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Jun 10, 2018, 03:05 AM
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Extreme Sports's Avatar
I had a slightly less mathematical way of working out the wind effect.

Imagine your plane could sit motionless in the air using zero energy. Now for every hour it sits there it will be blown downwind by the wind speed. So in a 10km/h wind it will be 10km downwind after 1 hour and will have to fly back that same distance to be in the same position as if there was no wind. So my range would be reduced by 10km for every hour I fly. At 50km/h a 100km flight will take 2 hours but in 2 hours I'll lose 20km of range. I think this is correct if I fly perpendicular to the wind rather than out and back. But the 20% penalty is a lot higher than yours!

The optimal of course is to fly into the wind at a speed higher than cruise and back with the wind at a lower speed. I can't do that since I fly LOS so in effect by flying big circles over my head I'm probably closer to the 20% penalty
Last edited by Extreme Sports; Jun 10, 2018 at 03:28 AM.
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Jun 10, 2018, 03:27 AM
Build straight - Fly twisty
Whiskers's Avatar
Yes, but out and back you gain back some of what you lost, or lose something of what you have gained. Assuming the wind remains constant. (which it won't)
Jun 10, 2018, 03:47 AM
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Extreme Sports's Avatar
Correct. My very simple calc is a worst case scenario. Even flying perpendicular to the wind one crabs along so won't see the full penalty. In geometrical terms you fly the hypotenuse of the triangle rather than the two sides which is what the 20% represents
Jun 10, 2018, 11:54 AM
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Extreme Sports's Avatar
Decided to stop being lazy and do the proper calculations. After all, it is just simple high school trig (though not so simple after all these years )

I considered 4 scenarios:
  1. Fly out and back straight into and with the wind (as SirRexAlot has calculated)
  2. Fly perpendicular to the wind. Here the ground track will be angled downwind, and at the end of the flight the plane will be downwind of its starting point, so will have to turn and fly back into the wind. This is what my simple calculation does
  3. Fly a ground track that is perpendicular to the wind. This means that the plane will 'crab' slightly into the wind on both the out and back legs - the stronger the wind, the larger the crab angle
  4. Fly SirRexAlot's track, but increase speed by 10% on the into wind leg, and decrease by 10% on the with the wind leg. I've assumed the extra current consumption on the fast leg is matched on the slow leg. There is probably some error in this.

So which are the best and worst strategies? If my calcs are right, flying option 2 (perpendicular to the wind) is by far the worst....this makes sense as one flies a 'dog leg', which is silly. Out and back at a constant speed is the second worst. The best strategy depends on the wind strength....at lighter wind strengths, it is better to fly out and back with a 10% percent speed differential. But as the wind increases, either one must increase the speed differential, or fly a ground track that is perpendicular to the wind.

Here are the results (multiple of extra distance to be flown in the air to cover the planned distance over ground). Best option in BOLD:

Wind % of AS---Out and Back---Fly perpendicular to wind---Perpendicular ground track---Change speed into and with wind
10%------------------1.01-------------------1.1-------------------------------1.005--------------------------1.0
20%------------------1.04-------------------1.2-------------------------------1.02----------------------------1.01
30%------------------1.1---------------------1.3-------------------------------1.04----------------------------1.04
40%------------------1.19-------------------1.4-------------------------------1.08----------------------------1.1
50%------------------1.33-------------------1.5-------------------------------1.12 ---------------------------1.19

Overall conclusion: In light winds, it does not really matter which way you fly, but in heavy winds. if you can't change your airspeed, it is better to fly out and back on a ground track that is perpendicular to the wind. Assuming of course that I've not made any major mistakes...
Last edited by Extreme Sports; Jun 10, 2018 at 12:01 PM.
Jun 11, 2018, 04:09 PM
Registered User
Just stumbled on this: https://www.scribd.com/document/2909...etry-selection

Lots of easy to understand pictures, which is nice if you are a self-taught visual learner like me. There is a wonderful explanation on page 22 about fat, thinner and very thin airfoils.
Jun 12, 2018, 12:44 PM
Registered User
I got another datapoint for my fpv plane. Turns out I don't have a 2 blade 4x4.5 bullnose prop, but I do have a 2 blade 3.5x4.5 bullnose. It only pulls ~7 amps max, so I flew around for a lipo at full throttle. It started off about 7.5 amps and at the end of the battery around 6.8 amps.

3.5x4.5 : 15.3km 17:03 min = 54 kph

The downside is this little prop is terribly loud, I could hear it out to 400 meters. The other problem is I flubbed a couple launches because this tiny prop has so little extra torque. I could learn to live with it if the numbers were amazing, but they weren't.

This 3.5" prop does speed with 7 amps that the 5" prop does with just 5 amps. So what do we make of this? Do I order a 4" prop and see if that's the sweet spot?
Jun 12, 2018, 02:54 PM
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Out of interest, is there any specific reason for the target speeds? I'd have thought that you would want to start with the 'speed for maximum range' and then find the prop that best matches that speed. That speed will depend heavily on the characteristics of your plane....and of course short of a wind tunnel, there is probably no 'direct' method to determine it. I'd love to be proven wrong on this

What I did was to use Wh/km as the main performance metric. Looks like you have all you need to get this number too. Do enough test flights at different speeds and with different props (obviously changing one input at a time, and taking note of Don's input on props) and you should get a pretty good feel for what that speed is and what the best prop and motor combo are to get the lowest Wh/km. If the optimal speed is not close to your 60km/h or 60mph, then you might be better off redesigning/ modifying the plane for your 60 - 60 speed targets.

Also, my experience is that max throttle speed is not all that useful (unless you plan to fly at max throttle, which is already sub-optimal) - average current (in the absence of Wh/km) at the best cruise speed is what matters when deciding which prop is best.

I'm eager to compare notes and hear if there are any shortcuts/ more analytical approaches to this problem....it's been on my mind for most of the year
Jun 12, 2018, 06:57 PM
Registered User
Nice round numbers, an easy goal to remember. That and flying fpv slow is a little boring. I think it's super cool what the guys are doing with the Wingwing and mini Goblin, but bobbing along for 2+ hours isn't my thing.

I have a super simple delta that flies fast and close. It's got a mid engine so the propwash flows right over the control surfaces which makes it very responsive. 70-80 kph at a meter off the ground is a rush! If I crack it up, easy to repair or replace.

I flew that tiny prop full throttle because I was amused it only took 7 amps. I had test flights at 3 amps and 5 amps, 7 seemed reasonable.

My plane flies fine with a 5000 mAh, a bigger lipo than that starts to get spendy. I want to run it for 20 mins; 3c.
5 Ah * 3c * 0.80 (max discharge) means I have ~12 amps to go 60 kph. Is my math good?

If so, I'll put the 5" prop back on, kick the throttle up, and see what she does.
Jun 13, 2018, 01:35 AM
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Extreme Sports's Avatar
The calc you want to do is multiply the mAh available by the voltage. The 3 of 3S on its own is meaningless. So the calc you want is 5,000 x 0.8 x 11.1 = 44,4 Wh. This is a measure of energy, similar to a Joule and gives a number you can compare regardless of battery voltage. Divide by km and you've got a number that you can compare across different flight distances, different battery voltages and with other people using different planes. I did a google search and came across a RCG thread where someone has kept a record of Wh/km for long range flights - very helpful as he also lists the plane type and average speed, so it not only gave me a good idea that my plane was already 'in the ball-park' for good range, but also just how much better I could expect to get if I redesigned the plane. I'll post if I can find it again.

Regardless of the metric, the trick is to see which prop takes you the furtherest at your desired speed with that 44Wh. If you can get close to the average 1Wh/km I get, then you've got 44km range already. What I've been doing is to measure the mAh put back when I recharge after the flight and from that and the flight distance calculate the Wh/km for that flight configuration. If one is more interested in duration than distance (as the OP is) you just divide by the flight time.

I get what you say about the need for some speed. The reason why I suggest trying to find the best flight speed for range is that it will tell you how close your airframe is to being 'right' for the task. By pure chance my wing seems most suited to a speed that is very close to what you want. Be useful to see where yours slots in, and if not close enough, what can be done to reduce drag or if it will be easier to redesign for the target speed. Or just use a larger lipo (and in that case, how much larger).

Care to post a pic of your plane? I suspect the km/h target is within reach of most planes, but the mph target may require a more purpose designed airframe.

PS: I am aware we are moving OT but we are still on the theme of maximizing flight distance/ duration for simple scratchbuilds and I suspect Gil (the OP) will find this discussion interesting. Happy to move it if he asks us to.
Last edited by Extreme Sports; Jun 13, 2018 at 02:22 AM.
Jun 13, 2018, 04:26 AM
jofro
I find my home built deltas rather in-efficient carrying 3-8 kilos of lipos to handle up to 240 Amp current draws for short 3-5 minute flight time but they turn every one heads around every time I fly one.
Jun 13, 2018, 03:36 PM
Registered User
jofro if I saw those giant intergalactic spaceships come out at my fly field, I'd run for cover!
Jun 13, 2018, 05:06 PM
Registered User
Quote:
Originally Posted by Extreme Sports
The 3 of 3S on its own is meaningless.
That was 3c, meaning I'm going to draw down the lipo at 3c, or (60/3=) 20 minutes. Often I make up my own math. Sometimes it works. Rarely does it make a lot of sense to anyone else.

Here's my mAh used for my test flights. Yes 3s but that's common to all my lipos so we can factor it out and look at the mAh to see how much lipo I need.

44:16 min, 33.5 km, 2666 mAh = 45 kph @ 79 mAh / km (3 amp with 5045 prop)
26:11 min, 22.5 km, 2117 mAh = 52 kph @ 94 mAh / km (5 amp with 5045 prop)
17:03 min, 15.3 km, 1822 mAh = 54 kph @ 119 mAh / km (7 amp with 3545 prop)

That's the reading from my onboard ammeter, calibrated as best I can. I checked it against my charger once and it was within about 10%.

To get to my magic 60 kph with 5045 prop, it's going to take:
60-45 = 15 / 45 = 33% more speed 1.33^3 = 2.35 times more amps x 79 = 185 mAh / km @ (2.35*3=) 7 amps
60-52 = 8 / 52 = 15% more speed 1.15^3 = 1.52 times more amps x 94 = 142 mAh / km @ (1.52*5=) 7.6 amps

My 3300 mAh lipo won't quite do 20 mins, but my 5000 mAh will and with plenty to spare. I'll try 8 amps this weekend.

Here's my plane. She's had a cam and vTx taped to the top up front, small gps taped to the wing, the rear center fin removed, LRS wire whip added, and the wings were moved foreward about 3" on the fuse for balance since the pic was taken.
Jun 14, 2018, 04:31 AM
jofro
Quote:
Originally Posted by SirRexAlot
jofro if I saw those giant intergalactic spaceships come out at my fly field, I'd run for cover!
No need to run for cover, they are all under my full command.
M4H00605 (3 min 58 sec)

M4H00847 (7 min 59 sec)
Jun 14, 2018, 06:07 AM
gpw
gpw
“There’s no place like Foam”
gpw's Avatar
Cool !!!
Latest blog entry: Lost plans
Jun 14, 2018, 10:01 AM
An itch?. Scratch build.
eflightray's Avatar
Quote:
Originally Posted by SirRexAlot
I got another datapoint for my fpv plane. Turns out I don't have a 2 blade 4x4.5 bullnose prop, but I do have a 2 blade 3.5x4.5 bullnose. It only pulls ~7 amps max, so I flew around for a lipo at full throttle. It started off about 7.5 amps and at the end of the battery around 6.8 amps.

3.5x4.5 : 15.3km 17:03 min = 54 kph

The downside is this little prop is terribly loud, I could hear it out to 400 meters. The other problem is I flubbed a couple launches because this tiny prop has so little extra torque. I could learn to live with it if the numbers were amazing, but they weren't.

This 3.5" prop does speed with 7 amps that the 5" prop does with just 5 amps. So what do we make of this? Do I order a 4" prop and see if that's the sweet spot?
Have a look at this thread ---

Prop Slot Noise Reduction Design and Tests

Although it's about prop-in-slot models, most of the information is equally valid for pusher props that are close to the trailing edge.

Also high speed props can get extremely noisy if the prop tips are getting near the speed of sound.

Annoying noise from model planes is often the precursor to local resident complaints and getting a flying site banned.

.


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