GA Crash at PDK

[Pinecone]

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.

Uuh, the Coefficient of Lift changes, so the amount of lift at a given speed and AOA increases. You have changed the camber of the wing or the camber and area with Fowler flaps.

 

[Half Fast]

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.

When I was training for my Sport Pilot ticket, we were doing an engine out simulation and I was trying to set us down on the runway at Orlando North. I was using very little flaps (maybe none) and as I got close it became apparent that I wouldn't quite make it and we were going to touch down in the grass about 50' short of the threshold. When we were maybe 10' off the ground, my instructor reached up and hit the flap switch. My immediate thought was "Why do we need flaps now?!" But sure enough, we ballooned up a bit and had just enough additional altitude to make the runway.

I said, "Neat trick," and my instructor said something like "I learned it from Ernest Gann."

 

[GMascelli]

Glad to hear they both survived.

Watching that video sure made the hairs on my neck stand up..... I remembered instantly being in that seat, with everything happening in what seemed like a super accelerated state. Not a time to think, just react, hopefully your training kicks in. Don't stall and get it wings level went through my mind that day like a loop recording.

 

[dtuuri]

Uuh, the Coefficient of Lift changes, so the amount of lift at a given speed and AOA increases.

Yes, indeed, but unless you're bolted to the floor in a wind tunnel I don't see how you can hold AOA at the "given speed". It's gonna slow down.

 

[Rcmutz]

Take a look at this diagram. https://wright.grc.nasa.gov/airplane/ldrat.html Doesn’t matter it is for the Wright Flyer, discussion is germain. It shows how from L/D one can find the best angle, the smaller the angle, the longer the glide distance. Also from the drag and L/D curve shown here https://en.wikipedia.org/wiki/Lift-to-drag_ratio one can determine the indicated airspeed that gives that angle. With flaps deflected the max CL is affected and so is drag, so one would have to plot an L/D curve for flaps deflected. You pick the flap deflection you want to figure the glide angle and speed out for. Keep in mind, deflecting flaps will change the effective camber of the wing section, so a very small deflection may reduce the drag of that section of the wing. Also, if the minimum drag point is effected, the airspeed at which that occurs may also be different than without any flaps hanging out. Keep in mind, I am not talking about large flap deflections, but small, probably under 5 degrees. It all depends on the airfoil shape of the basic wing, and the type of flap system, hinged or Fowler, etc.

 

[Rcmutz]

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

Yes. Bottom line is each configuration will have its own best L/D, hence glide ratio, angle, and speed. Is one more than another, you would have to plot the data up for each unique configuration to determine the answer to that question.

 

[nauga]

Is one more than another, you would have to plot the data up for each unique configuration to determine the answer to that question.

That's kind of the crux of my issue - if they've given 'best glide' for a configuration that's not the best glide for the airplane at *any* flap setting then what really is best glide and why did they choose something less for the engine out procedure? If L/Dmax really is at TO flaps then is max range cruise (which occurs at L/Dmax for a prop) also at TO flaps and is this in the perf charts? These are the bits of missing information I was referring to.

Nauga,
and the difference between global and local optima

 

[Hang 4]

Or more precisely, Polars. Flaps generate lift, but they also generate drag. The question is does a small deflection generate more lift than drag. No real way to know that without plotting a polar. What would be really interesting is to look at a polar of a typical GA craft vs a HP glider. Competition gliders rarely fly at best glide. They focus on things like headwind vs tailwind, strength of the thermals (fly faster if the thermals are strong so you can cover more ground in the strongest part of the day). The other thing is that gliders can fly a lot faster than best glide without paying a large penalty in sink rate, which shows up as a very flat polar.

A GA aircraft on the other hand, would have a much steeper polar, so flying faster than best glide would have a significant penalty in glide ratio vs a slick glider. Adding flaps would have a slower L/D max speed as both lift and drag are increased. That would likely steepen the polar even more. What would be really interesting is to figure out the min sink speed for a GA aircraft vs L/D max in different flap conditions. Min sink is useful in an engine out if you have a landing option nearby as it gives you more time to figure out your issue and restart your engine, assuming that is possible. Since we don't seem to have available polars, it's difficult to understand if there is an effective difference between L/Dmax and min Sink.

 

[brcase]

you asked for pictures, here is one...
Below is the polar for my glider which shows data for +10 to -5 degrees flaps. I wish it showed data for 20 and 30 degrees of flaps. I added L/D numbers at various point.

But my take away from the data below is that between 48 and 55 kts I can use any of the flaps settings with no penalty due to flaps.
Between 48 and 64kts I can use +5 to -5 flaps with no penalty. 10 degrees flaps over 55kts starts to give me a reduction in L/D.


Brian
CFIIG/ASEL

 

[RyanShort1]

Or more precisely, Polars. Flaps generate lift, but they also generate drag. The question is does a small deflection generate more lift than drag. No real way to know that without plotting a polar. What would be really interesting is to look at a polar of a typical GA craft vs a HP glider. Competition gliders rarely fly at best glide. They focus on things like headwind vs tailwind, strength of the thermals (fly faster if the thermals are strong so you can cover more ground in the strongest part of the day). The other thing is that gliders can fly a lot faster than best glide without paying a large penalty in sink rate, which shows up as a very flat polar.

A GA aircraft on the other hand, would have a much steeper polar, so flying faster than best glide would have a significant penalty in glide ratio vs a slick glider. Adding flaps would have a slower L/D max speed as both lift and drag are increased. That would likely steepen the polar even more. What would be really interesting is to figure out the min sink speed for a GA aircraft vs L/D max in different flap conditions. Min sink is useful in an engine out if you have a landing option nearby as it gives you more time to figure out your issue and restart your engine, assuming that is possible. Since we don't seem to have available polars, it's difficult to understand if there is an effective difference between L/Dmax and min Sink.

This is a significant oversimplification, but imagine you are in a Cub, and lets just say your best glide is 50mph... and you are in a 50mph headwind.

https://static1.squarespace.com/sta...139fadb82/1524679788505/NC6128H+Checklist.pdf

Yes, most of the time you should choose best glide, but if the best landing spot was into the headwind, the only way to get there - if you have the altitude - is to go faster than best glide... The point being that best glide is USUALLY the best option, but not always. If you are approaching a field, for instance, and realize you might hit powerlines, it may be counterintuitive, but it would probably be better to go under them than over them.

 

[geezer]

Earlier we had an extensive calculation discussion of the difference in Kinetic Energy when crashing, Cessna 172, clean or 40 degrees of flaps.

Yes, drag goes up, but you have just swapped KE from 53 Knots, for altitude at 47 Knots. That is swapping 22% of your energy for altitude! Following that important gain, you do descend more steeply, but if you were just a little short of a safe touchdown, you are now over the pavement, or cleared the chain link fence, or whatever you were going to just barely hit.

22% is not much KE, but it should not be wasted in the last moments before contact with the earth.
Using it also reduces the severity of the impact with what ever you wished to avoid.

Geezer
Who has used that 22% at less than 10 feet AGL

 

[RyanShort1]

And this is why you should get a tailwheel endorsement and get good at wheel and three points... I kid, mostly.

 

[Pinecone]

Yes, indeed, but unless you're bolted to the floor in a wind tunnel I don't see how you can hold AOA at the "given speed". It's gonna slow down.

Uuh, because it means you have have the same amount of lift at a lower AOA and/or speed.

 

[dtuuri]

Uuh, because it means you have have the same amount of lift at a lower AOA and/or speed.

Right. The same lift at a slower speed due to the added drag results in a less efficient glide angle after the plane stabilizes.

 

[David Loftus]

Well done to get back to the field. Personally, I probably would have put it on I-85 South. That was my preplanned ditch at PDK when the active was 21R. HWY-141 or I285-E if taking off 3L. But then an overweight PA-32 with a clogged fuel line tried 141 in 2015 with disastrous results. The instructor probably had that knowledge.

 

[Pinecone]

Right. The same lift at a slower speed due to the added drag results in a less efficient glide angle after the plane stabilizes.

No, same lift at lower speed with the same drag, or a tiny bit more.

If the lift increases 10% and drag only 5%, you have a gain in Lift over Drag.

 

[nauga]

If the lift increases 10% and drag only 5%, you have a gain in Lift over Drag.

That's arithmetic. For aerodynamics you'd drop the "IF" and show us drag polars.

Nauga,
the polars bear

 

[Mongoose Aviator]

If my single engine loses all engine power than I am dropping the nose for POH airspeed for max glide [per memory] and trimming. Decide on a landing spot and turn in that direction while maintain airspeed for max glide per POH. If flaps are out then retract. Communicate to ATC. Check winds. Consider if there are alternative landings sites. If time allows then troubleshoot and maybe get lucky and get engine power. When over landing site and landing assured then full flaps to land with minimal airspeed without stalling.

I am not a test pilot to try and figure out a better way to do it.

 

[luvflyin]

If my single engine loses all engine power than I am dropping the nose for POH airspeed for max glide [per memory] and trimming. Decide on a landing spot and turn in that direction while maintain airspeed for max glide per POH. If flaps are out then retract. Communicate to ATC. Check winds. Consider if there are alternative landings sites. If time allows then troubleshoot and maybe get lucky and get engine power. When over landing site and landing assured then full flaps to land with minimal airspeed without stalling.

I am not a test pilot to try and figure out a better way to do it.

If it happens low like just after takeoff, you may want to think about adding flap so you can crash at the lowest possible speed.

 

[Mongoose Aviator]

If it happens low like just after takeoff, you may want to think about adding flap so you can crash at the lowest possible speed.

Not a chance. Other people can play around with their L/D stuff. I am just not that experienced of a test pilot.
Maximum glide distance.
Once landing assured then full flaps.

If other techniques work for more expert and experienced pilots that is great and more power to them.

 

[dtuuri]

No, same lift at lower speed with the same drag, or a tiny bit more.

If the lift increases 10% and drag only 5%, you have a gain in Lift over Drag.

How does "...lift increases 10%" if you have "...same lift"?

 

[luvflyin]

Not a chance. Other people can play around with their L/D stuff. I am just not that experienced of a test pilot.
Maximum glide distance.
Once landing assured then full flaps.

If other techniques work for more expert and experienced pilots that is great and more power to them.

Cool. Landing assured, get slow.

 

[Pinecone]

That's arithmetic. For aerodynamics you'd drop the "IF" and show us drag polars.

Nauga,
the polars bear

Each aircraft will be different. I will go play with mine one day and see what I get.

 

[Pinecone]

How does "...lift increases 10%" if you have "...same lift"?

One more time, if you increase the lift, you can then reduce AOA or speed for the same lift as before. The 10% increase is in available lift.

 

[dtuuri]

...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.

Sorry, missed this somehow. I downloaded the Katana POH and didn't see range information, just endurance values at different gph, IIRC.

 

[dtuuri]

One more time, if you increase the lift, you can then reduce AOA or speed for the same lift as before. The 10% increase is in available lift.

This thread has caused me to do some reflexion (see what I did there? ). Not being used to flaps that retract into the negative range, things have confused me a little. If I had experience as a glider pilot I would certainly be a better human being. However, I think things are becoming more in focus now that I've reviewed the facts. I would defer to Nauga, Dana or any other aero engineer, though, for correction. The way I see it is like this: The Katana's flaps reflex (come up to a "trail" position where they do no work). This negative position decreases both drag and lift. After the plane accelerates to equilibrium, lift is restored to the value of "weight" and drag balances out thrust. A new L/D ratio is thus established which is not the Katana's L/D(max) because the wing was originally optimized for an unreflexed minimum sink rate glider. Adapting the wing to a power-plane required reflexing so it could fly faster in cruise. In other words, flaps change airfoil characteristics and the reflexed position, while faster, has a less optimum L/D ratio than the original airfoil (T/O position) and it won't glide as far and it will reach the ground quicker. But I don't think anything I've written until now is wrong.

 

[Hang 4]

The key concept is that lift isn't "free". Adding more lift to a wing by using flaps does increase lift, but it also increases drag. In most cases (absent seeing a polar, I would argue all) the incremental drag is greater than the incremental lift.

Competition sailplane pilots want more than just L/D max, they want L/D max to be as fast as possible. That's why they use water ballast and some have negative reflex. The key to winning (or just going far XC) is to get to the next thermal as fast as possible. Ballast doesn't change the glide ratio, it just changes the airspeed.

If drooping a flap increased glide ratio, you'd see it done much more often. The Katana example really lacks some context, and reading the POH doesn't provide that context. It just lists a configuration and a speed. A speculation would be that there is little glide ratio penalty in the TO flap position, but there is an improvement in the sink rate, providing more time to figure crap out.

 

[FormerHangie]

Well done to get back to the field. Personally, I probably would have put it on I-85 South. That was my preplanned ditch at PDK when the active was 21R. HWY-141 or I285-E if taking off 3L. But then an overweight PA-32 with a clogged fuel line tried 141 in 2015 with disastrous results. The instructor probably had that knowledge.

Even if you managed to get it down on I-85 without hitting anything, you'd wind up colliding with at least one car or truck, and they're a lot stouter than our airplanes. I think the pilot made the best choice he could have, there's not much in the way of open space near PDK.

 

[Dave Theisen]

I took off on that runway in a Bonanza and it just reinforced that that was the best possible outcome.

 

[Spinka]

The Katana example really lacks some context, and reading the POH doesn't provide that context. It just lists a configuration and a speed. A speculation would be that there is little glide ratio penalty in the TO flap position, but there is an improvement in the sink rate, providing more time to figure crap out.

According to the answer I got from Diamond you nailed it above:
"The DA20-A1 has its best glide performance with flaps in UP position as you were correctly guessing. But the AFM tells differently and I will tell you why. The flaps TO setting is simply used because of one important factor --> time. It gives the pilot more time in the air for decision making. The glide ratio is marginally reduced when using T/O flaps. The emergency procedure in the AFM instructs to set T/O flaps for best angle of glide- certainly with different speeds depending on the weight.

The glide ratio is 1:14 with flaps in T/O position and a windmilling propeller."

 

[Hang 4]

One more bit of context, 14:1 is an excellent glide ratio with some flaps in. Most GA trainers (172, PA28) are in the 9:1 range clean. Add 10 degrees of flaps and it's going to degrade by a point or two.

Point being, I would not make the assumption that adding a bit of flaps to a basic GA aircraft is going to have a negligible effect on glide, like the Katana example above.