im working on a custom build using 4 400mm carbon fiber EDF's and trying to choose the right Kv range to turn these monsters. inrunner or outrunner? im not too worried about battery options as it will be powered by a kerosene microturbine/generator set and a voltage regulator (and a small lipo pack to start that). what i am worried about is overheating due to maxing the motors load (turning too slow) or over powering it which could also lower the life of the motor. i was thinking about using dynamite's fuse 9700Kv inrunners but that might not be enough power. any suggestions? i will know more about the impellers weight once it is made but it will consist of a milled aluminum hub with carbon fiber blades estimated weight will be about 150-200g.
edit> thermoelectric coolers and heat sinks will be installed on the motors as well.
I will be modifying a KJ-66 microturbine to turn it into a turboalternator (turbogenerator). nice price for parts and plans at marchlabs.com.... http://www.marchlabs.com/kj66_jet_engine_s/29.htm $375 for a parts set with plans.
dual ( dual mount gearbox) turnigy E500 4000kv motors? 8 total for the 4 EDF's looking at a 12 tooth gear on each motor then a 96 tooth main gear on the fan mount. i got a guy that will cut me titanium gears cheap enough as well as mill the mount/gearbox once i get it drafted... ill be running 12.4v through the system from the turbine. or should i double up EDF motors like turnigy's 2610 5000kv EDF motors. ill cut 8g off my max weight and boost a little in the max rpm but ill have to run at a lower voltage which means more cooling for the voltage regulator so ill add a few grams in heatsinks and coolers. any ideas guys?
i think im going to go away with the dynamite 9700kv motors, ment for cars/trucks id probably burn them up in one flight....
on second thought i might have to rethink that gear ratio.... i would be getting just under 7000rpm from that ratio at max rpm on the motor.... might have to use a 32 tooth main gear or less. 20,000 rpm should be good enough...
Interesting project. Since I am working on something similar, I hope to follow a build thread if you have one. Of particular interest is you generator setup.
On the motor/fan matching subject, this is fairly complex and you are wasting large amounts of energy if the motor and fan/prop are not well matched. For a vehicle that must continually lift it's own weight, efficiency is pretty important.
Did you do any simulation to find out what you are getting with the area, shape, speed, and pitch of your impeller blades? This is critical info for choosing a motor. Every fan has a RPM range in which it will operate efficiently, and requires certain power and torque to operate within that range. You need this info to choose or design a motor or motor/gearbox to match.
FYI - I looked at EDFs as a part of my early research. I came to the conclusion that their thrust-to-weight ratio is too low to be practical. They also use around twice the current per gram of thrust compared to a good open prop setup. Now to qualify those statements, I have not attempted to fab my own impellers as I do not have the ability to do the performance modeling. Also the latter may not be such an issue to you since since you are using a gas/electric setup where I am opting for an all battery solution.
im looking at a 42* pitch at the base of the blade rotating to a 35* pitch at the tip of each blade. the shroud surronding the fan is reduced by a 20* angle to begin compression. as for the turbogenerator its a pretty simple design using a diametricly poled magnet (with poles around the circumfrence) on the turbine shaft and the coils surrounding that. a heat shield of stainless with either glass fiber or aerogel mat will keep most of the heat from the combustion section form killing the coils and magnet. +/- wires would be guided through stators in the fan section. the magnet used would have to be SmCo so it can withstand high heat without loosing its magnatism. ive attached 2 quickie (mspaint) drawings of the the rough blade design (2 bolt with a guide pin) and the motor mount/gearbox.
Sounds like a very clever design, and that you've put a lot of thought into it.
I have one suggestion, not based on experience but rather on other gas turbine designs I've seen on the interwebs. Have you thought about putting your PTO (power take-off) on the turbine inlet end? This solves most of the thermal issues and greatly improves the generator life. It complicates inlet ducting somewhat, but that is less of a problem than outlet ducting and thermal management.
On your original question about motor selection, I highly recommend that you find a way to characterize your impeller design. It is possible that you could find someone on this forum with access to the requisit software. They may be able to import your 3D model and simulate it. When you have prop constants you can use simple r/c motor/prop calculators found on the web to figure out the best motor and gearing to use. You will also need to know the max RPM that the prop can handle. If you don't have specific knowledge of how to design for high centrifugal force, I recommend finding someone who can do stress modeling for you. A 400mm fan at 10,000 RPM could generate lethal shrapnel if it frags.
I've spent the last few weeks doing prop calcs for my project. However I've also limited myself to off-the-shelf motors and props for the time being.
My cautions included, I'm very interested in the progress of your project. It's a novel approach to a challenging design problem. Your solution has potential advantages over my project.
ill have to draw up the blade core but it consists of a 2mm titanium (Titanium 6AL-4V alloy) skeleton with pins and bolts/nuts made of the same alloy that extends 70mm into the blade its self which is carbon fiber. the main shroud is foam cored with the inside wall made up of 4mm of a bi-drectional continuous strand kevlar composite that should stop any frag as it is designed to come apart/unwrap to catch projectiles and the outside of the shroud and stators made of carbon fiber. as for the PTO to the generator that might be a possability but i would have to modify the exhisting parts i planed on using, that would mean welding a spline shaft to the impeller shaft. that could cause some unstability leading to failure unless it is perfectly balanced. the design i have incorporates the magnet into the allready exhisting and balanced shaft and would simply have to be braised onto the shaft before it is magnatized eliminating the need to weld extra materials to the impeller face. though i do have a machinist that could possibly TIG weld and turn the spline onto the main shaft and balance it. ill have to look into it for an alternative. it may be better that way since i would be able to purchase a pre made generator rather than fab one myself.
here is the blade core. also the continuous strand kevlar can be found here: http://www.thethreadexchange.com/miva/merchant.mvc?Screen=PROD&... the resin being used is Z-poxy a 1:1 2hr+ set time 24hr full cure. every now and then you can find the same kevlar thread on american science & surplus (www.sciplus.com) for $14.99 (1250 yd)
Like I stated, you have an impressive design and it's obvious that you've put a lot of thought into it. I have no doubt that your blades, shrouds, and hubs will be very strong. However, no one can just tell you what motor and gearing to use. You will either need to find a way to simulate your design or use potentially expensive and dangerous trial and error to find a motor and gears that are in the sweet spot for your impeller. Furthermore, it would be wise to know the failure point of the impeller design and set your max RPM limit at half of that or less.
To me, there is no substitute for real engineering. Unfortunately for us, I'm an electrical engineer, not an aerospace or mechanical engineer. I don't have the skills nor tools to give you the info you need.
Is there anyone on this forum that is willing to characterize/flow model Ian's impeller design so that he can choose a motor and gear ratio?
Ian, Can you provide 3D models of your fan components in common file formats? You don't need to post them for the world, just have them available to PM a person who is willing to help you.
To share a little about my project...
I'm working on a "tri-copter" design that uses 3 contra-rotating prop pair. Since there are six electric motors and props, it's technically a "hexa-copter", but has the advantage of being much more compact. Having an even number of motors (6 compared to 3) also has the advantage of giving me yaw control without a yaw servo. The contra-rotating props right next to each other are a little less efficient than 6 separate props, but I think I make up for it with compactness and simplicity of design.
The smaller version that I am building first uses motor pairs that generate 2000+g of thrust each. So I have around 6000g thrust total. This is a short duration absolute max thrust figure. In order to accelerate upward at 1m/s/sat full throttle, total vehicle weight can be 3000g. My calculated powerplant weight (including battery and motor speed controls) is about 1300g, leaving 1700g for airframe, control electronics, and payload.
My weight target for the airframe is 300g, and control electronics should not exceed 100g including r/c, xBee, and auto-pilot. The airframe is shaping up to be a mix of carbon-fiber and aramid composites like yours.
My current task is designing the airframe components. I'm just learning 3D modeling and trying to make use of free tools for the task. It hasn't been easy for me and my progress is painfully slow. It's especially painful to run into the capability limit of one program and start over in another. When I make a little more progress I intend to have the masters 3D printed and use the masters to make molds to lay up my composite airframe components.
Then I have some parts in hand I figured I would start a build thread on it. For now, it's still vaporware.
it should be easy enough to draw up te rest in a day or two its only another 3 simple parts to complete the impeller hub and blade locking rings, i could do a basic cutaway layout of the shroud as well. ill include measurements in the new drawings too. here is another source for a good material: nomex honeycomb