FT versa underpowered, need a few suggestions

[njking]

hey all,

great to be here, and I owe this to flite test for my first successful flight with the ft flyer, since then I have made an Ft blunt nose versa and got it in the air a few times, it flies great but underpowered for sure, I have to fly it on full chatter most of the time and a vertical runs out of steam very quick. the plane is heavy as I made it from depron covered in packing tape, its very strong but I need a bit more poke. this is the setup iam using:

motor turnigy 2730 1500kv brushless
http://www.hobbyking.co.uk/hobbykin...730_Brushless_Motor_1500kv_UK_Warehouse_.html

40 amp titan esc, turnigy 3s 2200 mah battery

7x3.8R propeller

at full power after a 10 minute flight time the battery went from 12.40v to 11.45, which is not a lot of power draw flying on full chatter.

I would like some help to increase thrust, iam not particularly keen on buying a new motor so is there a way I can use a bigger prop with this motor to increase thrust? I don't want to make it a rocket ship but 20% more thrust would be ideal, at the same time I don't want to fry my electronics

plus a bonus question :-D

do all the 1500 kv motors generate the same thrust? it seems to me that two motors of same kv rating can be immensely different regarding power delivery, how can I tell?

sorry for the long winded post and thanks for reading, any help would be great

regards
naj

 

[joshuabardwell]

The kv rating is simply the ratio of rpm to volts. A 1000 kv motor will spin at 1000 rpm when supplied with 1 volt; 2000 rpm when supplied with 2 volts; and so forth. But that same motor will generate more thrust and draw more power when provided with a larger prop, than a smaller one.

Have a look at the specs for your motor. It's rated for a max current of 8 amps and generates up to 410 grams of thrust with a 7x3.5" prop. Now have a look at this one. It's rated for 15 amps. When supplied with an 8x4" prop, it will generate about 800 grams of thrust. Now have a look at this motor. It will generate 1200 grams of thrust with a 9x5" prop, pulling 23 amps.

All of those motors have about the same kv rating, so they will be trying to spin the prop at about the same rpm, but as the prop size goes up, so does the power the motor pulls and the thrust it generates. To make an analogy, think about car/truck engines. If you had an engine spinning at 2000 rpm, is it a given that the engine will make the same power/torque and use the same amount of gasoline? Absolutely not. The engine's displacement matters too. Bigger motors can draw more power and make more thrust without burning themselves up.

The first step in determining what size motor might be right for you is to know the all-up-weight (AUW) of your craft. This is the craft's weight, including all gear that it takes into the air, including battery. Once we know the AUW, we can determine about how much thrust you need to fly and work from there.

 

[joshuabardwell]

If I had to pull something out of my butt, I would say try something like a 2212 kv1400 with an 8x6 prop. I used that setup very successfully on my PopWing, and had more thrust than I knew what to do with. The SunnySky 2212 kv1400 should generate about 755 grams of thrust pulling 18 amps with an 8x6 prop.

A SunnySky 2212 kv980 with a 9x4.7 prop should generate about 850 grams of thrust pulling 12 amps. With a 10x4.5 prop, it shoudl generate about 1000 grams of thrust pulling about 16 amps.

You may notice that the 980 and the 1400 kv motors generate in the neighborhood of the same amount of thrust, but the 1400 pulls the most power. The difference is that the 1400 will make a much faster plane. The 1400 kv motor with an 8x6 prop has a pitch speed of about 59 mph. The 980kv motors will thrust just as hard (in terms of vertical performance, for example, and getting up to speed after launch), but the plane will not be moving as fast at the time. I have found that it's sometimes the case that higher-pitch props actually use the same or less power because, although the plane WOULD pull more power at wide-open-throttle, I can cruise at a lower throttle setting and it evens things out.

 

[herk1]

That motor looks like a twin to the "Blue Wonder"...i.e. this one:

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=34097

They're the same size, weight, kv, appearance, etc. I suppose it's just different branding.

I used it (the Hextronik one) in my Hobbyking Reaktor for many flights with an 8x3.8 prop and it was fine. It pulled 12.25 amps on my wattmeter, which is over the stated amps -- but I've found that all the different-kv "Blue Wonders" that I've used have performed well on 12 amps without getting hot, when they're mounted out in the open air for good cooling. So yes I think it's fine to put a little more prop on it. Just test fly it a short flight at first and feel the motor right after to make sure it's not getting hot.

The thing that strikes me as a problem on your setup though, is that you're using a small lightweight motor combined with an awfully big, heavy battery. If you wanted to do a lightweight Versa build, that motor would be OK, as long as you use it with a much smaller battery (such as an 850 mAh or even a 500 mAh), and preferably take additional steps to keep the AUW down (use a smaller ESC -your plane sure doesn't need that big 40-amp!, maybe some small servos, take it easy on the glue and tape in the build, etc., etc.). Just to compare: on my "regular size" Versa Wing, I use a motor twice that size and usually fly it with batteries less than half the size (850 mAh and 1000 mAh).

 

[njking]

If I had to pull something out of my butt, I would say try something like a 2212 kv1400 with an 8x6 prop. I used that setup very successfully on my PopWing, and had more thrust than I knew what to do with. The SunnySky 2212 kv1400 should generate about 755 grams of thrust pulling 18 amps with an 8x6 prop.

A SunnySky 2212 kv980 with a 9x4.7 prop should generate about 850 grams of thrust pulling 12 amps. With a 10x4.5 prop, it shoudl generate about 1000 grams of thrust pulling about 16 amps.

You may notice that the 980 and the 1400 kv motors generate in the neighborhood of the same amount of thrust, but the 1400 pulls the most power. The difference is that the 1400 will make a much faster plane. The 1400 kv motor with an 8x6 prop has a pitch speed of about 59 mph. The 980kv motors will thrust just as hard (in terms of vertical performance, for example, and getting up to speed after launch), but the plane will not be moving as fast at the time. I have found that it's sometimes the case that higher-pitch props actually use the same or less power because, although the plane WOULD pull more power at wide-open-throttle, I can cruise at a lower throttle setting and it evens things out.

Thanks very much for the great reply, I have learned something here, specially being a petrol head myself your car analogy hit the spot, next time iam on hobbyking I will buy a couple more motors and keep what you told me in mind, I have been looking at the wrong criteria and missed the important bits,
Thanks once again mate

 

[joshuabardwell]

To carry the car analogy forward a bit, a prop's size is roughly analogous to a car's torque. In the same way that torque is what gets a car off the line quickly, or allows it to gain speed quickly when passing, thrust is what causes a plane to accelerate quickly. Thrust is important when hand-launching a plane, because it gets the plane above its stall speed before the plane hits the ground. Thrust is important when doing a go-around on a failed landing. Thrust also determines your maximum climb angle.

A prop's pitch, combined with the motor kv rating, combined with the battery voltage, is analogous to the gearing. RPM = kv * volts. Pitch * RPM = pitch speed, which is a good way of comparing how fast a plane will fly. In reality, RPM will only equal kv * volts when the motor is completely unloaded. When you put a prop on a motor, it slows down and pulls more current. The bigger the prop, the slower the motor goes and the more current it pulls. So if you really want to calculate pitch speed, you need to get an actual motor and put an actual prop on it and measure the rpm with a tach. There are some programs, like eCalc, that can estimate pitch speed, but those results don't always line up with reality.

To size a motor for a plane, I would start with the AUW. Let's say you have a 1 kg plane.

Next, decide desired thrust-to-weight ratio. Actual TWR can be all over the map, but in general, I would suggest thinking about a minimum of 0.5:1, 0.75:1 being a bit more aerobatic, and ratios from 1:1 to 1.5:1 being more extreme, with enough thrust for unlimited vertical. So let's say that we want about 750 grams of thrust, for a TWR of 0.75:1.

Next, let's think about power-to-weight. This is just a rule of thumb, but it goes like this:

• 50 W / lb - Slow flyer.
• 80 W / lb - Powered gliders, basic park flyers and trainers.
• 120 W / lb - Sport flying, basic aerobatics, scale warbirds.
• 180 W / lb - Getting into the 3D and scale jets range.
• 200 W / lb and up - Massive power

Let's pick 120 W / lb as our desired power level. 1 kg = 2.2 lbs * 120 W/lb = 264 W. So as a baseline, we are looking for a motor that will produce around 750 grams of thrust, with a rating of around 264 watts.

The process of picking the desired kv and prop pitch is a little more involved. You calculate your stall speed based on your plane's wing loading, then you estimate that your maximum speed should be about 2.5-3x the stall speed, then you look for a kv/pitch combination that gives you close to the desired maximum speed at full throttle. This is actually a little tricky, because it's often hard to find real-world RPM results for lots and lots of motors.

Having a tool like eCalc (eCalc.ch) is really helpful for tweaking different permutations of motor and prop without breaking the bank buying a bunch of equipment. I highly recommend it.

 

[njking]

great info form both of you guys, very thankful, JOSH after reading your posts around 5 times I went to hobbyking website and checked on specs on a few motors and now it makes sense, being a newbie I always looked at KV rating, now I know what to look for.

HERK and JOSH: I will try the 8x6 prop as its the easiest solution I can do for now and keep an eye on the motor, BTW how do I find out how much amps my prop is drawing, I have a multi meter that's all, don't have a tach.

you guys asked about the insane setup :-D so let me explain, I crashed 3 planes with combined flight time of 20 seconds or less and was about to say goodbye to the hobby, with the final two boards of depron that I had left I decided to build ft flyer and versa, the motors I had already bought so the blue wonder went into ft flyer and the turnigy into versa, to be honest I wasn't even sure it was going to fly, but fantastic turn of events and both planes flew, its so exciting I cant believe it.

had to make it heavy with packing tape (experimental airlines method) because here in UK depron is the best option for scratch building and while is cheap and easy to work with its also very brittle, even a modest impact and it shatters in pieces, the versa has now crashed a few times but the packing tape and fibre glass tape works miracles and its still in one airworthy piece, I think its weight compromise that's worth it.

I am using a big battery because cant get CG right otherwise, even with the 2200 I have to tape some coins to the front to get it balanced, which is why I would like to get some guidance on the following from you guys

1. if I use a more powerful motor on versa it means to balance the cg I will have to add a second 2200 or equivalent weight which makes the place heavy again rendering the extra power moot, doesn't it take me back to square one or is there something iam missing here.

2. are there special tractor and pusher props or iam ok with my current setup of numbers facing forward for both applications? and the letters after the numbers indicate to what, for example SF, R, APC

3. I have already damaged a prop landing the versa, might be a stupid question but is there anyway to make sure the before landing the prop stops parallel to the wing to save it from scratching on the ground?


kind regards to you both for taking out your time and helping out.

NAJ

 

[joshuabardwell]

There are tractor and pusher props, but they mostly matter for gas/nitro engines, which always spin in one direction. They also matter for multirotors, which require that some motors spin opposite to other motors. With a brushless electric motor, you can reverse the motor direction by swapping any two motor leads, so whatever kind of prop you buy, you could just swap the leads to make the motor spin the right direction. Best practice is to buy standard (tractor) props, because they are easier to find in more variety than pusher.

As for landing the Versa, just make sure you are at zero throttle when you touch down, and the prop should move out of the way as you land. There is no easy way of forcing the prop to stop at a fixed position. Part of your problem may be that your motor is so underpowered that you are having to use high throttle to avoid stalling when landing. I usually establish a glide slope at around 15%-20% throttle on my Popwing, and so there isn't much energy in the motor when I chop the throttle for landing.

 

[joshuabardwell]

Regarding your question about CG, are you sure that your CG is correct? How much up trim are you using to fly level? Have you tried pushing the CG back until the plane starts to become unstable? It may be that you are more nose-heavy than you really need to be.

I would think that the tape would make the plane heavier, but it wouldn't change the CG because it would be all over. So it's a little confusing that you are having to go to extremes to get the plane properly balanced.

 

[njking]

didn't really go to extremes to get it balanced, but I will try a 1000 mah later on today and see how that works out, as for flying, I launched it on up trim but I kept going up like pendulum so I brought the trim down to near parallel with the fuselage and its nearly spot on now, although even now if left on its own it tends to climb so I think iam not nose heavy enough, but its easy to fly so iam not much worried about that.

the CG location is exactly what was on the plans for the blunt nose versa, didn't really experiment with moving it at the fear of getting completely lost :-D

 

[njking]

just took for freezing flight, flew great, still underpowered off course but tracks well, as per your instructions managed to land it without any damage, great tip regarding the glide slope worked like a charm but found out something really spooky I don't know if you come across it, as I was landing it, inspite the fact that I completely turned the throttle off the motor was still running, and I don't mean the prop spinning I mean the motor running with the buzzing sound, and everytime I flared it it would slow right down and on nose down started buzzing again, that's with throttle completely off. kept going until touch down, any technical explanation for that mate or have I got a ghost motor :-D

 

[joshuabardwell]

If it wants to climb, experiment with down trim. As the CG moves back, you will notice that the plane becomes more and more responsive, especially in pitch. If the plane becomes tail heavy, it will be almost impossible to control, but if you are careful with the CG, and only make small changes (perhaps moving the CG only 5mm at a time) you should be able to see this coming before you get into trouble. As you move the CG, adjust trim and if necessary also reduce throws. Eventually, you will find the exact point for your plane where you are sure the CG can't go back any further. Especially on a scratch build, you may find that the exact right CG for your plane is not 100% the same as where the designer put it, so don't stress too much about the marks. Pay attention to how the plane flies and work from there.

It would be very unusual for a flying wing to need down trim, but since this is a scratch build, who knows what other aerodynamic effects your specific build has brought into play. For example, maybe your motor has some upwards thrust angle, causing the plane to pitch up when you throttle up. Try this: set the trim to center the control surface, then take off and fly up high, and turn off the motor and glide. Does the plane glide relatively flat, or does it "porpoise" in the air, trying to climb, stalling, diving, and trying to climb again. If it flies flat in glide, and climbs under throttle, it's your motor's thrust angle that is the issue.

For myself, if the plane was stable and controllable in pitch, I would want to have the CG as far aft as possible, if only because it would bring the overall weight down, but also because the more neutral a plane flies, the more efficiently it flies.

 

[joshuabardwell]

The motor is not actually running. That's just the prop free-wheeling in the wind. You can turn on ESC braking if you want (assuming your ESC supports it), which will cause the ESC to stop the prop when the throttle is at zero. But there is no way to guarantee where the ESC will stop the prop. Either way, the prop should move out of the way on landing, as the "brake" is not very strong. It is mostly intended for folding props, where if the prop kept spinning it would never fold, but once it folds up, it has no more tendency to spin. On most of my planes, I leave the ESC brake off because a free-wheeling prop has more drag, and I like the fact that the prop helps the plane slow down when I'm landing. On gliders, ESC brake is essential because they are trying to minimize drag.

 

[joshuabardwell]

A random question: do you have FPV gear in the plane? Since the blunt nose was designed to hold FPV gear, it may have been designed with a CG intended for more nose weight than you have. That may be part of the problem.

 

[njking]

excellent, thanks a lot, I got it now, I will leave it as it is, plus to be honest I have a titan esc and no programming instructions for that anyway, you have been a great help mate,

the three posts you sent regarding wing loading, motor power and matching props is already printed out and on my work wall, sure to save me from many future mishaps

thank you

 

[njking]

A random question: do you have FPV gear in the plane? Since the blunt nose was designed to hold FPV gear, it may have been designed with a CG intended for more nose weight than you have. That may be part of the problem.

interesting point, and no I don't have any FPV gear whatsoever.

two 9 gram servos, orange receiver, titan esc, two servo extension wires and finally the battery, I made it blunt nose just caz I liked the shape

 

[makattack]

If the prop wasn't just free spinning/windmilling, you may not have adjusted the esc's throttle range. You mentioned the motor buzzing and maybe twitching after landing, which made me think of that.

Most esc's set the throttle range by powering up with the throttle maxed, and after a series of beeps, lowering fast to minimum throttle. Of course, do this with the prop off in case it doesn't work for any reason.

 

[joshuabardwell]

If the prop wasn't just free spinning/windmilling, you may not have adjusted the esc's throttle range. You mentioned the motor buzzing and maybe twitching after landing, which made me think of that.

Hmmm.... this might also explain having to fly at full throttle all the time! Maybe he's not actually at full throttle!

 

[njking]

food for thought there, I will do what you suggested, after powering up I will wait for a few beeps and then bring the stick down, as I don't know the programme parameters on the ecu ill just do it after the first few beeps, hopefully that will establish full line of throttle.

 

[joshuabardwell]

It looks like you can pull the manuals for your ESC from here:
http://www.aircraft-japan.com/en/datasheet/hp/titan/titan.htm

Usually, the way it works is, the ESC will beep a code that indicates what parameter is being programmed. It will beep the code a few times and then move to the next parameter. The ESC is put into programming mode by powering it up with the throttle at full. The parameter is selected by lowering the throttle. The first parameter to be configured is usually the throttle range, so you would power up the ESC with the receiver on, connected, and the throttle at full. The ESC would begin to beep--typically a single beep. After it beeps four (or however many) times, it will move to the next parameter. This is relevant because if you wait too long to lower the throttle, you may end up changing some other parameter (braking on/off, ESC timing high/low, battery voltage warning, etc...) without meaning to. If you accidentally move past the first parameter without lowering the throttle, just pull the power without lowering the throttle and start over.

As was mentioned, do this with the prop off, or with the plane restrained and the prop clear, because if these assumptions are wrong, then the plane will go to full throttle when you apply power, which could be a Bad Thing.

EDIT: Very strange. The manual doesn't seem to list throttle range as a programmable parameter. That makes no sense...