|Oct 12, 2013, 08:57 AM|
MotoCalc Performance Modelling for a FPV Bixler2
Hello aero scientists, I just ran my proposed Bixler FPV build drive-train choices through MotoCalc, which gave me the summary of advice below. To get a report at all, I had to change the prop from an 8x6 to a 7x6, but now that I have this report, I am still uncertain how this bird will perform, or for how long it will stay aloft before battery power drops too low to sustain flight.
I sure would be grateful if anyone here who has used Motocalc could be so kind as to glance over my choices below, and offer any advice on ideas to reduce the overheating of the motor, besides just minimizing full throttle flying. To take the weight of all the FPV gear into account, I added the combined weight of the Arkbird, Video Transmitter, Video Camera, RC Reciever and ESC to the airframe weight of the Bixler on the Motocalc form.
Any pointers for heating reduction would be much appreciated. Finally I haven't spotted the projected flight time of this Bixler anywhere in the report, and I would appreciate it if anyone can point me towards where Motocalc displays flight-time information.
Below is Motocalc's verdict on my Bixler2 FPV build.
MotOpinion - Bixler2 FPV Build With Arkbird PROP 7x6
Sea Level, 29.92inHg, 59°F
Motor: NTM Propdrive 2826; 1350rpm/V; 18A no-load; 0.011 Ohms.
Battery: Zippy Flightmax 3000 mAH, 20C; 3 cells; 1000mAh @ 3.7V; 0.004 Ohms/cell.
Speed Control: Turnigy Plush 30A; 0.003 Ohms; High rate.
Drive System: Direct Drive - No Gearbox; 7x6 (Pconst=1.06; Tconst=0.995) direct drive.
Airframe: Bixler2; 325sq.in; 46.2oz RTF; 20.5oz/sq.ft; Cd=0.044; Cl=0.49; Clopt=0.69; Clmax=1.07.
Stats: 105 W/lb in; 30 W/lb out; 22mph stall; 28mph opt @ 59% (4:34, 545°F); 33mph level @ 65% (4:02, 600°F); 404ft/min @ 9.5°; -198ft/min @ -4.6°.
Possible Power System Problems:
The full-throttle motor current at the best lift-to-drag ratio airspeed (28.6A) is lower than the motor's maximum efficiency current (132.1A). A higher current level would improve system efficiency.
The steady-state still-air motor temperature at the hands-off cruise airspeed and throttle setting (600°F) is extremely high, which will significantly affect efficiency and possibly damage the motor unless very good cooling airflow is provided, and/or run times are kept very short, and/or flying is primarily done at lower throttle settings.
The full-throttle steady-state motor temperature (988°F) is extremely high, which will likely damage the motor unless full-throttle is used sparingly and cooling is good (even then, damage is possible).
The estimated steady-state still-air battery temperature at the hands-off cruise airspeed and throttle setting (approximately 146°F) is higher than the suggested maximum temperature for this cell type (122°F). This could result in battery pack damage unless adequate cooling airflow is provided and/or run times are kept short.
A lower current would also decrease the battery temperature.
Current can be decreased by using fewer cells, a smaller diameter or lower pitched propeller, a higher gear ratio, or some combination of these methods.
Current can be increased by using more cells, a larger diameter or higher pitched propeller, a lower gear ratio, or some combination of these methods.
Due to the conflicting suggestions to both increase and decrease the current, this particular combination of power system components is not ideal (but not necessarily unusable).
Possible Aerodynamic Problems:
The diameter (7.0in) to pitch (6.0in) ratio is less than 1.5:1, which will result in reduced propeller efficiency at low speeds (the propeller is stalled). Although this is not likely to affect flying characteristics, it may make take-off or hand launching difficult.
The static pitch speed (65mph) is within the range of approximately 2.5 to 3 times the model's stall speed (22mph), which is considered ideal for good performance.
With a wing loading of 20.5oz/sq.ft, a model of this size will have flying characteristics suited to an experienced pilot. The plane will fly fast, and be readily able to handle fairly strong winds.
The static thrust (13.9oz) to weight (46.2oz) ratio is 0.3:1, which will result in long take-off runs, especially on grass surfaces. Hand launching is recommended if the surface is not smooth.
At the best lift-to-drag ratio airspeed, the excess-thrust (8.3oz) to weight (46.2oz) ratio is 0.18:1, which will give good climbs and acceleration. This is a good in-flight thrust to weight ratio for a basic trainer.
This analysis is based on calculations that take motor heating effects into account.
These calculations are based on mathematical models that may not account for all limitations of the components used. Always consult the power system component manufacturers to ensure that no limits (current, rpm, etc.) are being exceeded.
|Oct 12, 2013, 10:40 AM|
What performance are you after?
280 Watts input to a 7X6 will move a Bixler very briskly.
46 ounces sounds a bit heavy for a Bixler - what's in it?
|Oct 12, 2013, 12:52 PM|
Thanks for the reply, Tom. I am hoping to build a stable, slow-flying, long flight-time FPV platform with this Bixler2, but I would like it to be powerful enough to take off reliably from a hand-throw, obviously with the FPV gear that I'm now preparing to connect and test aboard. I am looking for a system that will drain power slowly enough for the Bixler2 to stay aloft well beyond the 20 minute mark, if that could become possible with changes in prop size or pitch, for example.
The science of choosing the right prop is new to me, so I pretty much went ahead and ordered a motor/prop combination I had seen used often with the Bixler, namely the NTM PropDrive 2826 1350kv motor, turning an 8x6 prop.
Still much of a noob in all things RC, I recently acquired a watt meter, and hopefully will better understand its use by the time I get to testing the circuitry that will ultimately get installed in this plane. Hoping to avoid heating issues, I looked to Motocalc for a preview of what preformance my combination will be capable of delivering, with the weight and current drain of all the FPV gear taken into account.
My FPV gear will comprise Arkbird OSD (45.5g weight), RC 72Mhz Reciever( 70g), Video Camera(70g), Video Transmitter and Antenna(89g). Adding the above to the 760g weight of the empty Bixler2, I wound up with 1043.5 grammes, which equals about 37.25 ounces, battery included. Also, I intend to go with a larger ESC than the one in my Motocalc listing above.
I am not sure whether Motocalc will automatically add in the weight of the four flight control servos, or how I go about adding the weight of the two camera pan/tilt servos, and the pair of flaps servos, but I left those figures out initially, to simplify my Motocalc performance estimate.
In summary, I seek a slow flying ultra-low power consumption Bixler2 FPV platform whose motor and prop are selected to deliver the bare minimum propulsive power that will enable the bird to maintain steady flight in a mild crosswind. Any advice that will educate me about how best to create such a flying machine will be much appreciated.
Thanks in advance.
|Oct 12, 2013, 03:17 PM|
Thanks for this link Eflighttray. That is one of the few Bixler FPV threads that I hadn't found before now, and I'm seeing some useful facts there.
It is good to see that the specs of my equipment choices aren't too far off the selections made by more experienced RC plane builders than me.
|Oct 12, 2013, 08:31 PM|
Some of your numbers make no sense at all. Such as;
The best LEVEL flight lift/drag typically occurs at speeds a little faster than the minimum sink speed. And for most models the minimum sink occurs at a speed a couple of mph over the stall speed. So if Motocalc is correct and the stall is around 22mph then Vms would likely be at around 24 to 25 mph and the best L/D would be likely at something a touch under 30 mph. The exact speed is tough to nail down since it's related to the airframe and is not something that Motocalc can calculate. You'd want to input the design into something like XFLR5 to get an idea of the speed for best L/D.
But there's another possible misconception at play. You want to fly at the Vl/d for best distance covered in a flight. But your best duration will occur at the Vms or minimum sink speed. These are the two peak efficiency flying speeds. In practice you would likely spend some time at each during a flight depending on what you're doing at any given moment.
You'll want to get a good feel from closer in flying in non-FPV mode to find out the trim that gives you the minimum sink rate during a power off glide. Then you'd find out how many clicks it takes to up the glide speed by a noticeable amount without entering what seems more like a shallow dive. Pay particular attention to the number of trim clicks this requires. With these two points set play around with how much throttle you need to just barely achieve level flight at each trim setting. You'll find that you need a nudge more power for the best L/D speed trim to achieve level flight. Proof that it may let you cover more ground but it'll cost you flight time.
It's all pretty nebulous anyway since unless your onboard system has a decent airspeed sensor or a GPS link to provide ground speed readings guessing the speeds from the ground is vague at best. In practice you'll find out where it flies just before the stall and then trim it to fly some noticeable amount faster without becoming TOO fast. And then hope that you're getting most, if not all, the benefit from flying fairly close to the best L/D.
Now you know the two trim points and associated throttle settings to use for each of these optimum flying conditions.
Now to the part that simply doesn't make sense. From your wording it appears that you think you need to run at up around 28 to 32 amps to get to where the motor is most efficient. But if you do that you'll be ripping around at high speed and using up your battery super fast. Motor efficiency numbers aside you're going to find that your two trim and power points noted above will occur at FAR less than full throttle.
The rule of thumb for sport style powered models is that level flight takes somewhere around 13 watts/lb. So you're going to find that you can just maintain level at somewhere around 26 to 32 watts input depending on which trim and power setting you are using at any given moment. These numbers will climb a bit on windy days simply due to needing a little more power to punch through the turbulence and due to the need to stir the control sticks around a little more often. So let's say you need 35 watts for level flight as a nice round number.
On the note about your battery you said;
Assuming that this is actually a 3000mah 3S pack at 11.7 volts you only need to draw 35/11.7= 3 amps. Assuming that the onboard systems do not use more than 0.5 amps in addition to the 3 for the motor you're looking at a good 40 minutes of flight and still land with a reasonable 10 minute safety reserve. Maybe 35 given the added power used early in the flight for the climb up. Any up and down you do during the flight would further cut down on the flight time depending on how often it's required to climb back up to height. On blustery days the added power used to fight the turbulence and still maintain height would cut this down by another small amount.
Now if part of the fun is optimizing the power system to get the most from the onboard pack then you need to start looking at motor efficiency in this 30 to 35 watt operating region while still having a high enough peak power rating to provide a reasonable climb. What is reasonable? Well that depends on a few things. But generally you can get an acceptable climb rate where it takes about 60 to 90 seconds to climb to around 500 feet from as little as 50 to 60 watts/lb of model weight. It may well be that such a smaller motor would then be somewhat more efficient at the 30 to 35'ish watt cruise power level.
Oh, those temperature values for the motor are likely if there is no air cooling. Certainly by the time the motor got to 988°F the metal would be glowing a dull red and the foam around the motor would have melted away. In practice a little cooling air passing thru the motor makes those temperature values mean little or nothing.
|Oct 13, 2013, 01:04 AM|
I am extremely grateful for this detailed response, BMathews. I have copied and pasted your words above so that I can very carefully read more than once, to better understand all the points you have made, so that I can hopefully go back and amend my Motocalc input data before coming back here over the next day or so to seek more knowledge.
Once again, your reply here is very much appreciated, and will help clear up a lot of my noob confusion, particularly with respect to the correct format for data entry into Motocalc's equipment summary pages. I admit I did try to wing it somewhat, just to get Motocalc to generate a preliminary result that I could work with as a starting point.
Finally, I am hugely relieved to hear that the high operating temperatures, quoted by Motocalc for just about every power train combination I entered, are no cause for alarm, thanks to the cooling effect of air-flow in flight.
I'll be back with more questions, I'm afraid, but will be sure and do some more homework beforehand to ensure that my enquries are more focused and educated ones than the first lot I asked here.
|Oct 13, 2013, 03:53 AM|
Joined Nov 2003
You might do better continuing your queries in the Power Systems forum where the results of the various *calc programs are discussed more often and by many more people who have done considerable testing of motor/prop/ESC/battery combinations. This forum is used more often for aerodynamics discussions.
|Oct 13, 2013, 04:33 AM|
Ah, I hadn't noticed the power systems discusion area, though it was no doubt there under my nose all along.
Much thanks much for this link, Steve, though I will no doubt have the odd technical issue about which I may seek clarification here, a bit later when I begin testing the flaps in this Bixler. It is drive train selection that has me scratching my head for now, as I ponder the multiple options, and the trade-offs of each combination.
I've been correcting my data entry mistakes and now will run Motocalc again with a wider range of props from 7 to 10 inch diameter, alongside prop pitches ranged from x4 to x8, so as to narrow down on a drive train about which I can seek more specific advice on how to attain acceptable flight times with minimized heat generation, over at the Power Systems Forum.
|Oct 13, 2013, 12:45 PM|
The Bixler may not be the best platform for everything you want to do. My Bixler is limited to about a 7 inch dia prop. That gives very good performance but no latitude for prop selection.
Also the Bixler is too fragile to handle heavy loads. I reinforced the inside walls and floor of the fuselage with plywood, but that does not help the tail boom and flying surfaces.
I think the Bixler is an excellent starting point. Keep it as light as possible for first flights. Add components slowly and use the experience to plan a larger model. In fact you can post your results here to get participation on a new design. It works, I've done it twice.
|Oct 13, 2013, 03:01 PM|
This timely reminder that the Bixler2 needs a fair bit of reinforcement is appreciated, Tom. Apparently the wings are liable to snap on windy days, if they are not reinforced adequately.
Having watched a few Youtube videos that showed the addition of multiple parallel carbon rods in the wings, along the fuselage, and in the tail's horizontal surface as well, I intend to try and follow those guidelines to improve the strength of my Bixlers wings, tail and fuselage as best as is possible.
Regarding the prop size limitation to 7-inch diameter or less, I acquired the adjustable metal Bixler engine mount that is sold by Small CNC Machines, and this should enable me to position the engine higher than the stock position, such that anything up to a 9-inch diameter prop will fit with sufficient clearance. This engine mount also places the engine out of the foam and in the cooling air-stream, which ought to help prevent overheating on longer flights.
If anyone else here has words of advice or cautions about the Bixler2, I would be very grateful to hear those thoughts, so that I can hopefuly learn from the mistakes of those who have tried this bird for FPV, or even general line-of-sight flying.
|Oct 13, 2013, 05:30 PM|
Raising the thrust line brings on it's own troubles. All in all the bixler is not the optimum platform for much in the way of modifications along this line.
|Oct 13, 2013, 06:09 PM|
Agreed, the Bixler is about optimum as it's delivered. Notice that it already has a thrust adjustment to compensate for the stock high thrust line. You may find that the mount is not strong enough to handle the larger prop and added power. In any case BALANCE THE PROP before you run it.
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