Prop choice

The thrust may be greater, but it will drop off faster at any sort of flying speed.

You don't say what speed your plane flies at, or what form of motor is driving it, but frankly a 10x7 is usually a better bet than an 11x5 ..and a 12x6 will load the engine up a lot more.

(a) If you're happy with the performance, why change?

(b) What are you trying to do?

If you go to a lower pitch, higher diameter prop your static thrust will increase. However, the lower the pitch of the prop the sooner the prop will 'unload' and cease to deliver thrust. The rule of thumb for sport flying is to choose a pitch speed three times the stall speed of the aircraft.

Using a three bladed prop of lower pitch will have similar results, except that you will lose some efficiency. If it's really static thrust that you're after, at the expense of high speed then go for it. 3D models are a good example of where you _do_ want lots of static thrust and don't necessarily care about top-end speed -- and indeed those models have large, low pitch props.

You're both right, I should provide more info. I am flying an electric powered by an AXI 2212/34. The plane weighs 21 ounce. As for flying speed, the sort of speeds sport fliers do. That is sometimes flat out at about 20 Amps draw, sometimes very slowly. What I'm trying to do is some verticals, nothing wild but just for the fun of it.

The batteries I use are 3S Thunderpower, 11.1 V and provide 2100 Amps. My friend flies all sorts of planes, electric, foamies, glow, heli, but he tested for static thrust only (He got about 26 ounces of thrust).

I did question his proposal for me to use the larger diameter 12 x 6, as I have used 12 x 8's and found them lacking. I have tried various props at different pitches and settled on the 10 x 7. This on the same kind of motor and batteries.

So why do I want to change? Well, nothing ventured, nothing gained, I guess. Would this be enough information?

Wan

Wow... That would give you 23,310 watts.. ;)

David, you're right. I meant "and provide 2100 mAh" I guess I didn't have my head on straight : )Thanks for being kind and making it fun. Wan

Wan,

Did you try running your variables through MotoCalc or a similar program to get an estimate of the resulting performance? It is pretty interesting to plug in your information and see what comes out! (MotoCalc has a free

30-day trial. I would highly recommend taking a look at it.)

Good flying, desmobob

Yeah, I knew it was a typo, but I couldn't resist. Kind of like the Wal-Mart ad a couple of years ago advertising their 500 'megawatt' Talkabout walkie-talkies..

snipped-for-privacy@toast.net wrote in news:1142464725.505609.280470 @p10g2000cwp.googlegroups.com:

If you're drawing 220W on a 21 oz sport plane, you should have unlimited vertical. I have a 23 oz sport plane (a converted Laser Stick) drawing

130W and it has a T/W of around 0.9 - not quite hovering, but very close.

So - with electrics (and with gas, for that matter) - the prop numbers don't tell the whole story. What brand and series of prop are you using? And what's the kv on that motor?

Static thrust is interesting, but it's easy to take it too seriously.

12x8 sounds big to me, unless you have a really low kv or you're running a gas prop (or maybe an APC E-series?)

Oh, hey, I switch out props all the time. Very easy & relatively cheap & safe way to experiment.

We're getting there.

Ted shuffled out of his cave and grunted these great (and sometimes not so great) words of knowledge:

The 3 blade prop is going to be less efficient.

If you are worried about ground clearance, put on some larger wheels and try the prop out. 1/2" larger diameter (or more if you want) should give you enough ground clearance to try the prop. If you like what it is doing, then you can decide if you want to stay with larger wheels or go to a longer landing gear.

If you're drawing 220W on a 21 oz sport plane, you should have unlimited vertical.

I was told to write the numbers down, but was not quite thorough, so I forgot how many watts it was drawing during the test. All I know is the static thrust of about 26 oz. gives my 21 oz. plane a 26/21 T/W ratio. This should give decent verticals.

What brand and series of prop are you using? And what's the kv on that motor?

The 12 x 6 prop was an APC E series. I may be naive, what is a kv?

Oh, hey, I switch out props all the time. Very easy & relatively cheap & safe way to experiment.

Switching props is fairly easy task and I learn from experimentation. That's why I was interested in the larger 12 x 6 prop. But I am informed here that static thrust is not the whole story for flying. Wan

Don't assume swiching props is a cheap and safe experiment on electrics. The power demands on your setup can change dramatically, so have your wattmeter hooked up if you have one. On my biggest plane, going from a 16x8 (1500 W, unsafe) to a 15x8 (1080W, safe) made a huge difference.

kv is a way of measuring how many rpm per volt you're going to get. Not an exact number, but good for comparisons....

PCPhill

Just a quick calculation but a 3 blade 11 x 6 will give about the same engine loading and thrust as the 12 x 6 and give you some increase in ground clearance. Triming the 11 x 6, 3 blade, to 10.8 inch dia should give the same engine loading and thrust as the 12 x 6.

Ray Shearer

OK, all. With all this discussion, I've changed my mind and will make a longer landing gear, try different diameter and pitched props and take notes. After all we could learn by experimentation.

Now to order one each of the props I want to try. The LHS does not have a very good supply of APC e- props in stock.

While Watt meters, Motorcalcs can provide a lot of preflight information, its the inflight data that really makes sense. True?

Wan

True. But without a Watt meter and MotoCalc, you might only make one flight with your motor, ESC, or battery....

Good flying, desmobob

My point about changing props not necessarilly being cheap.... Overtaxing your electronics gets expensive fast.

PCPhill

Indeed. The purcahse I have made that have really heloped me are

G2 simulator Whattmeter Permagrit sanding block.

For very different reasons, these unlikely pieces of stuff have saved me pounds and vastly increased my modeling satisfaction.

Static thrust is a meaningless figure unless all you are interested in is hovering or pulling out of a hover. A lot of props that give high static thrust firures don't provide much forward speed and can cause the engine to run hotter.

Fly the plane with different prop combinations until you get the flight characteristics you like.

Paul, What you said made a lot of sense. I have been trying different props on my electric planes. Actually, some props were disappointing. My friend who did the thrust test had been flying foamies and he enjoys hovering and all the things that foamies can do. I am interested in an airplane that can fly fast and true, sometimes aerobatics and a few tricks.

That's why I am looking to try other props that I have not tried, to see if I could be doing better for sheer enjoyment. Part of that enjoyment is the discovery and testing new things and approaches to flying. When I find that combinaton, life is good.

Thanks to everyone, Wan