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By DavidScott
#386482
I really doubt they would have chosen a fan if it cruised at 35 mph. The other electric aircraft I know of all use 2 bladed props and look like they were designed to fly much slower.
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By W9GFO
#386506
Watson wrote:It looks like W9GFO knows what he is talking about, while me and AIRBUS got it wrong:

http://www.airbusgroup.com/int/en/innov ... craft.html
They didn't get it "wrong". However, their aircraft would climb faster, and have further range for the same amount of energy if they would have chosen a well designed open prop power plant. They chose ducted fans because of the 'coolness" factor, because they work, and because it is a step towards their E-fan concept which they want to use on larger aircraft. Not because it was the most efficient use of electrical power to propel what is effectively a motor glider at 160 kph.

I will concede that if you have some size constraints, such as needing to mount the thrust producers on either side of the fuselage, that you may be better off ducting them. But if your number one priority is efficiency, you will not choose a ducted fan for slow speed applications. Other reasons may make them a reasonable choice.
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By flyit
#386516
" I will concede that if you have some size constraints, such as needing to mount the thrust producers on either side of the fuselage, "

My thinking exactly ( my pod ) sleek power harness . EDF low profile, box pattern around my legs, battery pack laid out similar to E help 1 , along lower back . The combined EDF power is enough for a light pilot ( 80 kg ) based on my live two unit proximity test discussed above in this thread, and Reidar Berntsen concurred, agreeably battery / flight time efficiency is a down side , but could accomplish my goal to replace aero / winch tow to 2k altitude before esc shut down / low voltage

When I have extra money I will finish this EDF project, ( total estimate $1400 )
( 1/2 way there now, I am certain I would use 120mm 12 blade units, maybe forego the alloy version for the less expensive models, they produce nearly same thrust )

Im still interested in the concept a pod harness with rear mounted twin side by side 100cc rotomax pusher 28x10 props, one counter rotating to offset right thrust, should yield plenty of power, roughly 20 minutes flight time per Reidar Berntsen , E help twin prototype. ( he used 2 150 cc rotomax, but added hook in weight of full aero tow frame and caster / fixed to glider )
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By W9GFO
#386517
The EDFs may actually work for a hang glider, their compact size is a huge advantage, and they are generating respectable amounts of thrust - but please, let's not pretend they are more efficient than an open prop system.
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By flyit
#386523
The prop vs turbine is not a debate I can get in. I don't have that kind of technical expertise. Three vs two blade prop is another hair splitter with some folks. I am just hopeful that it can be an unusable harness when Im done.

My hope is that HG pilots will be able to use the more relevant conversations and test result information in this thread to advance the common use of powered hang gliding, expand the sport to routine FLFL.

I love this sport and I believe that a modern streamline powered harness might open doors to rebirth of a shrinking sport. Okay maybe Im a little dramatic here, but when people don't have to drive 1/2 day to get to a mountain or aero town site, hope there is not a 10 mph wind at the launch, the regular everyday participation and enthusiasm can change. HG clubs could spring up in every small town. As long as future pilots stay with proper training at a certified facility, why not support this opportunity with advent of new tech.

Ill probably sign off this conversation now until I have finished a harness and tested, success or fail I will post the results here for the good of those pilots in favor of FLFL
By RichDiamond
#386530
i don't know if i would want that many LiPo batteries on top of me
i do like the convenience though of Electric Assist
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By red
#386543
Campers,

Almost hate to bring facts to a good debate, but here goes:

Every Boeing 474 in commercial service uses ducted fans, driven by turbines. As a type, these engines are also called "high-bypass." The jet turbine exhaust is not the major source of thrust in these engines. Most of the thrust comes from the ducted fan.

Image

The Un-Ducted Fan engine is quieter at lower airspeeds, and it certainly works, but it has not replaced the TurboFans as yet.

Image

At the low airspeeds we use, I would expect a big slow-turning prop would be better than a ducted fan. The real advantage of a prop here would come in when the power was shut OFF. A prop can stop and fold down into a fairly sleek package. A ducted fan would be like flying with a drogue 'chute, when shut off.

:mrgreen:
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By Watson
#386561
Hey guys,

I was trying to compare ducted fans to propellers in a more simplified way. I understand that the ability of propellers to fold when not in use and, the compactness of ducted fans might drive a design away from a more basic comparison. Therefore, there are some things that I want to make myself clearer.

Talking strictly about aerodynamics, when both systems are in use, there will always be a more efficient ducted fan design than a propeller design for a given condition, but not the other way around. This is for the same reason that a glider in ground effect will glide more than the same glider not in ground effect. Basically, using tip vortex to your advantage, outweighs the duct drag.

However, when we talk about the propulsion itself, in order to turn these aerodynamic surfaces, there are more things that come to play. Ducted fans usually need to spin faster, while propellers need to spin slower. So, when only piston engines were available (lower RPM), propellers were the only plausible answer. But, with the invention of gas turbines (high RPM), ducted fans grew in popularity, however their cost is still too high for some applications.

Now, with electric engines, there has been a change in paradigm. The newest electric engines (with rare earth magnets) have a higher efficiency at high RPMs and are dirt cheap! You can still put a gear box to change from high to low RPM (or the other way around), but every time you go through one of those you necessarily loose efficiency through the system.

So, this is the reason why I believe we are going to see more and more ducted fans paired with electric engines. They really have unprecedented efficiency numbers, that even people in the area are not used to it.
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By W9GFO
#386562
Watson wrote:Talking strictly about aerodynamics, when both systems are in use, there will always be a more efficient ducted fan design than a propeller design for a given condition, but not the other way around. This is for the same reason that a glider in ground effect will glide more than the same glider not in ground effect. Basically, using tip vortex to your advantage, outweighs the duct drag.
You are correct that a properly designed duct will increase the efficiency of the blade. You are not correct in stating that a ducted fan therefore is more efficient than an open prop. The efficiency is lost because a ducted fan must accelerate the air to many times the flying speed. That is wasteful. On a slow speed aircraft you want to accelerate the air as little as possible. A large diameter propeller is the most efficient way to do this. It is the same reason you see long slender wings on a glider. They sweep a large amount of air, accelerating it a little.

However, as the aircraft speed increases the large diameter propeller gets impractical - tip speed become surpersonic and they become inefficient. The blades then need to be enclosed.

Also, as further evidence of the "move a lot of air a little bit" method, just look at the turbofans on todays modern airliners compared to the turbojets of yesteryear. The difference? Big, huge fans. They put those huge fans on them because it is more efficient. The amount of acceleration is less, but is imparted to a larger volume of air. That is what makes them more efficient. The same reason a large open prop will outperform a small ducted fan in slow speed applications.
Watson wrote:However, when we talk about the propulsion itself, in order to turn these aerodynamic surfaces, there are more things that come to play. Ducted fans usually need to spin faster, while propellers need to spin slower. So, when only piston engines were available (lower RPM), propellers were the only plausible answer. But, with the invention of gas turbines (high RPM), ducted fans grew in popularity, however their cost is still too high for some applications.
Many piston engines have reduction unit to further slow the prop speed - so they can swing the largest possible prop! Many thousands of gas turbines also have reduction units - so they can swing the largest possible propeller!
Watson wrote:Now, with electric engines, there has been a change in paradigm. The newest electric engines (with rare earth magnets) have a higher efficiency at high RPMs and are dirt cheap! You can still put a gear box to change from high to low RPM (or the other way around), but every time you go through one of those you necessarily loose efficiency through the system.
True, but no need to gear them down, just use the appropriate motor for the desired rpm (it's in the windings and diameter of motor bell).
Watson wrote:So, this is the reason why I believe we are going to see more and more ducted fans paired with electric engines. They really have unprecedented efficiency numbers, that even people in the area are not used to it.
Readily available efficient brushless motors will make EDFs gain in popularity. Still, if efficiency is your priority, you will choose a large open prop.

Please remember that an efficient motor does not mean an efficient thrust producer. It only means that the motor is very good at converting electrical energy into rotary motion. It does not mean that it is producing more pounds of thrust per watt than an open propeller can.
By blindrodie
#386571
So have you guys looked into the motor Manfred Rumor uses (10 kw Flytec (Geiger/ Eck system) Engine and a 1,4 carbon folding Propeller). His entire rig, wing and all is $11,000 USD plus shipping. Can this be modified to a FL harness?

The cheapest 103 trike with a 2 stroke from NW is $16,000+. NW claims that the simple electric trike is still not ready for "Granpa". I want to start an E Revolution locally as all my buddy's are quitting to fly a sailplane or have sworn off 2 stroke engines and they want to launch and land on wheels. Are "we" waiting for batteries to come down in price? I'll be too damn old by then...I want to fly a lite trike now with quiet power and no worry about (gas) engine failure.

Were is the "Manfred" of the USA?

Sorry if this is :offtopic:

8)
By DavidScott
#386585
The Flytec motor and electronics could be used more easily than the hobby stuff for a powered harness. It's only downside is the price.
By wacker
#386599
red wrote:
raquo wrote:One other thing I've been thinking about is how we don't have props mounted on the gliders anymore. We stopped doing that because in thermal air such a setup could exacerbate pitch-overs into tumbles.
However, with current technology it should be possible to equip the glider with redundant accelerometers / gyroscopes that would detect such events as they develop and immediately stop the propeller (or even make it spin backwards momentarily).
Storing the batteries on the glider rather than on your harness might give you a bailout option in case of battery fire – deploy chute, cut off burning glider.
Raquo,

There might be a better way. 8)

I think "exacerbate" is not exactly the right word there. CAUSE is a much better word there. I have seen the keel-mounted power units in flight, and with even minor turbulence, as soon as the pilot goes weightless for an instant, the glider is pitching over violently. NO HG pilot could do anything sufficient to prevent this dangerous reality. I am not talking about extreme turbulence here, either. Just the minor mechanical bumps (that you can get over almost any flat field) were enough to cause an instant vertical dive at very low altitude. I watched one (M.C., local HG instructor) pilot pulling sagebrush twigs out of his control bar corners, after a surprise dive during a flat-field take-off. That was too close! He then removed that killer power unit from his glider forever. His glider had fixed tips and luff lines, and it was probably more pitch-positive than many modern gliders today.

The cost and complexity of gyroscopes and accelerometers would stop many pilots from installing that equipment. If these items are mechanical rather than electronic, there would be on-going maintenance costs as well.

Now it may be possible to rig a weight-sensing switch along the length of the hang-strap, which would open instantly when the strap is not pulled fully tight by the pilot's weight. That switch could possibly be used to reverse the engine thrust when the straps go slack, as you have proposed, but in all cases I would want a dead-mans' switch under the pilot's hand as well, which would chop all power to the engine if released. These switches and wires do not need to be heavy-duty; they can be fairly small if they control relays to apply power to the motor. Even using heavy-gauge wire, you get a lot of power loss through long wires. Relays and short heavy wires make good sense, for this application. This entire power proposition needs some serious work, to be developed into a safe flyin' unit someday. I would still favor a prone powered harness, instead.

All IMHO, of course. YMMV.
You might be able to get compressed air in the keel. It could spin a propeller on its way out and gain a little extra forward momentum with a rear-facing exhaust. Make sure to release most of the pressure by the time you land, though, or you risk playing with explosives.

Of course you'd have to seal the ends better than with plastic caps, but the tube shape is simple and the materials/dimensions are well published so a mechanical engineer could make quick work of a rough calculation to see how much air it can hold. You could probably figure it out yourself, even. If you back that number off by a factor of two or three it might be safe enough to try??? :popcorn:

You could always use roman candles instead of compressed air if you're looking for something with a little more "oomph"! :mosh: :punch:
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By Watson
#386650
Hi, I just want to make a parenthesis here to talk about the danger of using gasoline instead of lipo batteries.

If you believe that lipo batteries are dangerous to have around you, don't use gasoline, it is many times more flammable and unstable. This is not a joke, just check any car review of the Tesla models and you'll see how safer an electric car is when compared to a gas one.


Now, back to our regular programing, W9GFO I agree that high aspect ratio and minimal air disturbance is beneficial at a propeller and on most aircraft wings, however that is not the case with ducted fans.

On a ducted fan, minimal air disturbance is not an issue because the spanwise airflow gets dealt with by the presence of the duct. In the same way that an ekranoplan can have really low aspect ratio wings.

The newer higher bypass ratio engines indeed have a bit bigger aspect ratios than regular ducted fans, but are still very far from what a propeller needs to have to get the same efficiency.

Gear boxes are a design choice trade-off, you know that you are reducing your efficiency, but the overall power train might cost less because you used a propeller instead of a ducted fan.

The efficiency numbers that I mentioned earlier were calculated from the power provided to the ESC to the trust force in the air, so including the efficiency of the electric engine plus the ducted fan.
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By W9GFO
#386691
Watson wrote:The newer higher bypass ratio engines indeed have a bit bigger aspect ratios than regular ducted fans, but are still very far from what a propeller needs to have to get the same efficiency.
I think you missed the point. It's not about the aspect ratio, its about accelerating a mass of air. You can accelerate a small amount of air a great deal, or you can accelerate a large amount of air a little bit. Both can produce the same thrust.

It's like paddling a canoe. You could develop just as much thrust with a teaspoon as with a paddle but you would have to paddle the teaspoon a hell of a lot faster. You could even optimize the teaspoon's shape to make it more efficient, hook it up to a motor and mechanism to automate the paddling, use the latest in brushless motors and controllers and you would certainly improve its efficiency - but it would still be no match for the paddle. No magical thing you ever do to that teaspoon will make it more efficient that the large slow moving paddle.
Watson wrote:The efficiency numbers that I mentioned earlier were calculated from the power provided to the ESC to the trust force in the air, so including the efficiency of the electric engine plus the ducted fan.
The efficiency numbers you quoted are unrealistic. The best motors are 90 something percent efficient. To get the 97% efficient number you quoted would require a 99% efficient motor, a 99% efficient controller and a 99% efficient ducted fan. Not. Gonna. Happen.

It is true that a ducted fan's efficiency can be better than an open prop - when the prop/fan diameter is the same. A small ducted fan will not be as efficient as a much larger open propeller.

I don't know what else to say to refute this efficiency claim. If you wish to further explore this topic, we should do so on a dedicated thread.
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By W9GFO
#386692
Watson wrote: If you believe that lipo batteries are dangerous to have around you, don't use gasoline, it is many times more flammable and unstable.
One huge difference between the two is that lipo batteries can catch fire simply from being used. No trauma required. Gasoline doesn't do that.
By cgoldy
#386693
Hi,

I am new to the site but an old dog hang glider pilot. I have been researching building my own winch so I can flat land launch from my front paddock but have just discovered E-Help. Looks like the go for me but if some of you smart younger blokes can answer a few questions for me before I start ordering parts, that would be helpful. Remembering, I want to flat land launch to 1500 feet and then happy to sniff out thermals


1) Where do I find the shopping list for the reider design E-Help?

2) Why is the turnigy 150 always on back order and is it likely to become available soon?

3) I have looked at the Flytec motor and it seems to be much heavier and prop looks too big for hang glider operations. What do you guys think?

4) I am prepared to pay for a good motor. Is there a better option to the turnigy. = or > 10KW and =or < 2.5 KG . Probably looking for a slower turning motor? I have looked into Pletenburg but have become totally confused and have come back to turnigy.

5) What prop combination for flat land launch with the turnigy 150?

Sorry if all this has been addressed elsewhere.

I am 85 KG and fly a Freedom 190.

Any other Australian pilots building or plan to build I would love to here from you -

Colin Goldsworthy
0437969364
mayatres@gmail.com
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By Watson
#386855
W9GFO, I understand and agree with the concept that you are trying to explain about props, but you are ignoring the effects of having a duct around a blade and, that bigger picture will render your conclusions wrong. Get yourself a enclosed duct to spin your teaspoon and you've got yourself the basis of a jet-ski :)


About the numbers that I was quoting, play a bit with the following website and, you'll be surprised:
http://www.ecalc.ch/fancalc.php


This is kind of embarrassing, but I don't know if you know that gasoline HAS to catch fire when in use. While electric systems don't even need to have a spark to work.
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By W9GFO
#386879
Watson wrote:W9GFO, I understand and agree with the concept that you are trying to explain about props, but you are ignoring the effects of having a duct around a blade and, that bigger picture will render your conclusions wrong. Get yourself a enclosed duct to spin your teaspoon and you've got yourself the basis of a jet-ski :)
No, I am not ignoring the effects of a duct. I acknowledged that a duct can increase efficiency. When you compare a ducted fan against a much larger open propeller, the open propeller wins. It's just physics. Just like a large ducted fan is more efficient than a small ducted fan, a large prop is more efficient than a small prop.

Why do you suppose that container ships use one large prop instead of a small ducted prop? Is it because all the naval architects are idiots? No, it is because that is the most efficient method of producing thrust. When you do see a ducted prop it is for a special application, not because it produces the most thrust for the least energy.

The jet ski is a good example. You know how much power jet skis require? My old modified jet ski (1976 400) was about 50 hp. It moved me pretty well. Put that jet ski in a tug of war with a regular 30 hp outboard and it is going to lose - big time. Sure, put the teaspoon in a duct and spin it real fast and it could produce more thrust - but that does NOT make it MORE EFFICIENT than the paddle. It would require a much greater amount of power to achieve the same result as the paddle. That is the whole point!

Go ahead and run some calculations for a 4" ducted fan. Let us know the pounds of thrust per watt. Then run some calculations for a (properly designed) 12" two bladed open prop. Please share the results.
Last edited by W9GFO on Mon Apr 25, 2016 4:41 pm, edited 2 times in total.
By freeflight
#386883
W9GFO wrote:
Watson wrote:W9GFO, I understand and agree with the concept that you are trying to explain about props, but you are ignoring the effects of having a duct around a blade and, that bigger picture will render your conclusions wrong. Get yourself a enclosed duct to spin your teaspoon and you've got yourself the basis of a jet-ski :)
No, I am not ignoring the effects of a duct. I acknowledged that a duct can increase efficiency. When you compare a ducted fan against a much larger open propeller, the open propeller wins. It's just physics. Just like a large ducted fan is more efficient than a small ducted fan, a large prop is more efficient than a small prop.

Why do you suppose that container ships use one large prop instead of a small ducted prop? Is it because all the naval architects are idiots? No, it is because that is the most efficient method of producing thrust. When you do see a ducted prop it is for a special application, not because it produces the most thrust for the least energy.

The jet ski is a good example. You know how much power jet skis require? My old modified jet ski (1976 400) was about 50 hp. It moved me pretty well. Put that jet ski in a tug of war with a regular 30 hp outboard and it is going to lose - big time. Sure, put the teaspoon in a duct and spin it real fast and it could produce more thrust - but that does NOT make it MORE EFFICIENT. It would require a much greater amount of power to achieve the same result as the paddle. That is the whole point!

Go ahead and run some calculations for a 4" ducted fan. Let us know the pounds of thrust per watt. Then run some calculations for a 12" two bladed open prop. Please share the results.
How much less efficient are we going to see? 50%?