Danger said:
Aircraft historians, ground-based observers, and R/C pilots all have different insights about flight in general, but would you guys please keep in mind that Andre actually flies the real thing. I'm more inclined to follow his responses.
True, but remember, the original topic is about firing bullets at Mach 3, which is a bit unrealisic.
Regarding knowledge of aerodynamics, excluding military trained pilots, I find that glider pilots, both real full scale and rc models, are more likely to be knowledgeable than powered aircraft piliots, since they don't have motors to ovecome pilot mis-judgements.
For example, there's a simple rule both full scale and model glidier pilots learn, when asking is my aircraft going to clear that object in front of me? One way to figure this out is to aim for the top of the object, if you're airspeed increases, you're going to clear the object, if you're aircraft slows down, you need an alternate plan.
Edit update: another glider rule, if a glider becomes unresponsive, the first control input is down elevator. Most of the time this is kind of obvious, because excessive AOA is due to trying to fly too slow. However when "hi-starting" a model glider, which bascially applies tension to a tow hook just in a bit in front of the center lift, the trim setup, or an updraft may create excessive AOA at high speed, causing a snap roll situation, where the model may "kite" right into the ground, and with very high tension to weight ratios, like 40g's for an F3J contest launches, time from launch release to impact is pretty quick (I witnessed one where launch to impact was less than 2 seconds, speed at impact about 90mph). Seeing the model rolling and/or yawing, the untrained instinctive response is to use ailerons and/or rudder, which are bascially useless. You pretty much have to anticipate using down elevator, and use it if you hint of getting of the model getting out of shape. Normally this is only an issue when trimming a new model, but if the launch setup is aggressive with a high AOA, thn launching through a thermal can push AOA beyond crictical. The main point here, is that contest rc pilots are well aware that stalling / snap rolls have nothing to do with speed and only to do with AOA. My own personal experience was with another
person's small slope glider, that had a large amount of elevator throw (unknown to me). While at a high and safe
alititude, I dove the model and then fed in a lot of up elevator, but there was no pitch response, just a roll response,
which was kind of cool once it was no longer a surprise, it was my first true snap roll with a model.
This leads to another rc model rule: a models altitude should be 2 or 3 mistakes high, especially when trying out
new stunts and/or with a new to you model.
Getting back to the normal glider pilot stuff, high end models allow camber adjustments that the pilot manually controls depending on the situation. Minimum sink mode while thermalling, best glide ratio while searching for thermals, and for the rc pilot bringing his model back from a downwind thermal, best glide ratio for high speed flight; for some model glider higher camber airfoils, as much as 3 degrees of reflex (entire trailing edge surfaces raised) is used. For models, the range of camber adjustment is pre-programmed via transmitter.
One of my pet peaves is how many web sites and books use "hump" theory to explain lift: "the hump on top of a wing makes the air travel further to catch up to the air below and faster moving air has less pressure". For example, older versions of Microsoft Encarta used "hump" theory, until the 2002 and later versions. You can still find a lot of web sites that still refer to "hump" theory.
I work with a group of programmers / engineers and I remember a co-worker with an instrument rating that believed in hump theory, as well as other co-workers, and it took a bit of convincing to get them to change their beliefs.
It's a two step process.
Step 1, explain how pressure differential is how air exerts it's weight inside a container. For example, putting 80 cubic feet of air into a scuba tank increases weight by 6 pounds, and it's the pressure difference versus alititude within the container that causes the net downwards force exactly equal to the weight of the air inside the container.
Step 2. Replace 1 pound of air with 1 pound model aircraft. It's a closed system, so as long as the center of mass isn't accelerating vertically, the total weight of the system never changes, regardless of what the model is doing, as long as the center of mass of the model has no vertical component of acceleration. If the model is flying within the container, then it has to increase the pressure differential by just enough to create a net downwards force within the container equal to the models weight. Therefore the model lifting surfaces are air pumps creating a downwards flow of air that results in the net increase in pressure differential within the container.
There's a third step for explaining lighter than air baloons within a container
Step 3. If the model is a helium baloon, then as the balloon is inflated within the container, it increases the pressure within the container, which increases the pressure differential. If the balloon is hovering,then the net increase equals the weight. IF the ballloon rests on the bottom, the net increase is less or zero. If the balloon is pushing against the top, the net increase in downforce due to pressure equals the weight of the model balloon plus the upwards force the balloon exerts on the top of the container.
A gentlemen on another site, not related to physics stated the following:
