Building a Jabiru J400 from a first-time builder's perspective

Useful Tips

Cold weather starting... stick a haridryer in your NACA duct!

Lately (1/21/2007) we've had some harsh weather including three snow storms in a row... anyway, temperatures are in the 20s F and for the first time, experienced cold weather starting. Funny thing is that after the first storm it started fine but I think the air on the ground has dropped even further lately because of the snow accumulation and so it just won't start. The solution? Stick a hairdryer on full blast in your NACA duct to the carb... the nice warm air vaporizes the fuel better and starts right up.. i've tried this once so far (yesterday) and it started in 4 prop turns with a nearly dead battery ... pretty effective

Why does an airplane fly? (seriously.. are you sure you know? it's not Bernoulli)

Amazingly, there's a lot of misconceptions about why an airplane flies. More specifically, there's misconceptions of why a wing produces lift. The popular explanation of lift actually misses the main point. In fact, I didn't quite realize how much I was missing until after I got my pilot license (it's not taught correctly during training either).

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For those of you who don't know how Bernoulli's explanation goes, here it is abbreviated.

the explanation says that wings need to be longer on top than the bottom (usually implemented with a curved top). Because it is longer path on top of the wing, the air is forced to travel faster over the top surface than the bottom surface as it needs to meet the 'same' air at the trailing edge that it separated from at the leading edge. This increase in speed reduces the pressure over the wing compared to below the wing
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Therefore, the Bernoulli explanation incorrectly explains that because the air travels on top faster than the bottom and this faster air on top drops the pressure, the higher pressure on the bottom lifts the wing.

So here's a brain teaser.. If the Bernoulli's explanation is correct, how does an airplane fly upside down? Wouldn't this airplane have to flip its wings in mid-flight since the 'curved' surface would be on the bottom now? The truth is that the top surface of the wing doesn't need to be any more curved than the bottom. Have you seen an aerobatic airplane's wing? It's symmetrical. In fact, even a flat plate would work as a wing. The curves in wings are for aerodynamic efficiency.

The true explanation of lift goes back to Newton's third law which states "for every action there is an equal and opposite reaction". Another words, for the wing to go up, something must be coming down and in this case the answer is air. A wing acts as a big air 're-director' that curves the direction of the air downward as it passes through it.

So a wing actually pulls down on the air above it (that's right.. pulls from above) instead of air pressure pushing the wing up as explained by the Bernoulli principle. It's this pulling down of the air above it that results in a lower pressure above.

Who says the air above the wing needs to travel faster on top? The top surface is not enclosed like a venturi, which is where Bernoulli's concept is borrowed from. If there's a void of air on the top surface of the wing, the void is filled with air from higher above the wing and hence you get a downward pulling of air.

Think about it, a rocket shoots up because a jet stream shoots down. A 747's turbojet goes forward because it jets air back. A prop (which is really a mini wing at high speed) thrusts forward because it pulls air back (you can certainly feel the propwash if you stand behind it... well that is air being pulled from the top of the blade which is facing forward on the aircraft to the bottom of the prop blade which is toward the back of the aircraft.... if it happens on the prop, it happens on the wing). A wing is no different, it goes up because it pulls the equivalent force in air down.

So think about this. If the Bernoulli Principle was correct and the true 'cause' of lift, then the high pressure air at the bottom of the wing would travel around the trailing edge of the wing and up unto the lower pressure above. This of course is what happens at the tips of the wing which results in vortices (and we know these lose lift). But if it were true all around the edge of the wing including the trailing edge you would have a lot of air being redirected up... and of course with Newton's 3rd law you'd be in trouble as the net force would have to be down (but a wing does lift up does it not?).

The funny thing is that a sail on a sailboat works the same way (redirects the wind's path). Except in this case the wing (sail) is vertical instead of horizontal. So really, the principle of lift was learned thousands of years ago. It was just not applied correctly and successfully to an 'air vehicle' until about 100 years ago. Also, there's an interesting 'sail' ship invented a while ago that doesn't have any sails... it uses vertical rotating cylinders to redirect the wind in its favor... sorta interesting... i guess the lesson here is that you can create 'lift' (thrust) many different ways by redirecting wind

There's a really good explanation of lift as mentioned above at the following link. I highly recommend reading through it carefully.
http://www.allstar.fiu.edu/aero/airflylvl3.htm

Added 7/15/2006: Recently i've gotten more interested in sailing...not sure why...maybe because it's another way to travel and all this high speed thinking has kicked in slow speed interest... even though i haven't done any sailing at all, i've watched a sailing video that explained the cause of lift (forward lift) on a sail also with the Bernouilli principle... interestingly, the narrator points out that it doesn't make much sense that the air over the front of the sail goes faster than the rear of the sail because of the thickness of the sail but that is the only explanation she knows of... you damn right...how thick is a sail? a few millimeters? What difference in speed could this possibly result in? does it make any sense?

the change in direction of the wind with the sail isn't too hard too prove...try this... take a tall drinking glass (a smooth one that is cylindrical) and stick it under the running tap horizontally (on its side) so that all the water falls on one side of the cylinder and the water follows the curvature of the glass... if you do it right...the water will actually go around and go against gravity on the other side (like it's trying to go all the way around back to the top of the glass where it fell on the glass) before being sucked down by gravity (can't stick to the glass any more as gravity is stronger at that point)... a sail and a wing do the same thing... the wind sort of sticks to the sail / wing and follows the curvature... in this experiment, there isn't even any water on the inside of the glass; nevermind it traveling slower as Bernoulli dictates (the two fluids never even touch...cap the drinking glass and it still works)... doesn't make sense... fluids follow curvatures... forget the "intelligent air molecule that speeds up to catch its buddy" explanation

btw, i don't mean to say that you can't use the Bernoulli equations to find the lift produced by a wing because i'm sure you can... there's no reason why you can't use the difference in pressure between the two sides of the wing to figure it out... my complaint is that the Bernoulli explanation doesn't correctly explain how the difference in pressure got there in the first place...