GA Crash at PDK

Half Fast said:
Even weirder that (1) it’s error got worse and (2) it’s explanation for the error is also an error. A^2 / B^2 does equal (A/B)^2.

Don’t trust this thing for math. Or anything else that’s important. And that’s AI in a nutshell.
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It’s almost like a human doing “mental math” and coming up short. Its original guess was “about 22%” which is close enough to prove a point about small changes in speed having magnified effects on energy. Rounding its new error up to 27% is quite significant. Seems like it should be able to check its work with a link to Wolfram Alpha or equivalent. I’m sure it will be tweaked going forward - this is beta software after all.
 
Pinecone said:
Glide distance is based on LIFT divided by drag. If you increase the lift more than you increase the drag, you get a longer glide.
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That extra "lift" only lasts a second or two, then it's back to equal to the weight of the airplane for the duration of the flight. In practice, though, you'll push the elevator forward for passenger comfort to offset the ballooning effect, unless you're trying to save the Taj Mahal, that is :), so there really isn't any extra lift at all.
 
Pinecone said:
Glide distance is based on LIFT divided by drag. If you increase the lift more than you increase the drag, you get a longer glide.
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That is correct, but you'll be hard pressed to find a flap design that actually increases L/D with conventional deflections. Maybe you could show us a drag polar of just such a design.

ETA: If theory of wing sections isn't your cup of tea, an example of published emergency procedures showing a non-zero flap deflection for max range glide would serve the same purpose.

Nauga,
and his pals Abbott and von Doenhoff
 
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dtuuri said:
I suppose anything is possible, but how would extending the flaps improve L/D max? Seems it would increase drag to me, else why not fly with them extended all the time?


Allow me to pick a nit for the student pilots: As to the part about "climb gradient", flaps only reduce ground run. Steeper airborne climb gradients require less flaps, a daily operational fact of life out West for air carriers.
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It all depends on the wing and flaps themselves. In most airplanes, the initial flap deflection will increase lift more than drag, therefore L/D increases.
 
Rcmutz said:
It all depends on the wing and flaps themselves. In most airplanes, the initial flap deflection will increase lift more than drag, therefore L/D increases.
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As before, if deflecting the flaps increases L/D why not build the basic wing to that airfoil and capture the efficiency gain? I certainly could be wrong but would like to see a quantifiable example. Or are you referring to the temporary increase that is then lost when in a steady-state glide? That's really an increase in lift giving you margin to trade more airspeed for a shallower descent in the short-term, with a steeper glide at the resulting LOWER L/D in the long term.

Nauga,
and curved air
 
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Rcmutz said:
It all depends on the wing and flaps themselves. In most airplanes, the initial flap deflection will increase lift more than drag, therefore L/D increases.
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Rather than answering in stereo with @nauga please read page #369 here: 00-80t-80.pdf (faa.gov)

"Since the airplane lift is essentially equal to the weight, the minimum angle of glide will be obtained when the airplane is operated at maximum lift-drag ratio, (L/D)max"

and,

"The highest value of (L/D) will occur with the airplane in the clean configuration."
​
You will do all you can to PREVENT an increase in lift upon initial flap deflection, so you don't upset your passengers' tummies.
 
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High performance gliders do not use reflex to increase L/D. I had a model glider that could reflex the ailerons and flaps to increase lift when flying slow thermaling. That was about as clean as you could get as you could add just a couple degrees of reflex. I would never use that on glide.
Sorry but I am very dubious that adding any amount of flap would increase L/D.
 
Hang 4 said:
High performance gliders do not use reflex to increase L/D. I had a model glider that could reflex the ailerons and flaps to increase lift when flying slow thermaling. That was about as clean as you could get as you could add just a couple degrees of reflex. I would never use that on glide.
Sorry but I am very dubious that adding any amount of flap would increase L/D.
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Correct flaps seldom increase L/D in a steady state. Of course short term gains can be made until things stabilize.

My current 15Meter (50ft) wingspan glider uses full span flaperons. While changing the flaps setting won’t increase the L/D, they can change best L/D speed with minimal negative effects to the L/D. I.e going negative flaps I can go faster at about the same L/D I would get at a slower speed with 10 degrees of positive flap.


Brian
CFIIG/ASEL
 
Only one I can think of that I've flown myself would be the DV20 Katana which was developed from a motor glider.
 

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Spinka said:
Only one I can think of that I've flown myself would be the DV20 Katana which was developed from a motor glider.
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I’m not sure that follows.

I think it’s saying if you have an engine failure w/takeoff flaps, you should use a slower airspeed if you decide to keep takeoff flaps in.

I don’t think it means to imply that you can glide farther at that speed. Keeping them in might make sense to avoid the momentary sinking that happens on flap retraction, depending on the altitude.
 
brcase said:
Correct flaps seldom increase L/D in a steady state. Of course short term gains can be made until things stabilize.

My current 15Meter (50ft) wingspan glider uses full span flaperons. While changing the flaps setting won’t increase the L/D, they can change best L/D speed with minimal negative effects to the L/D. I.e going negative flaps I can go faster at about the same L/D I would get at a slower speed with 10 degrees of positive flap.
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I wasn't going to go down the negative reflex path, but my 3M RC will do that as well. Think what is missing is the kinetic component. If you add flaps in an unpowered state, you'll convert some kinetic energy into lift. However, you've also added drag and once the aircraft slows to a new equilibrium speed, it will have a worse glide ratio. So in the example of trying to get over a fence at the last minute might work, I can't imagine a useful change in distance over the ground from adding flaps.
 
FastEddieB said:
I’m not sure that follows.

I think it’s saying if you have an engine failure w/takeoff flaps, you should use a slower airspeed if you decide to keep takeoff flaps in.

I don’t think it means to imply that you can glide farther at that speed.
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Well, it says flaps T/O for gliding, engine fire in flight and restart with windmilling prop in the emergency procedures, so I believe that is what they are saying. However, I can ask someone at Diamond about it.
 

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Spinka said:
Well, it says flaps T/O for gliding, engine fire in flight and restart with windmilling prop in the emergency procedures, so I believe that is what they are saying. However, I can ask someone at Diamond about it.
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Well, that is different - and intriguing. Could it be they’re factoring glide duration - at the slower speed - into the equation?
 
FastEddieB said:
Well, that is different - and intriguing. Could it be they’re factoring glide duration - at the slower speed - into the equation?
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It would be unfortunate if they chose the term 'best glide ratio' per post 93 to mean a glide ratio that wasn't the maximum.

Nauga,
best by test
 
FastEddieB said:
Well, that is different - and intriguing. Could it be they’re factoring glide duration - at the slower speed - into the equation?
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So “best glide” might refer to either the maximum distance to the crash site or the maximum time it takes for you to die?
:devil:
 
nauga said:
It would be unfortunate if they chose the term 'best glide ratio' per post 93 to mean a glide ratio that wasn't the maximum.

Nauga,
best by test
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Unit is speed. It's best glide in that configuration. What should also be included is the glide ratio at those two configurations. I'd be willing to bet the ratio is worse with the flaps out. I'm guessing that since it's a powered glider, they include that as part of the engine out emergency procedure and somewhere else it mentions what to do in an engine out in take-off configuration which might not include retracting flaps ( that's a lot off speculation, even for POA)
 
FastEddieB said:
To be fair, buying time to run checklists and perhaps resolve the issue seems worth factoring in in some situations.
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I'd rather know the capability of the airplane and let me choose how to tailor it to the situation. For example, In some cases the 'optimum' descent is a *minimum* time profile. Min/Max time, max range, max endurance...? I'd bet the majority of GA pilots would assume best glide (with no other caveats or qualifiers) is max range, i.e. L/Dmax.

Nauga,
the gamma man
 
FastEddieB said:
Well, that is different - and intriguing. Could it be they’re factoring glide duration - at the slower speed - into the equation?
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That would be "minimum sink" which is different than best glide. Minimum sink is the speed at which a gliding aircraft descends at the slowest rate. It's slower than best glide. My aerodynamic skills are inadequate to guess at what might be the difference between the two in a GA aircraft. If you are over an airport, or have one an easy distance away, min sink might be useful to run checklists and try to get the engine to restart. Having said all that, there's likely little difference since most GA aircraft glide like crap.
In my hang glider, min sink is about 22mph, best glide is ~35mph. Sink rate at min sink is 200fpm, at best glide about 400fpm. So if I wanted to hang out over a spot (say looking for a thermal), I would have twice the time at min sink that I would at best glide. If I want to get to a place far away, best glide.
 
Hang 4 said:
Unit is speed. It's best glide in that configuration.
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But it says "speed for best glide" and then specifies the configuration. Maybe I'm parsing it too fine but that to me implies that the latter follows the former, i.e. "this is best glide, and THIS is the configuration for it." I agree that giving the glide ratios (and the speeds for them) for different configurations would clear it up.

Hang 4 said:
I'd be willing to bet the ratio is worse with the flaps out.
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That's my expectation as well, which is why I'm pulling on this thread.

Nauga,
dragging
 
@nauga I agree with you except for your parsing the configuration. It is missing a lot of context, but I would interpret is as "Best glide in this configuration" - if not why would two different configurations and two different speeds be listed?

Hang4
Wanting to have a new clever line at the end of each post
 
Gotta say, last time I lost my engine, at 200’, NONE of this really crossed my mind. Mostly concentrated on not hitting things.

Prolly shoulda broke out the POH?
 
Hang 4 said:
It is missing a lot of context, but I would interpret is as "Best glide in this configuration" - if not why would two different configurations and two different speeds be listed?
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I agree with the missing context. I read the 'T/O flaps' confguration as applying to 'best glide' and the two speeds applying to the different weights - notice the ratio of the speeds is roughly 91% and the square root of the ratio of the weights (or 1/) is roughly...91% - which is what you'd expect for a weight change with no configuration change. It's all in how you interpret the lines in the table :)

Nauga,
tabled
 
Tools said:
Gotta say, last time I lost my engine, at 200’, NONE of this really crossed my mind. Mostly concentrated on not hitting things.

Prolly shoulda broke out the POH?
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Tell me you never had a ready room 'discussion' about some insignificant bit of NATOPS trivia :)

Nauga,
and the width of a line on a perf chart
 
I relooked at the table and see what you mean. The big block is all one thing and the variable in the two smaller ones are weight. I never realized an engine out could require so much reading skill.
 
Tools said:
Prolly shoulda broke out the POH?
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That's why we pre-brief and do scenarios, right?

And practice. If you know from practice it takes 300' of altitude to make it work when you know it's going to happen in given conditions, you can set a margin of x - in my case 100' extra for my Luscombe, and you already know you will NOT have a good outcome if you are below that and can make the best choice ahead of you knowing you'll be hurt if you turn back.
 
I googled a POH for a DA20 and agree the table is suggesting best glide is with flaps in T/O position. I'd love to see the polar for the aircraft in cruise and T/O though.
The procedure for a "Precautionary landing" with engine power available is kind of interesting. They want you to find field, overfly it at 500 feet, climb back to 1000 feet, then do a low pass at 100 feet, then climb again to 1000 feet and do your landing. I'm hard pressed to come up with a scenario where that would be the right thing to do. If I can do all that climbing and maneuvering, why the heck am I landing out?
 
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FastEddieB said:
To be fair, buying time to run checklists and perhaps resolve the issue seems worth factoring in in some situations.
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Oh absolutely! You might already be over a suitable landing location and not need distance, but more time could be a good thing. Best to select a landing site within easy distance anyway, even if it’s less than ideal, to allow for maneuvering, obstacles, winds, etc.
 
The whole "polar" thing was mentioned to me by a glider CFI when we were doing some practice in an LSA Apollo Fox back in my 5C1 days. There are times when going faster into a wind is better than hanging at best glide... and keep in mind if you are doing a turn-back you're more than likely going to be landing downwind. When we did our practice at Llano on Saturday, my rollout was pretty much half of the grass runway doing a wheel landing with a bit of a left-quartering tailwind in the Luscombe. Not something I'd recommend for a rookie.
 
nauga said:
Tell me you never had a ready room 'discussion' about some insignificant bit of NATOPS trivia :)

Nauga,
and the width of a line on a perf chart
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Busted, just being snarky! I got to Fairbanks AK at 1:30 am, it’s 6:30 now and I start class at 8. I started this adventure at 4am eastern yesterday…. I’m all kinda punchy! Ha!

Cool old planes stacked like cordwood ever where…
 
Spinka said:
I can ask someone at Diamond about it.
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Please do, I'd really be interested in the explanation. My thoughts are that "zero flaps" probably is "negative flaps" which might be cleaner at cruise AoA, but dirtier than "T/O Flaps" at approach AoA. :dunno:
 
dtuuri said:
Please do, I'd really be interested in the explanation. My thoughts are that "zero flaps" probably is "negative flaps" which might be cleaner at cruise AoA, but dirtier than "T/O Flaps" at approach AoA. :dunno:
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What if you want to cruise and/or glide at best range (i.e. L/Dmax)? Do we really think the POH makes assumptions about hope the POH is making assumptions that change the definition of conditions for "best glide"? Or is it stated in the POH that max range is with TO flaps? It seems straightforward if you just blindly accept the numbers but there's something that isn't quite right without more information.

Nauga,
conditionally
 
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Tools said:
Gotta say, last time I lost my engine, at 200’, NONE of this really crossed my mind. Mostly concentrated on not hitting things.

Prolly shoulda broke out the POH?
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You're spot on. That's why it is imperative that pilots brief this before takeoff. Even something as simple as turning downwind instead of upwind may be the difference between success and failure.
 
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