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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 |
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Electric Duct Fan (EDF) Self-Launch Glider?
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 |
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Electric Duct Fan (EDF) Self-Launch Glider?
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. |
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Electric Duct Fan (EDF) Self-Launch Glider?
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 | |
#5
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Electric Duct Fan (EDF) Self-Launch Glider?
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 |
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Electric Duct Fan (EDF) Self-Launch Glider?
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. |
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Electric Duct Fan (EDF) Self-Launch Glider?
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 |
#8
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Electric Duct Fan (EDF) Self-Launch Glider?
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. |
#9
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Electric Duct Fan (EDF) Self-Launch Glider?
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. |
#10
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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) |
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