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Thread: 24 FPS Gust and Thermal

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    Re: 24 FPS Gust and Thermal

    Quote Originally Posted by proppastie View Post
    References from pilots of the actual aircraft, and I was not able to really grill them....The flaps are worthless, the reflex gave higher rate of sink (which makes sense) The -15 got you off faster and was better in the thermals. I am strongly leaning towards keeping the flaps and have in concept a simple better linkage design. The whole fuselage/boom is a different structure concept. Not even close to the original except perhaps for the shape, wind screen/panel, skid and tow release. I wonder why I need a tow release, but I might need/want it.
    Ummmm... Pretty sure those pilots misunderstood what reflexing the flaps is for. Reflexing the flaps is for moving the drag bucket to a lower angle of attack (CL) so that the airplane is running lowest-drag when operating at best L/D between thermals. The speed for best L/D always has a higher sink rate than Vminimum-sink, so an increase in sink rate is to be expected. Running the flaps reflexed at Vminimum-sink is going to be worse than running with them neutral but, again, that's not what reflexing the flaps is for.

    Aesquire has it right above. I'd keep the ability to reflex.
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    Re: 24 FPS Gust and Thermal

    -15 degrees sounds unbelievable.
    -5 is more common.

    Vb is not Va.
    Vb=Vra=max rough air speed.

    Can be higher or lower than Va. With high wing loadings it's often higher, with low wing loadings it's often lower than Va.
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    Re: 24 FPS Gust and Thermal

    Quote Originally Posted by autoreply View Post
    -15 degrees sounds unbelievable.
    -5 is more common.

    He has his sign convention desrever.
    Conventional wisdom and practices yield conventional results. If that is good enough for you:
    Problem solved.

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    Re: 24 FPS Gust and Thermal

    Gliders fly in thermals and near ridges at or near minimum sink speed.

    There are 2 important things about flying that slow. You are close to stall, and need to be aware of that. Minimum sink speed is also maximum climb ANGLE for powered planes, and it's behind the curve. That means the angle of GLIDE is lower than a higher speed, and it takes MORE power to stay up than at max L/D speed.

    Any speed above max L/D speed takes more power, that's simple & intuitive.
    Any speed below max L/D speed takes more power. That's not as intuitive. It's possible to not be stalled, but hanging on too slow at full power.... and going down.

    This isn't as common in a Cessna as it is in an under powered, high drag old school ultralight, but it's real in both.

    So at slow speed you drop some flaps to increase lift, lower stall speed, and change the characteristics of the plane to one more friendly when slow. See all the STOL types including Cub variants.

    Increasing lift also increases drag. Induced drag from the lift, and often parasitic drag from the flaps "catching" the air more. ( this can be very desirable when landing in a short pasture )

    Reflexed flaps address the needs at the other end of the speed spectrum. By reducing the camber of the airfoil, you lose lift, but you also lose the induced drag of that lift. If you are flying well above max L/D speed, you've got plenty of lift, so you throw some away to get more speed.

    Here are some typical Polar curves... They are for hang gliders, so they are steeper and shorter than a Nimbus or a Carbon Dragon, but what's important is the SLOPE of the curve. The flatter, the better. The Max L/D is calculated from the tangent..... at different speeds that tangent changes.
    https://en.wikipedia.org/wiki/Polar_...(aerodynamics)

    Click image for larger version. 

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    You can see that the Falcon, a beginner training glider, is pretty much falling like a rock at 35 mph. It's very speed limited. But can be a fine soaring machine in light conditions if you are not trying to go anywhere. The Talon OTOH, a competition glider, is still covering ground ok at 35 and it's angle of glide falls off much "slower" as you speed up.

    Not shown on that polar is the Variable Geometry differences. The Variable geometry on the faster machines is mechanically very different than flaps, but the result is to change the camber of the wing, a flatter camber at higher speed, just as reflexed flaps give you. This allows the Talon to fly nice slowly, but not as well as the Falcon. And fly much better faster than the Falcon. ( more weight, more complexity, longer setup time..... TANSTAAFL )

    Here's one with a very different group of aircraft.

    Click image for larger version. 

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    Here we see that the P-51D Mustang is a better glider than a FW-190D, at350-375 kph but is worse at 450kph.

    The idea of reflexed flaps is to flatten the curve at the higher speeds.

    Which means they are the wrong thing to use for takeoff, landing, or flying slow.

    I'm sorry I can't seem to make this clearer. I just look at the curves and see it, but that's how my mind works with math with my training in Statistical process control.

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    Re: 24 FPS Gust and Thermal

    Quote Originally Posted by Aesquire View Post
    The idea of reflexed flaps is to flatten the curve at the higher speeds.
    Which means they are the wrong thing to use for takeoff, landing, or flying slow.
    sorry I got the sign wrong....with the flaps, never meant to say the reflex setting of 5 degree was for takeoff I meant to say the flap setting of 15 was reported better for takeoff and thermals... In my Cherokee and Mooney it is standard practice "for me" to takeoff with the flap partially set. I remember 2 notches in the Cherokee and there is a Take Off setting mark in the Mooney

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    Re: 24 FPS Gust and Thermal

    Quote Originally Posted by Aesquire View Post
    Gliders fly in thermals and near ridges at or near minimum sink speed.

    There are 2 important things about flying that slow. You are close to stall, and need to be aware of that. Minimum sink speed is also maximum climb ANGLE for powered planes, and it's behind the curve. That means the angle of GLIDE is lower than a higher speed, and it takes MORE power to stay up than at max L/D speed.

    Any speed above max L/D speed takes more power, that's simple & intuitive.
    Any speed below max L/D speed takes more power. That's not as intuitive. It's possible to not be stalled, but hanging on too slow at full power.... and going down.
    snip
    I think something is garbled here. For a glider, minimum sink IS the speed for minimum power to stay up. Think about it. A glider is gravity powered. The faster it's coming down, the more power it's using. If I understand the power curve, then as long as you're above minimum sink, you're not behind the power curve. In other words, if you're faster than minimum sink, you can slow down to that speed and use at least a little less power. The drag is a bit more at minimum sink, but you're flying more slowly, and in this case power is drag times speed. At best L/D, the drag is a bit lower, but you're going faster, and the drag times the speed is a bit more.

    If you get into powered aircraft, then the peculiarities of engines and propellers get into it. The best climb speed for a jet, or for an aircraft with less than optimal prop diameter for climb, will be considerably faster than the minimum sink speed.

    If you look at a polar curve, a line from the origin (i.e. 0-0) that is tangent to the curve will be tangent at the best L/D. For instance, if you look at the curve for the Spitfire Fvb in the second chart in your post. Unless my flat screen monitor is somehow distorting it, best L/D is at something like 220 kph and sink rate at that speed is something like 7 meters per second. (That's only 8.7:1 !!?!!) At 170 kph or so, the sink rate is something like 6 meters per second, but the L/D is only 7.9.

    I don't know what v1 and v2 are in the first chart, and I'm lazy so I don't want to print it out and add the lines for 0 fpm and 0 mph and pick the tangent curve for best L/D. But we know minimum sink is around 170 fpm at 25 mph, or about 13:1. I arbitrarily picked 30 mph, which probably ISN'T the exact best L/D. However, we get something like 185 fpm at 30mph, or about 14.3 to 1.

    Operating manuals for powered airplanes may show lower best glide speeds than best climb speeds, but that's at least partly because of the drag of a windmilling propeller, which one doesn't have to cope with when climbing! The other part might be propeller efficiency considerations. I expect almost all props are at least a little smaller than would be best if they didn't have to stay clear of the pavement.

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    Re: 24 FPS Gust and Thermal

    P.S. If the air is bumpy enough, it may pay to be considerably faster than minimum sink, for the increased control power and margin above stall. Certainly it pays on some days for RC gliders. I suspect, though, that it would hurt more to do this. The on-board pilot in my RC glider is mute, so I don't know. ;-)

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    Re: 24 FPS Gust and Thermal

    If you are tring to get someplace, best L/D speed is minimum power speed to get there.
    If you just want to loiter on station like a spy drone, then minimum sink speed is the minimum power speed, I think.
    Call it minimum power loiter speed, for powered aircraft.

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    Re: 24 FPS Gust and Thermal

    "For a glider, minimum sink IS the speed for minimum power to stay up"
    Yes. Powered by the planet. Slower or faster you descend faster. Powered plane pilots have trouble with the concept of a glider having any power at all, so they don't usually figure in the 5.972 × 10^24 kg "remote mounted motor".

    "If I understand the power curve, then as long as you're above minimum sink, you're not behind the power curve"
    I meant powered airplanes where you figure this in LEVEL flight. Slower than max L/D takes more power to maintain level flight.

    " The best climb speed for a jet, or for an aircraft with less than optimal prop diameter for climb, will be considerably faster than the minimum sink speed."
    Best climb RATE speed is ( generally ) best L/D speed. Best climb ANGLE is Min Sink speed. The first is to get high quickly, the second to get high with a shorter distance, like clearing obstacles at the end of the runway. ( I had a hard time grasping this one at first )

    Yes, sometimes the actual best climb speed in a powered plane isn't max L/D speed because of propulsion efficiency at different speeds. A cruise prop or jet engine may give you a higher best rate of climb at a higher speed.

    "I don't know what v1 and v2 are in the first chart..."
    I just grabbed that off the web, first image that came up when I searched "glider polar". I haven't bothered redrawing it to get the zeros right ( a pet peeve of mine with some charts ) But knowing the aircraft involved I will guess that V1=stall speed ( or possibly Min Sink ) and V2= Va? You grasp the tangent part correctly. 14:1 is probably reasonable with a clean pilot pod type harness. Wills Wing states that just replacing the streamlined control bar base tube ( the horizontal ) with a round tube cuts max L/D by 0.5 ( usually done to put larger wheels on bar, or for more comfort ) so small things make a big difference.

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    Re: 24 FPS Gust and Thermal

    Quote Originally Posted by BBerson View Post
    If you are tring to get someplace, best L/D speed is minimum power speed to get there.
    If you just want to loiter on station like a spy drone, then minimum sink speed is the minimum power speed, I think.
    Call it minimum power loiter speed, for powered aircraft.
    Not really. Power is how FAST you're putting out energy, and at best L/D you're putting out it out faster than at minimum sink. However, unless something weird is going on, you'll get there fast enough that you end up burning less fuel anyway.


    Quote Originally Posted by Aesquire View Post
    snip
    "If I understand the power curve, then as long as you're above minimum sink, you're not behind the power curve"
    I meant powered airplanes where you figure this in LEVEL flight. Slower than max L/D takes more power to maintain level flight.

    " The best climb speed for a jet, or for an aircraft with less than optimal prop diameter for climb, will be considerably faster than the minimum sink speed."
    Best climb RATE speed is ( generally ) best L/D speed. Best climb ANGLE is Min Sink speed. The first is to get high quickly, the second to get high with a shorter distance, like clearing obstacles at the end of the runway. ( I had a hard time grasping this one at first )
    snip
    Well, maybe with prop efficiency figured in things look like that, but with the questionable assumption of equal prop efficiency, best angle of climb will be BELOW minimum sink speed and best climb will be AT minimum sink speed. If you're trying to maximize range, it would be best to climb at best L/D. Again, sink rate corresponds to power. And I'm talking about minimum sink when there's no prop or it's turning just fast enough for zero thrust. Stopped or windmilling props have considerable drag, so figures for that condition will be misleading.

    You mentioned level flight. At least for reasonably good L/D, the difference is very small. For instance, at 14:1, the direction of flight is only about 4 degrees below horizontal. The component of gravitational force perpendicular to the flight path is still sin (90-4) is still .998. Maybe if you're talking about a polyethylene covered bamboo hang glider, there's a significant difference.

    Let's look at the curve for the Ultra/Super from your chart. At a speed of 1980 fpm (33 fps), sink rate is around 195 ft/min (3.25 fps). At 2127 fpm (35.5 fps), sink is at about 185 fpm(3.1 fps) and at 2420 fpm (40.3 fps) it's maybe 205 fpm (3.4 fps). Corresponding L/D's are 10.2, 11.5, and 11.8. Let's again make the questionable assumption of equal prop efficiency (maybe we have the world's lightest, smallest constant speed prop), and put it at 50 percent, because we're using an obnoxious screaming 8hp two stroke. Thrust horsepower will be only 4 in each case. That corresponds to 2200 ft-lbs/second. Furthermore, let's say gross weight is 250 lbs. In the first case, thrust power required for level flight is (250/10.2) X 33 = 809 ft-lbs/second. That leaves 1391 ft lbs per second, divided by 250 lbs is 5.56 fps climb. That's a climb gradient of 5.56/33 = 0.169. For the next case, we get 772 ft-lbs/sec level flight, 1428 ft-lbs left over for 5.7 fps climb but a gradient of only .161. For the final case, where L/D is a bit better, we get only 5.4 fps climb and a gradient of only .134. It's kind of amazing how little power a clean, very light, slow flying aircraft requires. Of course we know that an 800 lb racing sailplane requires a similar amount of power for level flight.

    Obviously, in the real world, prop efficiency is going to be more important and going a little faster will be better. But if you were designing a human powered airplane with the possibility of a huge prop, it would be different.

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    Re: 24 FPS Gust and Thermal

    A better way to say what I meant is:
    Best L/D speed provides highest miles for one gallon of fuel.
    Minimum sink speed provides longest time aloft for one gallon of fuel.

    So minimum sink speed is the lowest power, level flight speed.
    Stick and Rudder has this info. I would need to dig it out from my book boxes stored in the basement, to double check.

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    Re: 24 FPS Gust and Thermal

    "...best angle of climb will be BELOW minimum sink speed and best climb will be AT minimum sink speed."
    no, below Min sink takes more power, as the lift drops off fairly rapidly. Best climb Angle is at min sink speed.

    "If you're trying to maximize range, it would be best to climb at best L/D"
    Yes, max rate of climb is at best L/D. You're faster, and cover more ground per unit time and fuel consumed. ( whereas at Min sink speed you minimize fuel consumed per vertical foot gained. )

    from BBerson "So minimum sink speed is the lowest power, level flight speed."

    Hey I could be wrong on this... Please correct me.

    I though best glide was min power.... but I think you're right.

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    Re: 24 FPS Gust and Thermal

    I'm glad I could clarify that.

    More simply:
    If you lose your engine but think you have a chance of surviving, fly best glide speed. If not, fly minimum sink and you'll live longer. ;-)

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    Re: 24 FPS Gust and Thermal

    Ok, I found my Stick and Rudder book. (airplane bible)
    I was right about Speed of Least Power Required, or Best Duration is near minimum sink glide speed.
    I was wrong about maximum miles per gallon at best L/D. It's actually quite a bit above best L/D.

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