WITH THIS ISSUE coming out during the
building season for most Navy Carrier
modelers, I’m sure there are some new
creations taking shape on your building
boards that would be of interest to this
column’s readers. I encourage you to send me
a photo or two of your latest project for
inclusion in future editions of this column.
The publication standards require a 35mm
film print (preferably on glossy paper) or a
file on CD from a camera with a minimum of
3 megapixels taken in high-resolution mode.
Try to have the model against an uncluttered
background and the background relatively
close to the model if you are photographing
with a flash.
This month I’ll continue the discussion of
fuel tanks, which I started in the November
column. A problem we face with pressure fuel
systems comes from the fact that the engine’s
crankcase pressure output varies significantly
with engine speed.
If you are using an intake throttle that
reduces air flow into the engine at lower
engine speeds as the primary means of
controlling the engine speed, the average
crankcase pressure can be even lower than
atmospheric at idle. If the tank end of the
pressure line is in contact with fuel as the
pressure changes, higher pressure in the tank
can force fuel back to the engine through the
pressure line when engine speed is reduced.
I’ve used a check valve in the pressure
line to keep tank pressure more constant, but
variations in pressure are too great to allow
optimum consistency in that mode of
operation. It is an effective means of keeping
fuel from flowing back through the pressure
line, but the other disadvantages outweighed
that single advantage.
One could also set up the tank so the
tank end of the pressure line is in a part of
the tank that is normally not going to result
in the end of the line being immersed in fuel.
That concept is good, but the realities of
turbulence, vibration, acceleration, etc. make
it hard to rely on.
In full-scale aircraft, especially larger
airplanes in which there can be significant
fuel expansion caused by temperature after
January 2006 139
The F-8 is equipped with a Johnson engine and a Vari-Speed
exhaust slide, which Bob designed and marketed at roughly the
same time as the original Flight Control three-line bellcrank. MA
Bob Smurthwaite designed and marketed this original J. Roberts
Flight Control unit. It revolutionized Carrier, and the concept is
still used today.
A “surge tank” in the pressure line can prevent fuel from flowing from the fuel tank to the engine when crankcase pressure is
reduced.
Pressure line
from engine to
this corner
Pressure line from engine
Pickup to fuel tank in this corner
Fwd
Up
tanks through the vent lines. As fuel is used or as the fuel temperature
falls in the colder air at cruise altitudes, the fuel that is captured by the
surge tank is returned to the main fuel tanks.
We can have the equivalent of the surge tank in our models’
pressure lines. Pete Mazur described this concept to me, and he has
been using it for many years to avoid fuel flow to the engine through
the pressure line. Pete calls it an air-fuel separator. It is nothing more
than a small fuel tank in the pressure line.
The shape and size of the tank are noncritical, and those aspects of
the design can be adapted to suit any particular model. There are only
two lines in the surge tank: one on the tank side and one on the engine
side.
There are basic requirements for this concept, one of which is that
the surge tank be large enough to accommodate any fuel that is forced
back through the pressure line as engine output pressure varies. If the
pressure line in the main fuel tank is in the upper inboard corner of the
tank, the surge tank can be quite small and still be effective.
The more important design factors are the location of the input and
output lines in the surge tank. The surge tank needs to be designed and
located so that the line on the fuel-tank side will collect any fuel in the
surge tank and return it to the main fuel tank as soon as the pressure in
the surge tank is greater than the pressure in the main tank. The line on
the engine side must be as dry as possible under all flight conditions.
The location and orientation of the surge tank must be such that
these conditions are maintained when the model is sitting on the deck,
accelerating for takeoff, decelerating after high speed (this is a critical
time), and during low-speed flight with the model in a nose-high
attitude (also critical). The photograph should help illustrate the
concept.
Included this month are some photos from Mike Potter. They include
two obscure designs from Bob (J. Robert) Smurthwaite: an AM-1
Mauler and a prototype Vought F-8 Crusader.
01sig5.QXD 11/22/05 12:21 PM Page 139
Edition: Model Aviation - 2006/01
Page Numbers: 138,139
Edition: Model Aviation - 2006/01
Page Numbers: 138,139
WITH THIS ISSUE coming out during the
building season for most Navy Carrier
modelers, I’m sure there are some new
creations taking shape on your building
boards that would be of interest to this
column’s readers. I encourage you to send me
a photo or two of your latest project for
inclusion in future editions of this column.
The publication standards require a 35mm
film print (preferably on glossy paper) or a
file on CD from a camera with a minimum of
3 megapixels taken in high-resolution mode.
Try to have the model against an uncluttered
background and the background relatively
close to the model if you are photographing
with a flash.
This month I’ll continue the discussion of
fuel tanks, which I started in the November
column. A problem we face with pressure fuel
systems comes from the fact that the engine’s
crankcase pressure output varies significantly
with engine speed.
If you are using an intake throttle that
reduces air flow into the engine at lower
engine speeds as the primary means of
controlling the engine speed, the average
crankcase pressure can be even lower than
atmospheric at idle. If the tank end of the
pressure line is in contact with fuel as the
pressure changes, higher pressure in the tank
can force fuel back to the engine through the
pressure line when engine speed is reduced.
I’ve used a check valve in the pressure
line to keep tank pressure more constant, but
variations in pressure are too great to allow
optimum consistency in that mode of
operation. It is an effective means of keeping
fuel from flowing back through the pressure
line, but the other disadvantages outweighed
that single advantage.
One could also set up the tank so the
tank end of the pressure line is in a part of
the tank that is normally not going to result
in the end of the line being immersed in fuel.
That concept is good, but the realities of
turbulence, vibration, acceleration, etc. make
it hard to rely on.
In full-scale aircraft, especially larger
airplanes in which there can be significant
fuel expansion caused by temperature after
January 2006 139
The F-8 is equipped with a Johnson engine and a Vari-Speed
exhaust slide, which Bob designed and marketed at roughly the
same time as the original Flight Control three-line bellcrank. MA
Bob Smurthwaite designed and marketed this original J. Roberts
Flight Control unit. It revolutionized Carrier, and the concept is
still used today.
A “surge tank” in the pressure line can prevent fuel from flowing from the fuel tank to the engine when crankcase pressure is
reduced.
Pressure line
from engine to
this corner
Pressure line from engine
Pickup to fuel tank in this corner
Fwd
Up
tanks through the vent lines. As fuel is used or as the fuel temperature
falls in the colder air at cruise altitudes, the fuel that is captured by the
surge tank is returned to the main fuel tanks.
We can have the equivalent of the surge tank in our models’
pressure lines. Pete Mazur described this concept to me, and he has
been using it for many years to avoid fuel flow to the engine through
the pressure line. Pete calls it an air-fuel separator. It is nothing more
than a small fuel tank in the pressure line.
The shape and size of the tank are noncritical, and those aspects of
the design can be adapted to suit any particular model. There are only
two lines in the surge tank: one on the tank side and one on the engine
side.
There are basic requirements for this concept, one of which is that
the surge tank be large enough to accommodate any fuel that is forced
back through the pressure line as engine output pressure varies. If the
pressure line in the main fuel tank is in the upper inboard corner of the
tank, the surge tank can be quite small and still be effective.
The more important design factors are the location of the input and
output lines in the surge tank. The surge tank needs to be designed and
located so that the line on the fuel-tank side will collect any fuel in the
surge tank and return it to the main fuel tank as soon as the pressure in
the surge tank is greater than the pressure in the main tank. The line on
the engine side must be as dry as possible under all flight conditions.
The location and orientation of the surge tank must be such that
these conditions are maintained when the model is sitting on the deck,
accelerating for takeoff, decelerating after high speed (this is a critical
time), and during low-speed flight with the model in a nose-high
attitude (also critical). The photograph should help illustrate the
concept.
Included this month are some photos from Mike Potter. They include
two obscure designs from Bob (J. Robert) Smurthwaite: an AM-1
Mauler and a prototype Vought F-8 Crusader.
01sig5.QXD 11/22/05 12:21 PM Page 139
