Lancair crash at SnF

In rec.aviation.student WingFlaps <Moreflaps@> wrote:
> On Apr 27, 6:04?am, Dan_Thomas_nos...m wrote:
>> ? ? ? ? Lots of people had the impression you were talking about the
>> dreaded downwind turn, with all the talk about the energy required to
>> accelerate to maintain airspeed. The energy required, as pointed out
>> in a much earlier post with several very good references, is so tiny
>> that it's not worth fooling with at all.
>
> Perhaps you could put a number on that? Could you try a gliding turn
> with stopwatch and altimeter and compare that to a straight glide?

In the optimal 45-degree-banked turn the load factor will be about 1.4.
Your best glide speed and min sink speed will increase by the square root
of that, or 20%. The glide angle remains the same if you increase your
airspeed appropriately, so your sink rate will also increase by 20%. So
instead of 650fpm you'll be coming down at 780fpm. At 78kts (65kts best
glide speed from previous post plus 20%) and a 45 degree bank you're
making a circle a bit over 500ft across which will take you 13 seconds to
complete half of. The extra sink rate from the turn will therefore cost
you 30 feet over what you would have experienced in a straight glide for
the same amount of time.

You'll also lose about 80 feet to accelerate from 65kts to 78kts. But
you'll gain this back at the end, so as long as the end of your turn ends
at a reasonable height it can be ignored.

The numbers will, of course, vary between aircraft but it would appear
that the extra energy loss due to the turn itself isn't all that
significant. If 30 feet is the difference between making it and not making
it you probably should not be turning around in the first place.

Did I miss anything?

--
Michael Ash
Rogue Amoeba Software

Michael Ash <mike@mikeash.com> wrote in
news:1209240958.122432@web1.segnet.com:

> In rec.aviation.student WingFlaps <Moreflaps@> wrote:
>> On Apr 27, 6:04?am, Dan_Thomas_nos...m wrote:
>>> ? ? ? ? Lots of people had the impression you were talking about the
>>> dreaded downwind turn, with all the talk about the energy required
>>> to accelerate to maintain airspeed. The energy required, as pointed
>>> out in a much earlier post with several very good references, is so
>>> tiny that it's not worth fooling with at all.
>>
>> Perhaps you could put a number on that? Could you try a gliding turn
>> with stopwatch and altimeter and compare that to a straight glide?
>
> In the optimal 45-degree-banked turn the load factor will be about
> 1.4. Your best glide speed and min sink speed will increase by the
> square root of that, or 20%. The glide angle remains the same if you
> increase your airspeed appropriately, so your sink rate will also
> increase by 20%. So instead of 650fpm you'll be coming down at 780fpm.
> At 78kts (65kts best glide speed from previous post plus 20%) and a 45
> degree bank you're making a circle a bit over 500ft across which will
> take you 13 seconds to complete half of. The extra sink rate from the
> turn will therefore cost you 30 feet over what you would have
> experienced in a straight glide for the same amount of time.
>
> You'll also lose about 80 feet to accelerate from 65kts to 78kts. But
> you'll gain this back at the end, so as long as the end of your turn
> ends at a reasonable height it can be ignored.
>
> The numbers will, of course, vary between aircraft but it would appear
> that the extra energy loss due to the turn itself isn't all that
> significant. If 30 feet is the difference between making it and not
> making it you probably should not be turning around in the first
> place.
>
> Did I miss anything?
>



45 degrees isn't enough unless you have a very nice gliding airplane or
are starting from a good height to begin with.


Bertie

On Apr 27, 8:22 am, Michael Ash <m...@mikeash.com> wrote:
> In rec.aviation.student WingFlaps <Morefl...@> wrote:
>
> > On Apr 27, 6:04?am, Dan_Thomas_nos...m wrote:
> >> ? ? ? ? Lots of people had the impression you were talking about the
> >> dreaded downwind turn, with all the talk about the energy required to
> >> accelerate to maintain airspeed. The energy required, as pointed out
> >> in a much earlier post with several very good references, is so tiny
> >> that it's not worth fooling with at all.
>
> > Perhaps you could put a number on that? Could you try a gliding turn
> > with stopwatch and altimeter and compare that to a straight glide?
>
> In the optimal 45-degree-banked turn the load factor will be about 1.4.
> Your best glide speed and min sink speed will increase by the square root
> of that, or 20%. The glide angle remains the same if you increase your
> airspeed appropriately, so your sink rate will also increase by 20%. So
> instead of 650fpm you'll be coming down at 780fpm. At 78kts (65kts best
> glide speed from previous post plus 20%) and a 45 degree bank you're
> making a circle a bit over 500ft across which will take you 13 seconds to
> complete half of. The extra sink rate from the turn will therefore cost
> you 30 feet over what you would have experienced in a straight glide for
> the same amount of time.
>
> You'll also lose about 80 feet to accelerate from 65kts to 78kts. But
> you'll gain this back at the end, so as long as the end of your turn ends
> at a reasonable height it can be ignored.
>
> The numbers will, of course, vary between aircraft but it would appear
> that the extra energy loss due to the turn itself isn't all that
> significant. If 30 feet is the difference between making it and not making
> it you probably should not be turning around in the first place.
>

I make the turn diameter bigger than that using the formula
rad=(knots^2)/(11.26 x tan(bank)) (assuming it's right) or about
1080'? So, what would you consider the minimum height taking decision
time into account and a 225 degree turn followed by a 45 to line up
back on the runway?

Cheers

On Apr 27, 8:18 am, Bertie the Bunyip <A...@AA.AA> wrote:

>
> I'm still not sure what you mean when you say the aircraft has to
> accelerate. Are you saying that turning downwind will cost more in
> acceleration than any other type of turn?
>
No, that's not what I'm saying.

Cheers

WingFlaps <Moreflaps@> wrote in news:167e4f31-1a4a-4bfa-98d0-
f45d163e58fa@z24g2000prf..com:

> On Apr 27, 8:18 am, Bertie the Bunyip <A...@AA.AA> wrote:
>
>>
>> I'm still not sure what you mean when you say the aircraft has to
>> accelerate. Are you saying that turning downwind will cost more in
>> acceleration than any other type of turn?
>>
> No, that's not what I'm saying.
>

OK

Bertie

In rec.aviation.student Bertie the Bunyip <AA@aa.aa> wrote:
> 45 degrees isn't enough unless you have a very nice gliding airplane or
> are starting from a good height to begin with.

A 45 degree bank will give you the minimum turn radius and thus minimum
altitude lost for any heading change done with a steady state turn. Are
you suggesting that there are more... interesting techniques which can be
used to turn around, or just that sometimes you are doomed?

--
Michael Ash
Rogue Amoeba Software

On Apr 27, 11:23 am, Michael Ash <m...@mikeash.com> wrote:

>
> My personal decision height for the analogous glider launch emergency is
> *usually* a shade under 200 feet. Conditions, performance, obstacles, and
> landing opportunities ahead can all modify this value.

What's your glide ratio? If it were 40:1 would that equate to 800' in
a 10:1 Cessna?

Cheers

In rec.aviation.student WingFlaps <Moreflaps@> wrote:
> On Apr 27, 11:23?am, Michael Ash <m...@mikeash.com> wrote:
>
>> My personal decision height for the analogous glider launch emergency is
>> *usually* a shade under 200 feet. Conditions, performance, obstacles, and
>> landing opportunities ahead can all modify this value.
>
> What's your glide ratio? If it were 40:1 would that equate to 800' in
> a 10:1 Cessna?

The things I fly regularly range from 20:1 on the low end to 37:1 on the
high end. The 200ft number remains the standard for all of them. Glide
ratio can adjust it but it's more a matter of having a comfortable safety
margin. I once talked to an instructor who did a 180 in a 34:1 machine at
only 100ft, and he said it was a piece of cake. With that kind of
performance the turn costs no altitude at all so the decision height is
basically all safety margin. Flying something with reasonable performance
out of an airport where the landing sites ahead look deadly then I
probably would reduce the decision height to something like 100ft.

A direct multiplication isn't going to work here for several reasons.

First because of the safety margin. You don't need four times as much of
that.

Second because of speed differences. The power plane needs to go
significantly faster to maintain best glide or min sink, resulting in a
larger turn, more altitude lost in that turn, and more corrective action
to regain the runway centerline after it. The power plane starts out
either around best glide or slower than it, whereas the glider generally
has extra speed which means extra energy. (This will of course depend on
the performance speeds of the plane in question, but most power planes
have faster performance speeds.)

Third, reaction time. When you hear a bang and your rope goes away it
makes the situation obvious instantly. Engine trouble can take more time
to diagnose. (One of the more difficult scenarios for a glider is when
this happens to the tow plane. You could end up low and slow before
realizing what's going on and 200ft may very well not be enough in that
case.)

Overall I don't think it makes sense to extrapolate from the glider case.
From what the power guys are saying, it sounds like you want at least
several hundred feet, with the exact number depending heavily on the skill
of the pilot and the performance of the airplane.

--
Michael Ash
Rogue Amoeba Software

> I can see you missed the point entirely. By the way, Vy is never at
> best glide (it is above that ~69knots in a 172) -perhaps you would
> like to revise  what determines Vy? My point was to ilustrate the
> impossible turn with some concrete numbers instead of the handwaving

If I read my aerodyamics book correctly my summary is Vy = the Best L/
D Speed Corrected for the affect of the engine running. The Correction
is usually small. So Vy is often very close to the Best Glide speed.

Brian
CFIIG/ASEL