yeah im going to get my hands on a adapter so that i acn charge the battery later today, and then we will see. also though the design of the first cheater hole was pretty cool but might make one without too see if there is a difference
Solved my journey to 3D print a JAS 39 gripen
- Thread starter NeonGreen
- Start date
-
- Tags
- 3d print airfoil wingloading
skymaster
Elite member
What would happen if you extended the wing let;s say to where the pilot seats?
Houndpup Rc
Master member
More powerful edf with more blades?
telnar1236
Elite member
Was the battery fully charged? Also, what is the diameter of the nozzle and what does the thrust tube look like? A bad thrust tube will kill thrust. Also, what size of battery are you using? Too small a battery won't give you enough current which can again kill thrust.
Generally, EDFs tend to produce less thrust than their rated value but that sounds like you're getting half the rated value. Worst case, I can point you to the 64mm Powerfun EDF unit. It produces about 1300 g of thrust with a good duct and nozzle on a 4S 2600 mAh pack which is about as good as 64mm EDFs get. It gets sold under a bunch of different names and pretty much any 3500 kv EDF which looks like the one in the picture will be the same regardless of the brand. This is from a quick search on Amazon, but it's sold all over the place.
Shurik-1960
Well-known member
I'm installing a 45С battery pack on a 3D aerobatics model. For your project, you will need a battery with a capacity of at least 60C, preferably 100C. Try installing batteries of the same capacity and with a different current output on an electric bike..- the ride will be different. I set up and flew on a similar model PNP with a friend, which flew only with a high-current battery. (100 C).I make all jet models with a motor + a screw.This gives you maximum traction.I sold all the impellers a long time ago and I'm not going to anymore.
Last edited:
was just about to report that i found out that it is the nozzle that is killing most of my thrust which i feel a bit silly about not thinking about sooner but you live you learn. Got 1000grams of thrust despite having done a few full thrust tests before so the battery was at around 16.2V.
going to do some more research and remodeling tomorrow but do you know any rule of thumb for how small i can make the diameter without losing too much thrust.
The battery is a 2300 mah 45C
L Edge
Master member
What brand edf, size, any specs given to push your 2.8 lb plane?
Did you try another fully charged battery? or another brand?
Like Shurik said, for jets especially from my own experience, always buy batteries at a much higher C rating. Many 64's run higher than 40 and can crap out in flight. Think that is one of your troubles.
Also concerned about length of flight, a 12 bladed 64mmEDF with a 2300 is going to give you a very short flight(especially if near open wide throttle-heavy) and if you have to go around on landing due to error?
Lowest I reduce is 90% for 64's and 70mm EDF's exit areas.
Did you try another fully charged battery? or another brand?
Like Shurik said, for jets especially from my own experience, always buy batteries at a much higher C rating. Many 64's run higher than 40 and can crap out in flight. Think that is one of your troubles.
Also concerned about length of flight, a 12 bladed 64mmEDF with a 2300 is going to give you a very short flight(especially if near open wide throttle-heavy) and if you have to go around on landing due to error?
Lowest I reduce is 90% for 64's and 70mm EDF's exit areas.
telnar1236
Elite member
Very good news. Generally, you get the best thrust across a reasonable speed range at about 90% of the fan swept area (FSA) which is the area of the EDF that is actually taken up by the fan blades. A simpler way to get a good estimate is to take 85% of the area of the fan assuming its specified diameter since this is a very good approximation for 90% FSA for the majority of modern EDFs (a difference of 2% for the F-106 I'm working on for example). As an example for a 64 mm fan here is how I would calculate it.
64^2 = 4096
4096 * 0.85 = 3481.6
sqrt(3481.6) = 59.01
So, for a 64mm fan, a 59mm diameter exhaust is roughly appropriate.
Because you're scaling and not worrying about exact values, the pi cancels out and you can use d instead of r, but if you were to calculate everything using the actual areas, like if you were calculating for a non-circular nozzle, you would need to use the actual formula A = pi*(r^2).
I am currently able to get 800 grams of thrust with what i have. I have tried making the nozzle shorter and tried a few different nozzle sizes and this is what i think i will be going with since i wanna try to get it flying this or next week. I'm wondering if anyone here has any last tips on how to make the canal more efficent inorder to get me a little bit more thrust since 800 grams is less than i want.
telnar1236
Elite member
There are few things I see right off the bat. First, your nozzle is probably still too big. For a 64mm fan, it should have a diameter of 59-60mm. Second, you want the nozzle to narrow down continuously from the EDF to the outlet. It looks like you have some messy geometry and the nozzle gets bigger before getting smaller. That causes turbulence and will lose some thrust. Third, it looks like the nozzle is out of line with EDF. That will lose a bit of thrust and also create a pitch up moment which could make the plane hard to fly.
Probably the biggest things to fix are the nozzle diameter and this section of the duct. I'm not completely sure I understand the geometry from your picture, but having those sharp steps will definitely hurt thrust.
Probably the biggest things to fix are the nozzle diameter and this section of the duct. I'm not completely sure I understand the geometry from your picture, but having those sharp steps will definitely hurt thrust.
The bottom part is a removable piece that holds the motor in place and allows for easy access. I noticed during testing that air was leaking out through the back, so I added those pieces to give the air a ramp over the gap, thinking it would only introduce a little turbulence. It hasn't been present during any of the tests; I just added it as a final touch since I thought its impact would be negligible. I'll remove it and just cover the gap with some tape.
As for the nozzle width, in my testing, I found that the 64.8 mm nozzle gave more thrust. However, it could be that I didn't charge the battery before testing with your recommended 59-60 mm. When I did four max-thrust tests in different configurations, I lost about 1V. My dad, who has experience with RC helicopters and cars, thinks the battery drains too quickly because it has deteriorated. It's an old battery that I got from a friend.
Do you think a nozzle design like this would improve the airflow?
I want to keep it aligned with the EDF to stay true to the original Gripen, but if you think the pitching moment will make it too difficult to fly, I'll have to redesign it.
quorneng
Master member
With a fast flying EDF it is the "in flight" thrust that is more critical than the static thrust.
Indeed the maximum static thrust is achieved with the bell mouth in free air and an exhaust nozzle set at the FSA.
In flight the equation is much more complex as the inlet geometry and flight speed come into play.
I do agree that what ever the configuration the duct it should be as smooth and free flowing as possible particularly after the EDF where the duct air speeds are higher and any "disturbance" has a bigger impact.
This does suggest that for any given total duct length there can be advantage in favouring inlet duct length over exhaust length but CG position may be a limit as well.
Indeed the maximum static thrust is achieved with the bell mouth in free air and an exhaust nozzle set at the FSA.
In flight the equation is much more complex as the inlet geometry and flight speed come into play.
I do agree that what ever the configuration the duct it should be as smooth and free flowing as possible particularly after the EDF where the duct air speeds are higher and any "disturbance" has a bigger impact.
This does suggest that for any given total duct length there can be advantage in favouring inlet duct length over exhaust length but CG position may be a limit as well.
I made some ideas for how to redesign the duct.
Nr. 1 is the same as the one shown before, but with curves. It has the downside of expanding to 72mm around the point where the line is, so it expands again before continuing to taper. The idea behind this design is that the end of the nozzle is somewhat symmetric, in order to avoid creating too large of a pitching moment, even at the cost of the extra widening.
Nr. 2 is the same as Nr. 1 but much smaller, which reduces the amount of expansion. However, it makes no effort to ensure the thrust comes out straight.
Nr. 3 continuously tapers off and is symmetric. This would go against the visual design goals, but depending on the performance, it might be a necessary compromise. I think I’m going to have to make some prototypes of these exhaust ducts in order to compare them against each other, since I don't know any other way to assess the real impact of the compromises.
Nr. 1 is the same as the one shown before, but with curves. It has the downside of expanding to 72mm around the point where the line is, so it expands again before continuing to taper. The idea behind this design is that the end of the nozzle is somewhat symmetric, in order to avoid creating too large of a pitching moment, even at the cost of the extra widening.
Nr. 2 is the same as Nr. 1 but much smaller, which reduces the amount of expansion. However, it makes no effort to ensure the thrust comes out straight.
Nr. 3 continuously tapers off and is symmetric. This would go against the visual design goals, but depending on the performance, it might be a necessary compromise. I think I’m going to have to make some prototypes of these exhaust ducts in order to compare them against each other, since I don't know any other way to assess the real impact of the compromises.
Attachments
L Edge
Master member
When I first saw the step in the EDF exhaust to the enlargen offset diameter downstream then compressed again to a smaller dia, I can see why you lost so much thrust in step/expanding flow, then going back to compressing the flow. That takes energy away from the thrust.
The concept is to try keeping the boundary layer going as long as possible in the exhaust tube. And that is straight, any deviations or splitting the flow , turning, whatever causes reduction in static thrust.
As far as reducing the exhaust dia according to FSA, most of the time I leave it the same. I have increased the static thrust by looking at the other end.
Essentially, with the Pressure static increased inside the EDF, it bleeds out the front blades and between the hub and blade edge. I decreased the leakage and improved the thrust. Want more explaining in detail especially why not the exhaust, ask me.
As far as the offset exhaust, yes, you will get a offset pitch movement, but it won't be that much that you can't use a down trim to counteract. I am working on the NYGAD problems to have a no gyro situation to fly.
One problem is If the exhaust flow is at an angle, how much pitch up does it cause?
I did actual testing and as you can see, the angle is sliced at 20 degrees for example. I would toss and fly(starting from 0 degrees and increment change every 5 degrees and sure enough the trimming needed more and more.
If you choose to flow exit out at an angle, be prepared to apply trim to correct.
Working with EDF's since 2006, found 2 ways that I use to determine if it maiden flys or dings/crashes.
If the wattage is not 180 to 200 per pound of airplane, it's hairy.
I always hold the jet vertical, add power until it sorta hovers (release/quick grab ) and if the throttle is 66% or under, if designed right, should fly. It is possible to get it airborne if throttle is 100, but I don't chance it. I will redesign.
Hope you video the maiden.
The concept is to try keeping the boundary layer going as long as possible in the exhaust tube. And that is straight, any deviations or splitting the flow , turning, whatever causes reduction in static thrust.
As far as reducing the exhaust dia according to FSA, most of the time I leave it the same. I have increased the static thrust by looking at the other end.
Essentially, with the Pressure static increased inside the EDF, it bleeds out the front blades and between the hub and blade edge. I decreased the leakage and improved the thrust. Want more explaining in detail especially why not the exhaust, ask me.
As far as the offset exhaust, yes, you will get a offset pitch movement, but it won't be that much that you can't use a down trim to counteract. I am working on the NYGAD problems to have a no gyro situation to fly.
One problem is If the exhaust flow is at an angle, how much pitch up does it cause?
I did actual testing and as you can see, the angle is sliced at 20 degrees for example. I would toss and fly(starting from 0 degrees and increment change every 5 degrees and sure enough the trimming needed more and more.
If you choose to flow exit out at an angle, be prepared to apply trim to correct.
Working with EDF's since 2006, found 2 ways that I use to determine if it maiden flys or dings/crashes.
If the wattage is not 180 to 200 per pound of airplane, it's hairy.
I always hold the jet vertical, add power until it sorta hovers (release/quick grab ) and if the throttle is 66% or under, if designed right, should fly. It is possible to get it airborne if throttle is 100, but I don't chance it. I will redesign.
Hope you video the maiden.
telnar1236
Elite member
No. 3 would be the best in terms of performance. It won't produce much pitching moment and it should give you good thrust at speed. No. 2 will similarly give you good thrust, but it will make the plane want to pitch up a bit. Should still be manageable though, and I've flown planes with worse. No. 1 will get the least thrust, though getting rid of that step after the EDF will still probably make things better. Like Quorneng said, any ducting after the EDF is very important to have smooth and straight since the diameter is narrower and therefore the air is flowing faster which means interruptions result in more losses. I suspect most of the losses in your design came from the step right after the EDF, and none of the three strike me as being awful.
In general, something like this is a pretty good duct design (ignore the outside of the nozzle it's entirely for looks).
If you have a cone on the back of the EDF motor, you can actually area rule the duct even more, like in the following picture, but with motors without a cone, there will be turbulence regardless so there is little point.
Designing the duct like this forces the flow to stay attached to the motor and boosted the thrust by about 5% on the old freewing outrunner motors I tested it on.
We maidened it today, and it took off!
I got some feedback from the guy flying it; he said it was tail-heavy and underpowered, but other than that, it flew well and somehow survived.
The CG shouldn't be an issue since the battery can easily be moved forward. In terms of power, I'm thinking of getting a 6S battery, since we had to connect two 3S batteries in series to get enough power to barely get it off the ground.
I got some feedback from the guy flying it; he said it was tail-heavy and underpowered, but other than that, it flew well and somehow survived.
The CG shouldn't be an issue since the battery can easily be moved forward. In terms of power, I'm thinking of getting a 6S battery, since we had to connect two 3S batteries in series to get enough power to barely get it off the ground.
Similar threads
