prop questions

[ghost civic]

I have previously only used lower kv, small motors spinning 8x4 or 9x5 slow fly props.

but I just flew a new DTFB profile I build with a 2212-6 2200kv motor. The prop that came with it was a orange 6x3.5 prop...this broke on the first attempt to fly (plane poorly trimmed)
The only other similar prop I had to try was a 3 bladed 7x3.5. The plane did fly, had plenty of thrust but not nearly the speed expected.(could have been as high as 75mph, but was more like 50) and above 60-65% throttle the sound changed, which I think was the prop blades bending. Both the 3 blade prop and the orange prop were much more flexible than the slow-fly 8'' props I am used to.

I understand that a smaller diameter props makes sense on a higher KV motor, but shouldn't that prop be at least as stiff as a slow fly prop? The 3 blade prop is sized to fit on my motor, but surely it is meant for a much smaller plane? Am I missing something?

Here is a pic of the motor and the original props.

If anyone can recommend a good prop for this type of motor....or maybe explain the different types of props (I notice HK has many different even for electric planes) that would be a huge help. The ones I have don't hold up on this motor and break very easy.

 

[PhenomPilot]

Hey goat civic. I don't have that particular motor but it is very similar in kv and amperage to 2204 quad motors. Most of those guys are running 5x4, 6x3 and 6x4. I would check out the 6x4.5 props some of these guys rave about for speed or some 7x4 props. Three bladed 7x3.5 might be too big and inefficient for that motor. The noise you hear could be it bogging down.
As far as flex goes, I don't think it should matter too much because the outward force at those kinds of rpm should keep that prop from flexing too much. The gemfan 6x4.5 props are supposedly stiffer than the other cheap 250 quad props though. Get a few of each if you can and try them out.

 

[LooseBruce]

Hi, I am no expert by any means. I do try to avoid any plastic props because I break them so easily on my rigid wing planes.. I like the composite APC slow fly props the best and will go out of my way to get them. I also have to go out of my way to break them because they are MUCH tougher than the plastic props like GWS. I do however break them on occasion.
Just my 2 cents,
LooseBruce

 

[PhenomPilot]

Prop brands are also kinda like clothes brands. One size between two brands aren't always similar in fit. A lot of guys, will use a amp meter and scale to compare props before they go fly. I usually buy a few different ones and try them out to see which one I like. As long as the motor isn't too warm then I am happy.

 

[ghost civic]

I did figure the 3 blade 7'' prop was a bit much for the motor, but wanted to get the bird in the air...and it was only 15 degrees outside. I figured the cold temps would let the motor spin that prop without getting hot.

upon taking the plane apart, I did find one of the motor mount screws was loose. that could have explained the noise at full throttle.

still, I was surprised that this motor, with that prop wasn't faster. the AUW was around 500g. a thrust calc shows thrust could be in the 5 pound range...felt more like 3 pounds at most (guessing, throttling up with plane in hand)

Even if the prop was not flexing, I still want stronger ones, the crash that broke the orange one was rough but the landing that broke the 3 blade prop was not bad at all. But it was a belly landing with a 3 blade, so maybe the blade just didn't have anywhere to go.

 

[PhenomPilot]

15 degrees makes most plastics brittle. I haven't had luck with 3 bladed props either though. Maybe because there is less room at the hub to join the blade? When it warms up for you July, they shouldn't break as easily

 

[ghost civic]

I hadn't factored in the cold effecting the props. I supposed 15 degrees plus the wing chill (flying threw the air at 30-40mph) would make them pretty cold.

 

[johnmw]

i read somewhere on Master Airscrew guidelines on replacing 2 to 3 blade prop,
is that i need to drop diameter by 1-2" while increasing pitch by 1-2" to compensate the 3rd blade drag.
so a 6x3.5 is replaceable with 5x5 3 blade i presume.
could someone confirm this?

 

[PhenomPilot]

Johnmw, that sounds about right. I would try a 5x4 or 5x5 three blade in replacement to a 6x3.5 prop.

 

[ghost civic]

that's good info to know for future reference^

stupid cold, makes testing new planes a PITA. At one point I had to trek threw snow 3' deep to retrieve the plane. Then the cold breaks the prop.

about the only thing winter is good for in terms of RC, is for running my 1/6 and 1/7 scale rock crawlers in the snow. Other than that, im ready for summer.

 

[johnmw]

thanks PhenomPilot!