Biggles' Elder Brother - Moderator Posted March 12, 2012 Share Posted March 12, 2012 Wingman - I totally agree - a superb book. Looks a bit dated now I know, but at the time it was published it was years ahead of it time. A lot of wisdom in there and as the physics of flying hasn't changed still relevant today. One thing he discusses is the radical idea that the one single device that could be fitted to private planes that would have the biggest impact in saving lives is a chain! Connected from the instrument panel to the stick - so that the pilot couldn't pull back past a certain point! I think he was only half tongue in cheek about that! BEB Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted March 12, 2012 Share Posted March 12, 2012 Posted by Cyclicscooby on 12/03/2012 20:20:23: fit two airspeed peto tubes. Both near the TE of the wing, one Top, one bottom. By comparing the airspeed between the two, you'd be able to tell when it was about to stall.. (i.e. as the top surface breaks away, it's airspeed will drop, whilst the lower tubes speed wouldn't) No Chrissie I'm afraid it wouldn't. The airspeed over the top of a stalled wing is unchanged by the stall. What that device the guy is using does is simply detect flow in one direction as opposed to recirculating flow as you get in a stall. Strickly speaking a pivot tube measures pressure. Assuming you could get one into the upper and lower surface flows and they didn't act as flow trips (very unlikely that you could and that they wouldn't, if you could!) then what you would read in normal flight is low pressure above the wing and higher pressure below. Then, after the stall has happened, the low pressure region above the wing will collapse jumping up to something approaching atmospheric (or free stream) pressure and the slightly elevated pressure below the wing would still exist. So the pressure on one side would go up - not down - and it would only happen after the stall - bit late by then! There would be no warning of an approaching stall from such a device. Pitot tubes have to be located away from any lifting surface to work properly. Either way out in front or hung off a dogleg away from the surface. They measure the speed of the free stream - not the flow next to surface and they can't measure that! If they could I wouldn't have to spend a fortune on things like laser Doppler anemometers to measure the flow next to a surface! Now if you're really determined to do this - and frankly I don't see the point, but lets assume its for the sake of curiousity - then what you need is not an airspeed sensor - but an attitude sensor combined with a "direction of travel" sensor in order to work out if you are flying at a high AoA. The combination of an electronic inclinometer with a gyro system would provide you with the required information. What you could do with it is another matter! As has been pointed out model aeroplanes are small compared to the scale of the turbulance in the air - and so even a system like this is unlikely to be meaningfully accurate because short term local effects in the air flow would probably be a larger, and more important, factor in circumstances where this might matter. I still reckon the best tools you have to deal with this situation are your R/C piloting skills, a rudementary knowledge of aerodynamics and the mark one eyeball BEB Quote Link to comment Share on other sites More sharing options...
Steve W-O Posted March 13, 2012 Share Posted March 13, 2012 Physics is fine, and BEB, I did comment about the sink. We talk about stall speed because the airspeed is one of the basic instruments that are continuously and subconciously scanned. Maybe the stall speed should be renamed the speed at which the AoA to maintain level flight become to great and causes a stall, but now we are back to stall. There would be nothing preventing you from landing at cruise speed, apart from the strength of the u/c, stopping distance, keeping it on the ground if there was a gusty head wind, strength of the flaps and I'm sure quite a few other problems, so we need to land slower. The best would be to touch down as it stalls, but that could present problems with tip stalling, and leaves no margin for error. So, as speed is the figure we been to know for reasons other than stalling, we need to know the stall speed, or the speed at which the angle of attack becomes to great and a stall occurs. As this varies with temp, pressure, load etc, it is taken as a figure above what would probably be the real figure. Then as we need effective control, the approach speed is a bit above that. As the actual landing combines many requirements, the speed, whether it is the correct measurement or not, is the one to use. I'm not sure having an extra instrument to accurately give the angle of attack would offer any practical advantage, though it might be useful data for the black box on bigger craft. I've tried to put that in simple terms, as I think that speed will continue to be spoken about, however technically incorrect. Are there no ultrasonic air velocity measurment divices available? On in the planes horizontal plane, and one in the vertical plane would give the info to. Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted March 13, 2012 Share Posted March 13, 2012 Posted by Steve W-O on 13/03/2012 05:19:34: Maybe the stall speed should be renamed the speed at which the AoA to maintain level flight become to great and causes a stall, Now that statement I have no problem with at all! Except that there will still be those that confuse the two! BEB Quote Link to comment Share on other sites More sharing options...
Steve W-O Posted March 13, 2012 Share Posted March 13, 2012 Posted by Biggles' Elder Brother - Moderator on 13/03/2012 08:11:17: Posted by Steve W-O on 13/03/2012 05:19:34: Maybe the stall speed should be renamed the speed at which the AoA to maintain level flight become to great and causes a stall, Now that statement I have no problem with at all! Except that there will still be those that confuse the two! BEB Of course, that is my point, people will always refer to speed. I'm not sure that is is correct to say that opening the throttle won't unstall the wings. If you open the throttle, you increase the velocity along the axis of the chord, effectively decreasing the angle of attack. With many planes, and a lot more models, the thrust will quickly get the plane moving in the direction needed. The vertical component will also lighten the loading. Aother situation where it would prevent a stall is something like a 320 about to touch down, it would be descending at say 3deg, opening the throttle would stop the descent, so subtracting that 3deg from the angle of attack. Again the vertical component of the thrust (probably the thrust would be about 12deg from the horizontal) would lighten the wing loading a bit. The point is , that the OP's question can still be answered with speed, even if the physics is not 100% accurate, and his proposed device would still be effective, applying the relevent margins, use speed as the measured input. It would of course work better in some circumstances if you could measure the AoA, but most of those situations could be approximated well enough to do as intended with a G sensor. Quote Link to comment Share on other sites More sharing options...
Cyclicscooby Posted March 13, 2012 Share Posted March 13, 2012 Soz BEB... I assumed that a Peto tube mounted close to the surface would pick up the airflow, and likewise, when the boundry layer lifts, there'd be now't but turbulance across the surface... ?! Don't get me wrong, i'm not not suggesting you're wrong, just suprised i'm so wrong.. I supose, if it was that easy, it would have been done already.. Luv Chrisie.. xx Quote Link to comment Share on other sites More sharing options...
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