Electric Duct Fan (EDF) Self-Launch Glider?

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)

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 1/18/2011 9:09 AM, CLewis95 wrote:

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)

Can you give a link to the EDF you are considering?

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

On Jan 18, 6:42*pm, Eric Greenwell wrote:
On 1/18/2011 9:09 AM, CLewis95 wrote:

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)

Can you give a link to the EDF you are considering?

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

These are (2) units that I have been watching for ~ 2 years now..
hoping next generation would be stronger:

TF 8000 (28lbs Max Thrust)
http://www.ductedfans.com/TF_8000.html

DS 94 HST (22lbs Max Thrust)
http://www.ductedfans.com/Schuebeler...ed%20Fans.html

I just noticed in reviewing these links that the Max Thrust is stated
as 28lbs on TF 8000 ... I was using 20lbs for assumptions x 3 =
60lbs. In a previous reply I incorrectly said "3 x 38 = 108 rated max
thrust" .. I should have said "3 x 28 = 84 rated max thrust" ... again
I am using 60lbs for assumed thrust.

There are a number of purpose built electronic speed controllers for
these units.

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

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 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