Electric Duct Fan (EDF) Self-Launch Glider?

At 21:45 17 January 2011, Martin Gregorie wrote:
On Mon, 17 Jan 2011 11:59:05 -0800, CLewis95 wrote:

No numbers, but:
- multiple impeller blades destroy efficiency due to interference
between the blades. Its similar to the inter-plane drag than makes
biplane less efficient than monoplanes. As a result, the fewer blades
the better, hence the superiority of the two blade propeller provided
speeds are low enough to avoid tip compressibility problems.
- a bigger diameter impeller is better because moving a given mass of
air slowly is more efficient for generating thrust than moving it much
faster as is required by the smaller impeller.

Against that, about a ducted fan can offer is reduced tip losses.

That has to make an Antares-style pop-up system that turns a large, two
blade prop a better bet than a ducted fan system.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |


The duct does do a bit more than reduce tip losses - there's an
additional thrust component from the duct lip, which in the long run comes
from an increase in effective capture area. The airship people like them
because they are easier to vector for take-off ... plus there's the
reduced noise (acoustic shielding) and increased safety (blade
containment).

The big question for a self-launcher is how you retract a ducted fan - if
it's producing the same thrust as a prop, it's going to have a
similar(ish) frontal area, or else be really inefficient.

Doug

On Jan 18, 4:13*am, Doug Greenwell wrote:
At 21:45 17 January 2011, Martin Gregorie wrote:



On Mon, 17 Jan 2011 11:59:05 -0800, CLewis95 wrote:

No numbers, but:
- multiple impeller blades destroy efficiency due to interference
*between the blades. Its similar to the inter-plane drag than makes
*biplane less efficient than monoplanes. As a result, the fewer blades
*the better, hence the superiority of the two blade propeller provided
*speeds are low enough to avoid tip compressibility problems.
- a bigger diameter impeller is better because moving a given mass of
*air slowly is more efficient for generating thrust than moving it much
*faster as is required by the smaller impeller.

Against that, about a ducted fan can offer is reduced tip losses.

That has to make an Antares-style pop-up system that turns a large, two
blade prop a better bet than a ducted fan system.

--
martin@ * | Martin Gregorie
gregorie. | Essex, UK
org * * * |

The duct does do a bit more than reduce tip losses - there's an
additional thrust component from the duct lip, which in the long run comes
from an increase in effective capture area. *The airship people like them
because they are easier to vector for take-off *... plus there's the
reduced noise (acoustic shielding) and increased safety (blade
containment).

The big question for a self-launcher is how you retract a ducted fan - if
it's producing the same thrust as a prop, it's going to have a
similar(ish) frontal area, or else be really inefficient.

Doug

One technique to launch underpowered self-launchers is to auto-tow the
ship until it is airborne and then climb under power. The
acceleration and ground roll can be a significant problem at high
altitudes or on soft fields and the auto-tow is cheap and simple.

Mike

At 11:56 18 January 2011, Mike the Strike wrote:
On Jan 18, 4:13=A0am, Doug Greenwell wrote:
At 21:45 17 January 2011, Martin Gregorie wrote:



On Mon, 17 Jan 2011 11:59:05 -0800, CLewis95 wrote:

No numbers, but:
- multiple impeller blades destroy efficiency due to interference
=A0between the blades. Its similar to the inter-plane drag than
makes
=A0biplane less efficient than monoplanes. As a result, the fewer
blade=
s
=A0the better, hence the superiority of the two blade propeller
provide=
d
=A0speeds are low enough to avoid tip compressibility problems.
- a bigger diameter impeller is better because moving a given mass of
=A0air slowly is more efficient for generating thrust than moving it
mu=
ch
=A0faster as is required by the smaller impeller.

Against that, about a ducted fan can offer is reduced tip losses.

That has to make an Antares-style pop-up system that turns a large,
two
blade prop a better bet than a ducted fan system.

--
martin@ =A0 | Martin Gregorie
gregorie. | Essex, UK
org =A0 =A0 =A0 |

The duct does do a bit more than reduce tip losses - there's an
additional thrust component from the duct lip, which in the long run
come=
s
from an increase in effective capture area. =A0The airship people like
th=
em
because they are easier to vector for take-off =A0... plus there's
the
reduced noise (acoustic shielding) and increased safety (blade
containment).

The big question for a self-launcher is how you retract a ducted fan -
if
it's producing the same thrust as a prop, it's going to have a
similar(ish) frontal area, or else be really inefficient.

Doug

One technique to launch underpowered self-launchers is to auto-tow the
ship until it is airborne and then climb under power. The
acceleration and ground roll can be a significant problem at high
altitudes or on soft fields and the auto-tow is cheap and simple.

Mike


Yes, but you would still need to be able to stow the fan in cruise?

I like the idea of some model airplane fans on a stick waved out of the DV
window :-) ... unfortunately, looking at advertised thrusts for these
units, I don't think they'd be up to it even as a sustainer.

There's an old rule of thumb that says that you need take off thrust
of about 1/4 your gross take off weight for satisfactory performance.
This holds true for a remarkably diverse range of aircraft, from J-3s
to jets. You can struggle off on less under favorable conditions, but
not a great deal less.

There's another rule of thumb that says that you get roughly 4.5 lbf
_static_ thrust for every hp in a typical propeller driven light
plane.

At 60 kts, a reasonable efficiency estimate for a light plane
propeller is 75%, yielding right around 4 lbf thrust per actual
developed hp at takeoff. That's about 800 lbf thrust for an L-19 or
any other 200 hp tow plane on a warm but not hot day. Plug in the
weight of your tow plane (fueled, with pilot) and various gliders it
could be towing and now you have some good semi-quantitative insight
into the relationship between thrust, weight and take off
performance.

-Evan Ludeman / T8

There's an old rule of thumb that says that you need take off thrust
of about 1/4 your gross take off weight for satisfactory performance.
This holds true for a remarkably diverse range of aircraft, from J-3s
to jets. You can struggle off on less under favorable conditions, but
not a great deal less.

There's another rule of thumb that says that you get roughly 4.5 lbf
_static_ thrust for every hp in a typical propeller driven light
plane.

At 60 kts, a reasonable efficiency estimate for a light plane
propeller is 75%, yielding right around 4 lbf thrust per actual
developed hp at takeoff. That's about 800 lbf thrust for an L-19 or
any other 200 hp tow plane on a warm but not hot day. Plug in the
weight of your tow plane (fueled, with pilot) and various gliders it
could be towing and now you have some good semi-quantitative insight
into the relationship between thrust, weight and take off
performance.

-Evan Ludeman / T8

(some of you may see double post -- sorry about that: posted on wrong
account)

On Jan 18, 10:40 am, T8 wrote:
There's an old rule of thumb that says that you need take off thrust
of about 1/4 your gross take off weight for satisfactory performance.
This holds true for a remarkably diverse range of aircraft, from J-3s
to jets. You can struggle off on less under favorable conditions, but
not a great deal less.

There's another rule of thumb that says that you get roughly 4.5 lbf
_static_ thrust for every hp in a typical propeller driven light
plane.

At 60 kts, a reasonable efficiency estimate for a light plane
propeller is 75%, yielding right around 4 lbf thrust per actual
developed hp at takeoff. That's about 800 lbf thrust for an L-19 or
any other 200 hp tow plane on a warm but not hot day. Plug in the
weight of your tow plane (fueled, with pilot) and various gliders it
could be towing and now you have some good semi-quantitative insight
into the relationship between thrust, weight and take off
performance.

-Evan Ludeman / T8

(some of you may see double post -- sorry about that: posted on wrong
account)

Evan .. some have missed a few of the parameters I stated up front.
The "model" EDF unit I am refering to advertises 38lbs Static
Thrust ... I can cluster and raise/retract this "cluster" into the
large bay of the Genesis 2 area that was designed to house a BRS
system. (though I would experiment with fixed mount first if I ever
actually tried this)

So 3 x 38 = 108 lbs Thrust .. but I stated and proposed 60lbs Thrust
(56%) because..
1 - I did not want to push the envelope of the EDFs
2 - I felt the mfg specs were probably optimistic and under ideal
conditions
3 - I felt there must be SOME loss of efficiency having the intake
ducts clusters so close together (ie touching).
(I am still hoping to hear comments on this subject ... I cannot find
ANYTHING on the web)

I had already considered Mike's technique of short auto-tow to get
airborne and would consider that as an acceptable requirement.
Working backwards from your numbers, could I assume 60lbs Static
Thrust translates to about 15HP in flight? That is the kind of
estimate I am looking for.

thx Evan and All

Curt -95

On Jan 18, 9:40*am, T8 wrote:
There's an old rule of thumb that says that you need take off thrust
of about 1/4 your gross take off weight for satisfactory performance.
This holds true for a remarkably diverse range of aircraft, from J-3s
to jets. *You can struggle off on less under favorable conditions, but
not a great deal less.

There's another rule of thumb that says that you get roughly 4.5 lbf
_static_ thrust for every hp in a typical propeller driven light
plane.

At 60 kts, a reasonable efficiency estimate for a light plane
propeller is 75%, yielding right around 4 lbf thrust per actual
developed hp at takeoff. *That's about 800 lbf thrust for an L-19 or
any other 200 hp tow plane on a warm but not hot day. *Plug in the
weight of your tow plane (fueled, with pilot) and various gliders it
could be towing and now you have some good semi-quantitative insight
into the relationship between thrust, weight and take off
performance.

-Evan Ludeman / T8

(some of you may see double post -- sorry about that: posted on wrong
account)

There are a number of propeller calculators on the web. Entering the
prop diameter, pitch, airfoil etc. and the engine power and RPM
suggests a 235 Pawnee generates 400 Lbs of thrust at towing speeds.

On Jan 18, 1:08*pm, bildan wrote:
On Jan 18, 9:40*am, T8 wrote:



There's an old rule of thumb that says that you need take off thrust
of about 1/4 your gross take off weight for satisfactory performance.
This holds true for a remarkably diverse range of aircraft, from J-3s
to jets. *You can struggle off on less under favorable conditions, but
not a great deal less.

There's another rule of thumb that says that you get roughly 4.5 lbf
_static_ thrust for every hp in a typical propeller driven light
plane.

At 60 kts, a reasonable efficiency estimate for a light plane
propeller is 75%, yielding right around 4 lbf thrust per actual
developed hp at takeoff. *That's about 800 lbf thrust for an L-19 or
any other 200 hp tow plane on a warm but not hot day. *Plug in the
weight of your tow plane (fueled, with pilot) and various gliders it
could be towing and now you have some good semi-quantitative insight
into the relationship between thrust, weight and take off
performance.

-Evan Ludeman / T8

(some of you may see double post -- sorry about that: posted on wrong
account)

There are a number of propeller calculators on the web. Entering the
prop diameter, pitch, airfoil etc. and the engine power and RPM
suggests a 235 Pawnee generates 400 Lbs of thrust at towing speeds.

Well, that's just not correct.

Some useful relations:

1 hp = 550 ft*lbf/sec

60 kts = 101 ft/sec

Apparent power = thrust * speed = brake hp * efficiency

Real world efficiency numbers are below 80%, typically 65 - 75% in
climb. Most light planes hit their best propeller efficiency in climb
or cruise/climb conditions.

-Evan Ludeman / T8

On Jan 18, 1:08*pm, bildan wrote:

suggests a 235 Pawnee generates 400 Lbs of thrust at towing speeds.

That pawnee, assuming gross wt of 1900# and an optimistic best L/D of
10 (in zero thrust condition) needs 190 lbf thrust just to maintain
level flight at best L/D. I think it's obvious that it produces much
more than twice this amount of thrust under full power....

-T8

Dont forget the speed controller for this thing would be a monster
with all the timing issues to go with it just like the RC model kind.
And one of the issues with EDF is the friggen heat from these
brushless motors spinning at such high rpms.
To save the bearings you have to design in some sort of liquid cooling
or heat sink. Then there is the inlet design.
...meaning development of the EDF for this may not be that easy.