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10-03-2015 05:28 PM | ||
stegl |
Re: Bench tested some props - same power, more thrust? quite often users dwell too much on thrust and forget about airspeed as they both have to work hand in hand. Meaning if the pitch is very low with a long prop ; yes you will have gobs of thrust but little airspeed to get your aircraft flying. On the other extreme if you go with a short prop and lots of pitch ; the airspeed is there but no thrust. Has to be a happy medium. As suggest earlier, Ecalc and others are a good starting point and then it gets down to actual prop testing to get the most efficient prop for your application. |
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10-03-2015 01:08 PM | ||
gallenwolf |
Re: Bench tested some props - same power, more thrust? Hey all! So after assembling the Teksumo, it was way to fast for me to fly, so I DIY'd a slightly smaller wing with a much better wing loading (5.9oz/sq.ft) and I got her dialed in this morning. I'm running an Emax CF2822/1200Kv motor swinging an APC 7x3.8SFW. There is a very wide speed range between 35% ~ 65% throttle, but if I push out the throttle slowly to max, she doesn't seem to go faster, but makes a rather loud noise. Is this a prop stall? Should I switch to a 7x4E or 7x5E? Or even 8x4E? I actually care less for speed - I want my throttle to have more granularity, and not be limited to a very small area of movement on the throttle. Ecalc indicates that moving to a higher pitch prop decreases runtime and increases top end speed. Alternatively, I put in a smaller 80w motor (Cobra 2204/58 1080Kv) and the runtime goes up a fair bit, and the top speed also goes down, so perhaps I could have more throttle range use? Thanks! |
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09-27-2015 09:49 AM | ||
OLD PRAIRIE RED NECK |
Re: Bench tested some props - same power, more thrust? One of the best kept secrets in out hobby! |
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09-24-2015 08:19 PM | ||
moo |
Re: Bench tested some props - same power, more thrust? sorry, yes it was THAT prop, OS46LA |
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09-14-2015 07:10 AM | ||
OLD PRAIRIE RED NECK |
Re: Bench tested some props - same power, more thrust? Quote:
My experiment was RPM related. The engine was a weed eater that I tried to convert to aircraft use. I was having trouble getting it to rev up to a usable RPM value due to a designed in RPM limit(transfer port timing it turned out to be) and a 4" pitch gave me 7000 RPM. Anything steeper and it would drop to 5000 or less! Trust me, a 4" pitch at 7000 RPM is not much speed for a 7' span model airplane! Your 4" pitch on a .40 glow engine would be in the 10 to 12,000 RPM range if memory serves. A whole lot more forward speed and as you said far better suited to the application. I once had a Kadet MKII with an OS Wankel on it. The best prop after experimenting turned out to be a 10 x 3. Again more than adequate speed as the static RPM was 17,500RPM and it unloaded big time in the air! The original poster is dealing with electric motors and in my experience changing prop sizes does not produce the dramatic RPM changes that infernal combustion engines experience. The power draw is what changes big time on them and he is on the right track by testing power draw as he experiments with prop sizes. |
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09-13-2015 06:11 PM | ||
moo |
Re: Bench tested some props - same power, more thrust? Quote:
maybe as a propellor approaches zero forward speed it acts simmilarly to vortex ring state in helicopters and this article on static thrust of propellors |
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09-13-2015 04:34 PM | ||
moo |
Re: Bench tested some props - same power, more thrust? Quote:
I was flying my trainer inverted the next week. |
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09-13-2015 01:09 PM | ||
michaely |
Re: Bench tested some props - same power, more thrust? Quote:
Ecalc does offer an estimate of the "stall speed" above which the prop/engine will develop "full thrust". Low and high quality motors differences... my understanding, apart from malfunctioning such as losing magnets, is that higher quality motors are more efficient and therefore lose less power in waste heat and typically a 5% increase in efficiency means a multiple of 5% (15 -20%?) or so increase in end power. good luck |
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09-13-2015 12:44 PM | ||
gallenwolf |
Re: Bench tested some props - same power, more thrust? Hi Mervin, That is a very clear explanation! I think I understand now. Static thrust testing is more about measuring current draw with the prop, in the end we need both thrust and speed to keep our planes in the air, and flying them is the only way to know! Hi Michael! I googled prop stalling too, but I must admit I don't really understand how a prop stalls, only that some props/motor combinations require a certain airspeed (?) to get out of the prop stall condition for forward flight. I've subscribed to ecalc, so far it's proven most illuminating. It's also very interesting to see the performance of the cheaper motors vs the more expensive branded ones. Thanks all! |
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09-12-2015 11:36 AM | ||
michaely |
Re: Bench tested some props - same power, more thrust? Alvin, here are some threads that pop up when you google "prop stalling" https://www.google.ca/search?sourcei...eller+stalling The type of plane you are flying - racer, 3d or trainer/sport/scale - does influence how important static thrust, prop stalling etc are .... but these are generally pretty advanced topics I have found Eclac, a performance software for electric motors to be very useful, especially for screening motor purchases that would be good for a particular application and the software also generates lots of information on predicted flight performance.... there is a free version which has a limited number of motors you can find information on; the full version is $5 u.s. a year and I have found it very useful. btw, with a kitchen digital scale you can build a physical thrust measuring tool.... there are online build threads on this good luck |
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