Electric Duct Fan (EDF) Self-Launch Glider?

Electric Duct Fan (EDF) Self-Launch Glider?

I have pondered this for some time now. Electric Duct Fan (EDF)
propulsion systems have been making strides in the RC model jet world
and are challenging the use of mini turbine jet engines (now used on a
number of full scale SL sailplanes) Battery technology regarding
power density and safety continues to improve at a rapid pace. (auto
engineer recently stated that within 5 years batteries will approach
the power density of gasoline ... hard to believe but ?? ) EDF
systems do not have the tremendously high exhaust temperature (~700°C)
and decibel level issues. (Though not as loud, the EDF systems sound
very similar)

Assuming:
- L/D 40:1 850 pound Sailplane (in my case Genesis 2)
- Cluster of (3) currently available EDF Units producing combined ~60
pounds STATIC THRUST (AFTER taking into account loss of efficiency do
to close clustering of intake ducts)
- Battery capacity for ~10 minutes full power .. no reserve
- 2,500ft Paved Runway .. No Tailwind
- Sailplane pre-positioned on runway (not taxied to runway)
- Goal altitude of ~1,500ft AGL

As a "sustainer" I am fairly confident this would yield some
success .. if only buying you ~10 miles What I would like to hear
from the hobby-physicists out there are comments on these questions:

1 - How detrimental is the loss of efficiency/performance when
clustering duct fan intakes in very close proximity?
2 - With sailplane starting from rest, how long would it take to
accelerate to flying speed? i.e. Would I need 3 miles of paved runway?
and/or .. Would the batteries be dead before the glider left the
ground?

I certainly am not proposing a "replacement" for jet turbines .. only
curious if the above scenario is at all feasible.

Thanks for comments!

Curt Lewis - 95
Genesis 2
Loves Park, IL USA

Assuming:
- L/D 40:1 850 pound Sailplane (in my case Genesis 2)
- Cluster of (3) currently available EDF Units producing combined ~60
pounds STATIC THRUST (AFTER taking into account loss of efficiency do
to close clustering of intake ducts)
- Battery capacity for ~10 minutes full power .. no reserve
- 2,500ft Paved Runway .. No Tailwind
- Sailplane pre-positioned on runway (not taxied to runway)
- Goal altitude of ~1,500ft AGL

I should have added: "Starting from 1,000ft MSL"
Curt -95

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 |

thx for comments so far ... another clarification to my "proposition"
here though:

I do understand why large, slow turning prop is more efficient. My
question is "How feasible would it be to use currently avaiable, off-
the-shelf EDF systems to achieve stated, limited goals?" .. the
trade off being much lower cost, simpler design, triple redundancy,
etc.

This EDF approach may never compete commercially or performance wise
with Jet Turbine or current conventional gas or electric prop
systems ... I'm just curious if adapting small EDF's could achieve the
very limited goals in my proposed scenario.

- 60lbs Thrust
- 10 minute duration (no reserve)
- Climb from 1,000' MSL to 1,500' AGL
- Using Paved Runway
- No taxi .. prepositioned on runway
- No tailwind component

While certainly not a feasible commercial solution .. it would be a
really neat experiment

Curt -95

On Jan 17, 3:20*pm, CLewis95 wrote:
thx for comments so far ... another clarification to my "proposition"
here though:

I do understand why large, slow turning prop is more efficient. *My
question is "How feasible would it be to use currently avaiable, off-
the-shelf EDF systems to achieve stated, limited goals?" *.. *the
trade off being much lower cost, simpler design, triple redundancy,
etc.

This EDF approach may never compete commercially or performance wise
with Jet Turbine or current conventional gas or electric prop
systems ... I'm just curious if adapting small EDF's could achieve the
very limited goals in my proposed scenario.

- 60lbs Thrust
- 10 minute duration (no reserve)
- Climb from 1,000' MSL to 1,500' AGL
- Using Paved Runway
- No taxi .. prepositioned on runway
- No tailwind component

While certainly not a feasible commercial solution .. it would be a
really neat experiment

Curt -95

For a self launch - probably not. For a sustainer - maybe. Heck, you
could put 2 or 3 on a stick, poke them out a storm window and find
out.

On Jan 18, 11:20*am, CLewis95 wrote:
This EDF approach may never compete commercially or performance wise
with Jet Turbine or current conventional gas or electric prop
systems ... I'm just curious if adapting small EDF's could achieve the
very limited goals in my proposed scenario.

- 60lbs Thrust
- 10 minute duration (no reserve)
- Climb from 1,000' MSL to 1,500' AGL
- Using Paved Runway
- No taxi .. prepositioned on runway
- No tailwind component

While certainly not a feasible commercial solution .. it would be a
really neat experiment

It seems to me to be far too little thrust to be useful, except as a
sustainer.

Back in 2000 I ran some calculations for various thrust levels for
glider takeoff using engines of a type where the thrust doesn't vary
with speed (i.e. rockets and, to a large extent, jets):

http://groups.google.com/group/rec.a...484c08689379af

At the time I was not aware of any jet or rocket-power gliders, but
there are now quite a number. I concluded that anything from 100 kg to
250 kg (220 - 550 lbf) of thrust looked very usable.

I see the "BonusJet" in fact has 240 lbf of thrust on a two seat
glider.

Bob Carlton has 225 lbf on his Super Salto. His earlier Silent had
twin 45 lbf engines for 90 lbf total. It obviously worked, but the
videos I've seen make the takeoff look pretty anaemic. I can only
imagine what it would be like with only 60 lbf!

I think these machines verify that my calculations in 2000 were in the
ballpark.

My constant thrust calculations are not as relevant to a prop or
ducted fan where the static thrust is quite a bit higher than the
thrust at 50 or 60 knots, and they're really starting to drop off
after 100 knots.

One conclusion that will still be relevant is that you use less total
energy for the launch if you have a reasonable level of thrust. WIth
low thrust you spend so much more time dragging the aircraft through
the air that you use a lot more energy in total -- my figures showed
17% more fuel needed with 50 kgf (110 lbf) of thrust compared to 100
kgf (220 lbf). WIth only 60 lbf available it would be a lot higher
again, because you'd be using a substantial proportion of the
available thrust just to fly straight and level. 120 lbf for 5
minutes or 180 lbf for 3m20 would be much more useful than 60 lbf for
10 minutes.

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.

On 1/18/2011 3:56 AM, Mike the Strike wrote:

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.

Bungee launch without the hill! That would enable you to launch
unassisted: stretch the bungey out the runway, climb in, trigger the
release of the bungee, and WHHoommp! You are going 45 knots and off the
runway.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)