3 Blade Prop ?

[Bigeard]

Hello All ,
Ive finally decided after a lot of research to go ahead and buy the FT Spitfire speedy build (when Uk suppliers actually get them again ) as it will be my first foam build . When I do get one I intend to do a few slight mods to strengthen things up a little in certain areas . One of the other mods I would like to make is fitting a 3 blade prop with a Dubro spinner . I know regarding the prop it might mean a more powerfull motor ? and as Im not 100% sure can anyone give me advice on the motor I would need relating to what propeller size probably bigger than the one supplied ?
Thanks for any help with this .
M

 

[TheFlyingBrit]

Rather than over complicate this discussion (although I can send you the calculation if you want). Its fairly straight forward, for example: I used an Emax GT2215-09 1180kv motor in my Spitfire.
my selected battery at that time was a 3s 2200mAh as I had plenty of them.
The specification for this motor recommends with a 3s battery:
1) A10x6 prop twin blade, produces 1140g thrust at 24 amps
2) A 10x4.7 prop twin blade, produces 1250g thrust at 26 amps
The choice is yours which example you use.
If you select the option 1) 10x6 twin blade the equivalent tri blade is 9x6x3
If you select the option 2) 10x4.7 twin blade the equivalent tri blade is 9x4.7x3
I have done the calculation many times and it simply comes down to subtracting 1 from the twin blade diameter, the pitch remains the same.
So all you need to do is select the motor you plan to fit, from the motor specification find what twin blade the motor it requires (based on the battery requirement), then you can easily determine what size of tri blade to use.

 

[Bigeard]

Thanks for that FB ,Ive got to admit that maths is not my best subject but I do understand basically what your saying . I take it your s is also the FT Spitfire speed build ? what kind of flight differences would you get between the two types of Tri blades ?
Thanks
M

 

[TheFlyingBrit]

Never flown the Spitfire with the tri-blade to be honest and it is the Simple build not the Master build. The only reason I never fitted a multiple blade on my Spitfire was simply because I couldn't find suitable spinner at that time. Plus to be really authentic it needs to be a 4 blade not 3.

Theoretically the math's suggest the thrust will be the same. However, in reality there is some debate and most of the comments I read suggest that tri-blades are not as efficient. A 2-blade propeller produces two pressure pulses per revolution, whereas a 3-blade propeller will produce three smaller pulses per revolution for the same amount of total thrust. As a result, the 3-blade prop will be inherently smoother and therefore quieter. Is it as efficient I will let you test it on a rig and let me know

Why use one then ? Well with bigger more powerful engines being produced for fighters during WW2, it inherently meant bigger props would be needed. This caused a problem as it impacted ground clearance with the 2 blade prop design, so 3 blade and 4 blade props where introduced to allow smaller diameters to be used, thus eliminating the ground clearance problem.

I had one set up on my Sportster and I was of the opposite opinion, as I felt it had more thrust than the 2 blade version. I eventually reverted back to the 2 blade, as felt the performance with a 2 blade better suited the model and how I thought it should fly.

 

[mrjdstewart]

you planning on landing gear? if not, don't waste money on a 3 blade. you will break it almost every landing.

 

[TheFlyingBrit]

you planning on landing gear? if not, don't waste money on a 3 blade. you will break it almost every landing.

Good point, I put one on my Edge 540 recently and had a rough landing where my landing gear buckled, it sheared all 3 blades off.
As I fitted fixed landing gear on my Spitfire, didn't think about that scenario with a belly lander.

 

[Bigeard]

Never flown the Spitfire with the tri-blade to be honest and it is the Simple build not the Master build. The only reason I never fitted a multiple blade on my Spitfire was simply because I couldn't find suitable spinner at that time. Plus to be really authentic it needs to be a 4 blade not 3.

Theoretically the math's suggest the thrust will be the same. However, in reality there is some debate and most of the comments I read suggest that tri-blades are not as efficient. A 2-blade propeller produces two pressure pulses per revolution, whereas a 3-blade propeller will produce three smaller pulses per revolution for the same amount of total thrust. As a result, the 3-blade prop will be inherently smoother and therefore quieter. Is it as efficient I will let you test it on a rig and let me know

Why use one then ? Well with bigger more powerful engines being produced for fighters during WW2, it inherently meant bigger props would be needed. This caused a problem as it impacted ground clearance with the 2 blade prop design, so 3 blade and 4 blade props where introduced to allow smaller diameters to be used, thus eliminating the ground clearance problem.

I had one set up on my Sportster and I was of the opposite opinion, as I felt it had more thrust than the 2 blade version. I eventually reverted back to the 2 blade, as felt the performance with a 2 blade better suited the model and how I thought it should fly.

 

[Bigeard]

Thanks FB , really interesting and informative reply

 

[Bigeard]

you planning on landing gear? if not, don't waste money on a 3 blade. you will break it almost every landing.

Good point thanks

 

[Hondo76251]

You can get lost in the weeds in a hurry debating props, simple rundown as i understand it is:

If you go up 1 in pitch, go down 1 in length. If you add a blade go down one in pitch or length at least, maybe both.

Much beyond that and you should probably have a reliable way to measure the draw on the motor if you want to experiment with props.

3 blad props look sweet on the ground but for most pilots, the actual benifits are pretty hard to feel in flight. 2 bades will typically work much better.

 

[Bigeard]

You can get lost in the weeds in a hurry debating props, simple rundown as i understand it is:

If you go up 1 in pitch, go down 1 in length. If you add a blade go down one in pitch or length at least, maybe both.

Much beyond that and you should probably have a reliable way to measure the draw on the motor if you want to experiment with props.

3 blad props look sweet on the ground but for most pilots, the actual benifits are pretty hard to feel in flight. 2 bades will typically work much better.

Thanks Hondo76251 , Think I should only stick to a twin prop then

 

[TheFlyingBrit]

Thanks Hondo76251 , Think I should only stick to a twin prop then

Try both then decide yourself