Richard Whitcomb is the same guy that gave us winglets and the super critical airfoil.

It really applies to the speed range of Mach 0.7 to 1.2, so if this applies to the Bearhawk, I've seriously underestimated their performance envelope. It sure creates some sexy shapes in planes like the Northrop T-38 tho:



Otherwise, if you are not operating in that speed range, you can get away with shit like this:



YB

Another great example is the Falcon 50 where the #1 and #3 engines are mounted.

When they initially broke the sound barrier, (mach 1) it caused terrible vibrations and damage. The coke bottle design as well as other changes lessened that vibration problem.
The T-38, with the coke bottle fuselage, goes thru mach 1 with no vibration. There were times that I went thru mach 1 without knowing it. The people down below knew it.

Dick