RCU Forums - View Single Post - J3 cub, crash on take off nearly every time.
Old 10-25-2008, 04:56 PM
Join Date: May 2004
Location: Oklahoma City, OK
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Default RE: J3 cub, crash on take off nearly every time.

then explain to me why i can fly a plane into the wind at 0 ground speed, basically keeping it in one spot and even climb, but when i turn downwind i drastically lose altitude and it wont climb at all unless i throttle up. not to mention my cub met its doom by flying slowly into the wind and then turning downwind and not throttling up enough and my plane basically fell out of the sky.
Ummmmmm........you're not disagreeing with me here. You can "hover" in a wind because the plane is moving forward within that air mass. However, the air mass in this case is moving the opposite direction in relation to the ground. Consider my example of the plane moving in a mass of air, and the ground suddenly zipping by at the same speed as the plane. Say the plane is doing 20 knots true airspeed, heading east, then imagine the ground beneath it suddenly starts moving 20 knots east as well. Logically, the "speed" of the ground has no effect at all to the plane, but an observer on the ground would perceive no motion (plane is hovering). As to why your Cub crashed, you answered that question yourself: "not throttling up enough". Any time you bank, wing loading must go up if you are to maintain altitude. If you're in a slow-flight configuration (which you almost certainly were at the time, unless you had a mighty strong wind), then you were already somewhat close to stalling. To get the lift you needed for the turn, you needed to maintain airspeed while still initiating the turn...this would require dropping the nose, or adding power. All this is exacerbated by the fact that the ground observer/RC pilot is more likely to make a steeper turn when turning with the wind, because we're used to the plane making a certain turn radius and we will involuntarily try to keep the same radius (will require a more extreme turn when turning with the wind).

qldviking....wow, kudos to you for actually taking this to the field and trying it out for yourself. Your observations sound like what I'd expect, how do you think they contradict what I said before? Let's look at your results for a minute. First, keep in mind that your groundspeed while in a front-quartering crosswind (which is what happens when you are trying to maintain a straight ground track, as I assume you were for radar-measuring purposes) is going to be LOWER than your true airspeed, because you are essentially in a "partial headwind". This is basic vector math, and thus does not change with aircraft type (so a Learjet at 180 knots TAS in a 10*, 20 knot crosswind will have the same groundspeed as a Bonanza under the same conditions at 180 knots TAS).

Now, your speed difference upwind vs downwind is 7.25 vs 8.5....a difference of 1.25 MPH, which actually is just under 7% of the wind speed - well within the likely variations in wind speed during your tests. So the actual ground speed difference upwind vs downwind is 15.75 MPH. Consider the margin of 5% of wind speed, which is 0.9 MPH, on a run both upwind and downwind, for possible 1.8 MPH change, and we are very close to our known wind speed, 18 MPH. For an unofficial test, your numbers are actually very consistent. This completely supports the concept that the plane was moving within an ~18 MPH air mass the entire time. As a ground observer, you saw this difference and recorded it.

If you could somehow fly the plane at a heading exactly 90* from the wind course (would require a compass in the plane and knowledge of the exact wind course), you as a ground observer would see the plane moving "sideways" at 18 MPH. Something else to think about: when you are flying in a steady crosswind, does the plane continually try to weathervane into the wind, or does it maintain the same crab angle?