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.

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

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

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

"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

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

"Kevin Horton" wrote ...
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.

You're ignoring aeroelastic effects. Some of the plastic airplanes can bend
quite a bit.

Rich

" since KE is mv^2, .

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

You are correct sir, I didn't include the constant because I was
concentrating what the relative benefit of changing the factors of the
expresion rather than calculating the absolute energy. Maybe I should
have said the energy was directly proportional to mv^2.

And I'd like to hear some other possible light weight implementations
or examples in place today. The solution I suggested adds another
port on the fuel selector valve, and a piece of plastic tubing to the
low pressure port.

"Toks Desalu" wrote in message news:vGwyb.263612$mZ5.1937460@attbi_s54...
" since KE is mv^2, .

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

Earlier, (Jay) wrote:

...The solution I suggested adds
another port on the fuel selector valve,
and a piece of plastic tubing to the
low pressure port.

Well, even with a bit of help from a low-pressure area, you're going
to need additional pumping capacity to get the fuel overboard in a
reasonable amount of time.

According to _Firewall Forward_, FAR part 23 guidelines specify that
pumped fuel system be capable of delivering 125% of the takeoff power
fuel flow, and gravity flow systems 150%. That's open-port flow with
zero backpressure.

For example:

Say you're using a 180 h.p. or so motor that draws 18 gallons per hour
at max takeoff power (let's say). The most you can expect the
appropriately-sized fuel pump to move is about 25 gallons per hour.

But, more likely, you haven't got an hour. The vast majority of
concievable small aircraft emergencies are likely to be over in 15
minutes or less. That means you only have time to send 25/4 or about
6.25 gallons (37.5 lbs) of fuel overboard.

And that's if the pump runs wide open at the outlet. With any
substantial backpressure, the flow rates will be much lower. And you
might happen to need some of that pressure to apply fuel pressure to
the engine fuel system inlet. I suppose that you could add a rate
restrictor to the overboard port, so that fuel pressure is maintained
even while dumping. But that's going to cut the dump rate
substantially.

Of course, both of these points can be easily addressed. You can add a
separate dumping pump just to pump fuel overboard, and you can size it
to achieve the desired rates. You can also give the pump its own fuel
supply and overboard plumbing, again sized for the desired dump rate.
You can even plumb the dump system with a standpipe so you can't
inadvertantly run the tanks completely dry with it.

However, that pump weighs something, and the fuel, electrical, and
mechanical connections that service it also weigh something. And when
you add up all that weight, it is substantial, and it will have a
measurably deleterious effect on takeoff, cruise, and landing
performance. It also adds many points of potential failure, both
mechanical and human. And for those accidents caused by fuel
exhaustion (a big slice of the pie), it means that the airplane hits
the ground with more weight and energy, not less.

What you end up with is a compromise that balances constant and
measurable penalties (weight and complexity) against hypothetical
gains (safety).

Thanks, and best regards to all

Bob K.