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 Jan 17, 12:53*pm, CLewis95 wrote:
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

I think a large one meter EDF would make sense for a tow plane.
Ducted fans "sweet spot" is right in the towing speed range - that's
why they're popular with the new blimps. People talk about 8 - 10 Lbs
of thrust per HP at 60 knots. If so, that would allow a 60 - 80 HP
EDF tug provide the same tow performance as a 235 Pawnee.

As I understand DF's, the bigger they are the more efficient they
are. Retracting a big DF into a SL glider would be a problem but
maybe if the duct were short, it could be rotated 90 degrees before
swinging down into the fuselage.

Why would this be better than the Antares large prop? Speed might be
an answer. The DF's cruise speed is likely to be higher than a large
slow turning prop. Speed would be useful to get to a lift area some
distance from the takeoff point.

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 17, 4:12*pm, bildan wrote:
*People talk about 8 - 10 Lbs
of thrust per HP at 60 knots.

Well, y'know what they say about "talk".

Ole father physics says you get 5.43 lbf of thrust per hp (550 ft-lbf/
sec) at 60 knots if you can achieve perfect efficiency.

Reality will be substantially less than that.

-Evan Ludeman / T8

On Jan 17, 3:08*pm, T8 wrote:
On Jan 17, 4:12*pm, bildan wrote:

*People talk about 8 - 10 Lbs
*of thrust per HP at 60 knots.

Well, y'know what they say about "talk".

Ole father physics says you get 5.43 lbf of thrust per hp (550 ft-lbf/
sec) at 60 knots if you can achieve perfect efficiency.

Reality will be substantially less than that.

-Evan Ludeman / T8

This may be another thread, but I like the idea of a microjet with an
electric main gear for takeoff assist. The electric main gear helps
accelerate the glider quickly and efficiently until the wheel leaves
the ground, say 40 knots. This means a jet glider needs less runway.
Also a jet is most inefficient at slower airspeeds. Of course you
could recover a little of the energy with regenerative braking during
the landing, but most likely you would just need to recharge the wheel
battery from the mains.

Anyone have the knowledge needed to size this battery and motor? Lets
say for two take-offs (one for a landout), 850 lb glider, good tire
traction up to 40 knots. Also, what kind of acceleration would be
reasonable to assume? The thrust of the jet would be added to the
wheel thrust, of course.

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.

On 1/17/2011 11:59 AM, CLewis95 wrote:
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

I like the idea, but starting the quest with a sustainer, because 60 lbf
would be unsafe on a 850 lb glider. 2500 runway? You'd still be on the
ground, as it'd take ~40 seconds to get to 60 mph. One fan would just
keep you up; two fans would let you climb. You could try a single fan
most easily by mounting it externally - no retraction. You'd probably
learn a lot with little investment.

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