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JimC
01-11-2003, 07:19 PM
Fresno David wrote, "In the past, I've had 3 early model Cessna 180s and all 3 were faster with a back seat passenger... My Cuby's the same way... "

Generally this is to be expected. With the exception of tailless flying wings with trailing edge reflex, some sailplanes with trailing edge reflex, and planes with symetrical wings; wing airfoils are usually designed to produce a nose down pitching moment in order to enhance pitch stability. Therefore, to offset the airfoil pitching moment, the plane's tail is required to generate a download, usually very roughly about 10% of the weight of the airplane. So, making up a few numbers for illustration, if you, your plane, and your cargo weigh about 1500 pounds, your wings will be carrying about 1500 pounds + about 150 pounds, for a total of about 1750 pounds. Moving stuff to the rear will increase your cruise speed for three reasons.
1) moving the stuff aft reduces the required tail download necessary to offset the wing's nosedown pitching moment, so the tail generates less induced drag.
2) since the tail is usually uncambered and tail zero lift occurs at zero aoa, reducing the tail download also reduces the tail profile drag.
3) since the wing is now supporting the original 1500 pounds + a reduced tail download (say 50 pounds for talking purposes), the wing is now carrying only about 1550 pounds, and therefore generates less induced drag.
3b) since the wing is asymmetrical and unreflexed, zero lift is occuring at a negative aoa and cruise aoa is probably near zero, so wing profile drag is near its minimum. It may increase slightly or decrease slightly as the passenger moves aft, but probably not enough to be significant.

So, by moving stuff aft, you've reduced three sources of drag and your plane compensates by going a little faster, which reduces induced drag even more. Usually about 2-4 kts increase for moving a 200 pound passenger from the front to the rear seat(s). Hope my explanation didn't muddy this up too much.
JimC
P.S. I moved this to a new topic because the 105 topic was getting so long my antiquated computer was usually crashing when I attempted to download it, and I couldn't post to it at all.

supercub
01-11-2003, 08:45 PM
Good explanation Jim. Years ago, the chief flight instructor where I worked and an advanced student of his got into a discussion of which airplane was faster..........a Cessna 182 Skylane or the Grumman Tiger (the one with the 180hp engine, tiger or cheetah, don't remember). Anyway, we took the 182 and the other fellow took the Tiger, both airplanes had 2 people onboard. Anyway, we leveled off, side by side and both airplanes full power......believe it or not, it was a dead heat. After a few minutes I slid my seat back to the stops and we slowly started to nose ahead of the other airplane. Everything you say above, we proved, plus the chief flight instructor won the 1$ bet. :lol: By the way...both planes were nearly new.....that's when airplanes were still being built LOL.
Brian

Crash
01-12-2003, 03:02 AM
A few years ago I removed my heavy stock battery from the rear along with the battery box, vent tubes, plywood platform, strobe, strobe wiring (coiled up in a big coil) ADF antanna wiring (a mile of it) etc. The net was over 40 lbs removed from the rear. It took 28lbs off the tail wheel weighed before and after. The new dry cell battery was installed under the pilot's seat. When I flew it after this change I noticed about a 5 mph drop in the cruise speed with just me in the plane. At the time I thought I must be dreaming because it didn't make any sense that taking weight out of the tail would cause the plane to fly slower. Hummm. Now it makes some sense. Crash

JimC
01-12-2003, 08:43 AM
The tail weight effect on cruise on planes with conventional airfoil camber has been well-known and recognised by designers for about 60 years now. They haven't done a good enough job of spreading the word.
JimC

FlipFlop
01-12-2003, 10:50 AM
Back in the 20s or 30s (or both), designers installed movable weights in the fuselage to work with CG changes... The weight was attach to a cable on pulleys and they simply moved the weight where they wanted it...

Now, this is starting to sound like a great idea, except we don't want to add additional weight and we're not always going to have moose meat hanging on these pulleys... So, we devise a way to attach the battery to the cable and send the positive and negative electrons by radio signal from the battery to the firewall mounted solenoid, thereby effectively eliminating the weight of an elastic battery cable...

The only problem I forsee with this installation is if the pilot is equiped with a pacemaker... The floating electrons might raise havoc with the conductive current flow...

And you guys thought I was just another pretty California face :lol: