| Overview: What we would like for our successful electric launch is: 1) A bit of additional thrust at start to assist us in acceleration. We get this automatically since thrust is max at zero speed but we may need to reduce the throttle a bit so that we don't overload the motor. However, most motors/controllers can sustain about a 20% increase in load for about 20 seconds. 2) Maximum efficiency at our best rate of climb speed. This is about 35 mph for the Monarch with the assumption that the best rate of climb speed is a little above the minimum sink speed (my assumption). And therefore we will want the propeller designed with a pitch speed of about 40 mph. 3) Once we attain liftoff we will want to pull back on the stick to keep the flight speed at 35 mph. If everything is as calculated (including efficiencies), this should result in a climb rate of 400 feet per minute or better. Since the prop has unloaded by 20%, we may need to advance the throttle (more voltage) to achieve the max. prop loading required. 4) As we get to a higher altitude, the air gets thinner and the prop will continue to unload but this should not be too significant since we are climbing less that 2000'. The voltage will also be bleeding off so the climb rate will drop off a bit towards the end of the climb. How can this be improved? I am not a prop person, so I don't know what can be accomplished but we need at least 80% prop efficiency to make this work. 1) The thrust at start can be increased without overloading the motor by reducing the pitch. So if we had a variable pitch prop, we could increase the thrust quite a bit for takeoff and then increase the pitch as we approach liftoff so that we get the maximum efficiency on the prop. Good idea but adds complexity/cost. 2) Larger diameter lower speed prop will be more efficient. This can be done by lowering the Kv of the motor (more windings) or through a gear box. I am asking one motor vendor about this. The gearbox adds weight and cost and a little penalty in efficiency but might be worth it. Bottom line is we need an 80% or better prop matched to the power characteristics of the motor/gearbox. So where is that propeller expert? Let's get out of the 10-15HP mindset and into the 7-9 HP region. Electric power is precious and my calculations indicate that we can achieve 400 ft/minute climb with this amount of power if we can achieve high efficiency in the motor/propellor arena. I still think you would be much better off with one of the two stroke opposite twins, delivering around 20 hp max. and being fairly quiet if properly silenced. The total weight of the power plant would be MUCH LESS than with an electric setup and you would have power EVERY TIME YOU NEED IT. |
| Calculations:
Calculations for Electric launch Monarch G Specs: Empty weight - 175# fiberglass, 155# carbon Electronics, motors, batteries - 35# Total - 190# Pilot - 180# Gear - 30# Flying weight 400# Min sink 138 `/minute @ 30 mph HP for zero sink = 400#*150'/min/60min/sec/550 `#/sec/hp = 1.8hp HP for 200 `/minute climb @ 30mph (min sink = best rate of climb) = 2.4hp HP for 400 `/min climb = 1.8+2.4+2.4 = 6.6hp HP for two 3.5kw motors = 7kw/.75kw/hp = 9.3hp ( One Predator will do @ 85% efficiency brushless motors = 7.9hp With 90% efficient low speed props (1500-2500rpm) = 7.1hp With 80% efficient low speed props = 6.3 hp Since we need 6.6hp from above, we won't quite make 400'/min with an For 2000' climb @ 400'/min. we need 5 minutes of power plus run to Our motor(s) draw 7kw * .1 hour = 700watt hour of energy required. This corresponds to a 14,000 mah battery at 50 volts. The EMOTEK battery is 9600 mah which will provide 70% of what would be needed for 2000'. So we can only get to 1400' with this battery. Is this good enough for you Mat??? Mat's reply " yes this is quite adequate
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| Electric motor theory: And let me say that with a 7HP electric motor you probably get more power that you do with a 10HP gasoline engine (this is mainly due the how the power is measured and sustained). Your biggest risk in electrics is to fixate on peak power which can usually only be sustained for 20 seconds due to heat buildup. We are only interested in "continuous" power delivery The DC motor is a constant RPM device based on the applied voltage. The voltage constant Kv (RPM/Volt) is part of the motor design (the more the turns the lower the Kv). RPM is equal to Kv x V. This will be important to us because we will want a large prop for efficiency reasons and this means lower RPM.
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| Batteries: The LiPo battery will charge to 4.2 volts per cell but rapidly drops to 3.9 and maintains an average of 3.7 volts until it is about 80% depleted. Cell life is compromised when the cells are discharged beyond about 80% capacity. Bottom line is that the battery pack will supply the motor higher voltage at start and slowly bleed down during flight. This characteristic also applys to NMH and NiCad except the LiPo decay is flatter.
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| Electric motors: You will need something like 12 hp and 15 The electric setup would need a large speed reduction. You are looking at an all up weight of 30-60 lbs depending on design choices. I would guess costs I assumed .8 efficiency. at 30mph and 12 horsepower that would be 150 lbf. Thats a big prop. Probably too big to fit on the monarch. |
| Battery recharging: On the subject of recharge: 1) LiPo takes a minumum of one hour to recharge. I say minumum because to ensure max performance you need to balance the cells (this can be done automatically but that adds time). Also, I would recommend a minimum of 14,000 mah battery (two 7000 mah packs in parallel) so that you would not discharge the cells at greater that a 10C rate (even though the cells are 20C rated) to prolong life. If you did a 600-1000' launch, you might get two flights prior to recharge. You will also need to buy charges/balancers adequate for the job no matter what type a battery you use. 2) NiCads can be recharged in 20 minutes. Although they are a lot heavier, this might be an advantage particularly during the test period. The motor controller/motor doesn't care what the source of DC voltage is, so any type will work. You can potentially get a faster recharge with 220 vac, if that power is available.
Whatever the case, I'd recommend two packs minimum particularly during the prototype phase. |
| Turbjets:
Another alternative would be model turbojets. They are available in the 30-45 lb thrust class and cost $3-4k. The installation is easy and a gallon
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| Gas motors: The props listed at the bottom of the page will just fit into a Monarch as a pusher configuration behind the pilot.These are using direct drive also (Lighter). http://www.desertaircraft.com/engines_detail.php?Page=DA-150 The baseline propulsion would be small gasoline engine of about 10-20 hp. I think these run about $1000 or $2000 complete with reduction drive ready
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| Gas motors: The props listed at the bottom of the page will just fit into a Monarch as a pusher configuration behind the pilot.These are using direct drive also (Lighter). http://www.desertaircraft.com/engines_detail.php?Page=DA-150 The baseline propulsion would be small gasoline engine of about 10-20 hp. I think these run about $1000 or $2000 complete with reduction drive ready
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| Propellors: The propeller converts the energy provided by the motor into thrust. The power absorbed by the prop is a function of the diameter to the 4th power, pitch and RPM cubed. So small changes in diameter and RPM have more affect on power than pitch. Pitch speed a design factor in propeller design (based on RPM and pitch) that represents the air speed at which maximum efficiency is achieved. If the aircraft goes faster than pitch speed, efficiency props off at a high rate (like a stalled wing). Thrust is maximum at zero speed and reduces to half that amount at pitch speed. In the acceleration from zero to pitch speed, the propeller linerally unloads by about 20%. And if we over prop our motor so that it cannot supply the power to reach the RPM demanded by the voltage constant, it will overload the motor and destroy it (avoid at all costs).
Do you know what static thrust they are getting out of those props? I have a prop model that does a very good job of correlating performance at speed given diameter, pitch, HP & static thrust. I suspect that it is less than 50 lbs static and this will drop off with speed. We will probably have to gear it down and put on a bigger prop. I'll cut some numbers and see what kind of diameter would be needed. What is the largest diameter prop we can fit onto the Monarch? Maybe I should work backwards from there and calculate the needed horsepower to drive it.
Might wing mounting, either two propellers or a single offset thrust motor (radical but already achieved) provide better propeller protection and the possibility of larger diameters? Holger Rochelt and his group have vast experience in this arena. Sadly he has passed on. I suspect his compatriots will freely answer questions. It is their mission to spread solar electric flight. http://www.solair.de/ The propeller I designed for the power pack (I mentioned it in a previous message) had an efficiency of 78 % @ 3000 rpm., using 21 hp through a reduction drive Being a 41"x 28"( 1041 mm x 711 mm) it developed a max thrust of 178 lb @ 3300 rpm., using 25 hp. To obtain a higher efficiency one would have to use a prop with sonic tips, which I would not recommend because of high noise and strong structural solicitation. If viable (spacewise), I would increase the diameter of the prop to 48"or 52" and slightly reduce the pitch (say to 20-22"), BUT I SERIOUSLY DOUBT 7 HP WILL BE SUFFICIENT FOR LAUNCHING THE MONARCH. I think AT LEAST 15 HP will be required . The reduction drive I built for that power plant weighed 4 lb (1.8 kg) and it offered very low friction, hence not many losses. I still have that design as well.
the design of the prop (41"X 28"). IT ABSORBS 25.6 HP @ 3300 RPM, DELIVERING 178 LB OF STATIC THRUST
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It was optimized for a cruise speed of 30-40 mph at about 2400-2700 rpm. At low cruise (2400 rpm) the power plant was using about 14-15 hp, of which 20 % were absorbed by the reduction drive. To use less power you may be able to slightly narrow the blade, without changing its shapes and pattern. If you intend to increase the prop's diameter to achieve extra thrust, you will probably need to use a reduction drive, which in turns, as you know, will absorb power etc.