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Jan 24, 2018, 02:37 AM
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cioro's Avatar
Thank you Naterater!
Wow, this is something different. I'm stopping myself to hit the buy button just because I want to see more of your work.
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Jan 30, 2018, 10:10 AM
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Thread OP

Initial Data Analysis


The MTD testing is unofficially complete. Here are some thought provoking results - You can fly at least 5 hours on li-ion batteries. At 17m/s, that's 306km (190mi).
Jan 30, 2018, 02:34 PM
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Beerwiser's Avatar
Hopefully the titan comes in with those sort of numbers. My fat fingers love the room in there. Thanks for the pictures, really nice job on them.
Feb 06, 2018, 11:30 AM
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Thread OP

MTD data


Thank you to everyone that has shown support for this project. I present to you the first round of data from the MTD.

Bottom Line: The MTD is a very efficient platform capable of multi-hour flight with significant payload.

Test Setup:
MTD plane with all options outlined in earlier post. Skywalker 510kv motors.
METAR: 282054Z 34012KT 10SM BKN025 M04/M10 A3040
Test Altitude: ~1300 feet MSL
Air Density: ~1.26kg/m^3
Density Altitude: ~(-900 feet)
Air condition: Moderately turbulent for RC aircraft
Stall Speed: 12-13 m/s at 2300g payload, 14m/s at 3400g payload
Stall Characteristics: Slow stable descent with full up elevator
Average Cruise Elevator: Neutral
Collection Method: ~2 minute track with five right 90-degree turns and 1 left 90-degree turn. mAh used at start and end with average voltage recorded.

Test Notes:
Weights in the following charts are expressed as payload weight - Payload included Multistar Li-Po batteries and steel weights.
2300g payload = 4960g all up weight. Payload does not include the weight of motors, props, wiring, servos, autopilot, radios, etc. Empty weight was published in a previous post. All numbers are total numbers (not to be confused with per motor numbers). The numbers are also voltage-independent so that battery state of charge does not affect the data. Only the 12x12 and 13x8 props pulled more than half of the ESC rated amps during takeoff. Both of those props pulled about 30 amps per motor. This is good because the propulsion system has the capacity to handle the larger props (ex: 16x10) at a potentially reduced max throttle. Hand launching into the wind was very easy, and the aircraft wanted to fly out of my hand. No running or significant pushing was necessary. I did break a few tips off of some of my props during landing only when the ESC brake was not completely engaged. When the brake was engaged, the slick fabric runway pushed the prop out of the way without damage. When the brake was not engaged, the prop would strike the runway and dig into the fabric, resulting in tip damage. Height and energy control were managed by ardupilot plane 3.8.4 TECS, and the average error was less then 1m rms.

Data Analysis:
The higher pitched props significantly improve performance for fixed-wing aircraft. This agrees with prop performance data published by APC: https://www.apcprop.com/technical-in...formance-data/. Power draws below 100watts are possible at slow airspeeds and relatively light weights. This allows for significant area coverage in still air. With no wind, the theoretical maximum range of the aircraft appears to be about 350km total air distance over 5.6 hours (12x12 prop, 17.5 m/s cruise, 3600g payload - three 14,000mAh 6s Li-Ion packs). If you would like to review the calculations and assumptions, please PM me. This may sound "amazing," but keep in mind that any payload that is not battery reduces the range. In addition, the atmospheric conditions may be different for your location.

Slow airspeeds with heavy payloads are much less efficient. Induced drag at the higher angle of attack becomes a significant limitation.

Future Steps:
Verify the efficiency and endurance using li-Ion battery packs. Then remove components and test other aircraft using higher pitched props. I do not see any advantage to using lower pitched props, except for VTOL "Zero-speed" thrust.

Special Notes: Power (in watts) = Current (in amps) x Voltage (in volts); Energy (in joules) = Power (in watts) * time (in seconds). True battery capacity to me is calculated in watt*hours, not mAh. Watt*hours are calculated by the average power * the time to discharge. Li-ion batteries experience significant voltage drop while the battery is used, so using watt*hours is a more accurate measure of the true battery energy capacity.
Feb 07, 2018, 10:18 AM
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mike_o's Avatar
Very interesting, subbed. I'd expect you to get better results with 10x7 props. Unfortunately very few high pitched CW/CCW pairs are available. Banggood had some APC clones in 8x8 and 9x9 pairs, but I havent seen above 9".
Feb 07, 2018, 10:42 AM
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Thread OP
Quote:
Originally Posted by mike_o
I'd expect you to get better results with 10x7 props.
Do you have any data that would suggest that the 10x7 props are more efficient? The information that I have from APC data shows that the 12x12's are a little under 85% efficient at the speeds and power that I'm operating at. The 10x7 props top out around 80% efficiency for the same speed/power range, plus they spin 50% faster, so I'd need higher kv motors (not necessarily bad, but I've read about everywhere that a slow turning prop is more efficient).
Feb 07, 2018, 11:20 AM
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mike_o's Avatar
You mentioned using 12x6 in the earlier post, so that was what I commented on. I'm not surprised that the "square" 12x12 performs better.

I base my comment on many trials with 1500-2500mm FPV platforms. My models have performed best - efficiency wise - with smaller, higher pitched props than what most other posters use. My point of view is that the prop only needs to provide just the required thrust at a given (pitch) speed, and that most designs are optimized for ease of take off as well as efficiency. I tend to prioritize efficiency and pay the price when I launch the plane with a stalled out, high pithc prop and the plane crawls over the field If the prop is too large in diameter, there will be excessive drag from the propblade itself before sufficient "pitchthrust" is achieved.

Some examples:
My 1720mm Talon uses a 12x10 APCe
My 1340mm Skywalker Falcon uses a 7x6 APCe
Both with very good results

I talked my buddy Ivan into swapping the SW Wall-E2000 stock 10x6x3 props for APC 10x7, improving efficiency by some 20-25%, all other things equal.

But of course, you're looking into fairly heavy planes, which may require more thrust than my anaemic ones
Last edited by mike_o; Feb 07, 2018 at 11:31 AM.
Feb 07, 2018, 02:57 PM
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RocketMouse's Avatar
Did you compare 1720mm Talon with 12x10 APC vs 12x8 and 13x8 by any chance?
I believe that different props efficiency also depends on takeoff weight.
Feb 07, 2018, 03:25 PM
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Thread OP
Quote:
Originally Posted by RocketMouse
I believe that different props efficiency also depends on takeoff weight.
I do not believe this to be true. The system and airfoil efficiency may depend on takeoff weight, but the primary variables affecting the prop efficiency are the oncoming air velocity, air density, and rotational velocity of the prop. Additional weight may require more rotational speed of the prop to produce additional thrust, but it does not directly affect the propeller efficiency.

Based on the data above, additional weight requires a higher airspeed to be efficient. You can see the data above showing that high weights cause a power increase at low airspeeds. It's most likely that induced drag is causing the inefficiency at high weight and low airspeed.
Feb 07, 2018, 04:26 PM
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RocketMouse's Avatar
Let's make it simple: more weight - higher the speed, and so different propellers are more efficient at different speeds, if so then it does depend on takeoff weight.
Feb 07, 2018, 04:50 PM
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Thread OP
I do see that the weight may have an effect on speed, and the speed has an effect on the prop choice (we agree). I'm just saying that the effect of weight on prop efficiency becomes practically negligible compared to the efficiency of the primary airfoil in these tests. Maybe that's just because the efficient flight envelope is quite small with these aircraft.
Feb 07, 2018, 05:05 PM
Registered User
Nice work!
Feb 08, 2018, 02:30 AM
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Al Austria's Avatar
Quote:
Originally Posted by RocketMouse
Let's make it simple: more weight - higher the speed, and so different propellers are more efficient at different speeds, if so then it does depend on takeoff weight.
Making it simple, takeoff weight factors into the AIRCRAFT'S efficiency, not the propeller's.

An increase in weight requires an equal increase lift for a 0 net climb rate. For an increase in weight and a constant airspeed, you need to increase AoA to increase lift(greater induced drag=less efficient). Conversely, for an increase in weight and a constant AoA, you need to increase airspeed to increase lift(greater parasite drag=less efficient). Both of these scenarios assume you're already at the bottom of the drag curve.

Propeller selection is a balance of performance requirements. Higher pitch props will certainly realize efficiency gains at higher airspeeds, but static thrust will be reduced compared to a larger diameter, lower pitch prop. If your aircraft is heavy, a larger diameter, lower pitch propeller might be advantageous if the aircraft in question is to be hand launched as static thrust would presumably be better, but perhaps at the expense of cruise efficiency.

naterater, excellent work. Look forward to reading more!
Feb 08, 2018, 02:35 AM
Registered User
RocketMouse's Avatar
Well said!


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