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Fuel dump switch in homebuilt



 
 
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  #1  
Old November 30th 03, 11:49 PM
Jay
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Default Fuel dump switch in homebuilt

After reading some of the transcripts from the "Last Words" website,
where the flight engineers were dumping fuel when it became evident
that an emergecy landing was a certainty, it dawned on me that there
might be some benefit for a small plane as well. The less energy you
carry into a crash landing the better off you're going to be. And
since KE is mv^2, you get a proportional benfit from dumping the
weight of the fuel which might be 20% the weight of the airplane, and
the lower weight allows for a slower stall speed which cuts the V
factor, and since thats squared, it counts for a lot. And then of
course you may have a larger glide radius with that reduction in
weight in addition to the reduced fire potential upon landing and
breakup

Maybe a fuel selector switch that ports to a low pressure area near
the tail would act as a light weight solution to draw out the fuel
from the tanks. A safety wire that would have to be broken would be a
good idea so it isn't accentally selected. Like the WEP setting on
the WW2 fighters with water injection.
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  #2  
Old December 1st 03, 12:26 AM
Kevin Horton
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On Sun, 30 Nov 2003 14:49:37 -0800, Jay wrote:

After reading some of the transcripts from the "Last Words" website, where
the flight engineers were dumping fuel when it became evident that an
emergency landing was a certainty, it dawned on me that there might be some
benefit for a small plane as well. The less energy you carry into a crash
landing the better off you're going to be. And since KE is mv^2, you get
a proportional benfit from dumping the weight of the fuel which might be
20% the weight of the airplane, and the lower weight allows for a slower
stall speed which cuts the V factor, and since thats squared, it counts
for a lot. And then of course you may have a larger glide radius with
that reduction in weight in addition to the reduced fire potential upon
landing and breakup

Maybe a fuel selector switch that ports to a low pressure area near the
tail would act as a light weight solution to draw out the fuel from the
tanks. A safety wire that would have to be broken would be a good idea so
it isn't accentally selected. Like the WEP setting on the WW2 fighters
with water injection.


Interesting idea, but I think there are good reasons why people haven't
done this on light aircraft:

1. If you have a major problem and need to do an off-airport landing (e.g.
engine failure, fire, etc), you usually don't have much time to play with,
so you probably wouldn't be able to get rid of enough fuel to make much of
a difference. Anything you do in the design to increase the dump rate
only makes the consequences of an uncommanded fuel dump even worse (see
item 3).

2. If you only have a minor problem (i.e. you have to land, but you are
not in any big rush), then you should have time to get to a suitable
airport where you can safely land at the current gross weight.

3. If you design in the ability to dump fuel, you have added a failure
mode where fuel gets dumped when you don't want it to. This could cause
an off-airport landing. Sure, this failure mode wouldn't happen too
often, but it only has to happen once to really ruin your day.

All things considered, this system would probably decrease the overall
level of safety, not increase it.

Fuel dump systems are on some large jets because of the large difference
between max approved take-off weight and max approved landing weight.
There are also some failures that make it attractive to be able to greatly
reduce the gross weight. E.g. failures of flaps and slats mean up to a 60
kt increase in approach speed on some aircraft. They may also have
partial brake failures to contend with, etc.

Note 1: even though big jets may have a big difference between max
approved take-off weight and max approved landing weight, that does not
mean that it is unsafe to land right after take-off and max approved
take-off weight. You just need to do a smooth landing, and you need a
long runway. Landings at max approved take-off weight are done routinely
during the take-off performance flight testing, as this type of testing
consists of multiple take-offs at max weight, followed a few minutes later
by a landing so the test can be repeated yet again.

Note 2: the glide ratio does not vary with gross weight. I won't try to
explain the math or physics as there are lots of references available on
the web for your googling pleasure.

--
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com

  #3  
Old December 1st 03, 01:51 AM
Holger Stephan
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Kevin Horton wrote:
Note 2: the glide ratio does not vary with gross weight. I won't try to
explain the math or physics as there are lots of references available on
the web for your googling pleasure.


True only in theory (i.e. on high performance gliders). The added drag with
the increased max glide speed will decrease the ratio.

- Holger

  #4  
Old December 1st 03, 02:19 AM
Toks Desalu
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" since KE is mv^2, .

Small correction: KE is (1/2 mv^2)


  #6  
Old December 1st 03, 02:41 AM
Peter Dohm
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Holger Stephan wrote:

Kevin Horton wrote:
Note 2: the glide ratio does not vary with gross weight. I won't try to
explain the math or physics as there are lots of references available on
the web for your googling pleasure.


True only in theory (i.e. on high performance gliders). The added drag with
the increased max glide speed will decrease the ratio.

- Holger


That could go either way, depending on the porportion of parasitic drag and
induced drag. However, in most cases you are exactly correct that lighter
weight will improve the glide ratio, and that heavier weight will worsen it;
assuming no wind conditions, of course.

Peter
  #7  
Old December 1st 03, 03:02 AM
Vaughn
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"Holger Stephan" wrote in message
...
Kevin Horton wrote:
Note 2: the glide ratio does not vary with gross weight. I won't try to
explain the math or physics as there are lots of references available on
the web for your googling pleasure.


True only in theory (i.e. on high performance gliders). The added drag

with
the increased max glide speed will decrease the ratio.


From what I have been taught (and teach my students) Kevin is quite
right. Just because glide ratio (best L/D) does not change, that does not
mean that the aircraft performance is unchanged by gross weight. At a
higher weight, best L/D comes at a higher airspeed. If you happened to be
downwind of your intended landing area, you would be better off (very
margionally) by hanging on to the extra weight. As already mentioned, stall
speed decreases with weight and using that same formula (1/2 MV^2) reducing
the velocity on an off-field landing drastically reduces the energy that
must be disipated after landing.

On balance, I believe that fuel dumps in small aircraft are a bad idea.

Vaughn



- Holger



  #8  
Old December 1st 03, 03:55 AM
Del Rawlins
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On 30 Nov 2003 01:49 PM, Jay posted the following:
After reading some of the transcripts from the "Last Words" website,
where the flight engineers were dumping fuel when it became evident
that an emergecy landing was a certainty, it dawned on me that there
might be some benefit for a small plane as well. The less energy you
carry into a crash landing the better off you're going to be.


If you are making an emergency landing in a small plane, chances are
that you don't have any fuel left on board to dump.

----------------------------------------------------
Del Rawlins-
Remove _kills_spammers_ to reply via email.
Unofficial Bearhawk FAQ website:
http://www.rawlinsbrothers.org/bhfaq/
  #9  
Old December 1st 03, 04:18 AM
Kevin Horton
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On Sun, 30 Nov 2003 16:51:58 -0800, Holger Stephan wrote:

Kevin Horton wrote:
Note 2: the glide ratio does not vary with gross weight. I won't try to
explain the math or physics as there are lots of references available on
the web for your googling pleasure.


True only in theory (i.e. on high performance gliders). The added drag
with the increased max glide speed will decrease the ratio.

- Holger


The higher speed also gives more lift, and the lift to drag ratio (and
hence the glide ratio) remains the same. This assumes that both
conditions are at the same angle of attack, and the the changes in
Reynolds number and Mach number don't change the airfoils CL and/or CD.
This should not be a problem with typical light aircraft at their best
glide speeds at typical light aircraft altitudes.

But, if we have a windmilling prop, the windmilling rpm will go up
with speed, and the windmilling drag could increase quite a bit, and that
could affect the results. So, add an assumption of a stopped prop.

--
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com

  #10  
Old December 1st 03, 05:07 AM
Eric Miller
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"Drew Dalgleish" wrote
Probably a lot closer to 10% since most don't take off with full fuel
or have catastrophic failures at that loading. You can keep adding
systems until your plane will never fly.


At which point it is finally safe! =D

Eric


 




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