3 blades - easier?
 

I read that with a 3 bladed prop, it is easier to make softer, more controled landings.

Can someone explain why that is and how much difference it makes?

Cheers :)

RE: 3 blades - easier?
 

That is NOT correct.

2, 3 or 4 blades it makes no difference.

HOWEVER, SOME ( notably the Hangar 9 PTS ) trainers come with a special fairly oblique low pitch 3 blade prop.

The prop acts as an air brake when you bring the throttle to idle, making it easy to slow the plane down.

That is why they are standard in all of the PTS series of planes, including the Alpha's, P-51 PTS, etc.

Some ( misinformed ) people poo-poo these props, because they are so used to their higher pitched, larger diameter props producing far more power, that they assume that these are no good.

However they are perfect for what they are designed for... newbie pilots flying an otherwise reliable setup.

When used on the PTS Alpha as designed, they help slow the plane down during landings, and in flight making it easier for the neophyte to control the plane.

You, or they are mistaking the PTS 3 blade prop and ALL other 3 blade props...

They are NOT the same.

RE: 3 blades - easier?
 

Never heard that before, some props pull less air so you can slow down more but that is diameter and pitch not number of blades.
Some people say 3 blades are less efficient than 2 blades and I think some testing on air turbulence over the blades may tend to prove this.
All that said it depends more on the airplane and pilots skills than the prop. You can only slow a plane down so much before it stalls and that is certainly not good.
I have only used a 3 blade prop on one airplane, P-51 PTS by Hangar 9, and then switch to a 2 blade. I never noticed a difference in the way it landed. Again, I had to keep some speed up on the plane to keep from stalling.

RE: 3 blades - easier?
 

The power of a prop is directly related to it's diameter. You normally need to run a smaller diameter 3 blade prop vs. a 2 blade prop. The prop "disc" not only creates thrust, it creates drag. A larger diameter low pitch prop will slow you down.
This is largely true. At the small size props we run, three or more blades will reduce efficiency. Consider the control line speed guys who run at 30,000 rpm. They will almost always use a single blade prop.
Again, mostly true since the airplane can only slow down to the point the wings fail to provide lift. Then you get a stall. Biggest advantage of using a 3 blade prop is you typically gain ground clearance.

RE: 3 blades - easier?
 

Oh ok. The 3 blades that their RTF and PTS planes come with do look markedly different. Slowing down more easily could contribute to taking part of the challenge out of making a good approach.

In the same thread where I read this claim it was also mentioned that power is less impulsive with 3 blades, so changes in power are more gradual... I thought that might partly explain it... that is if you're making a landing where you need a bit of power input before touch down... you're somewhat protected from over compensating and then making wild changes up and down.

So 3 blades being less efficient would support that general idea.

RE: 3 blades - easier?
 

In the "sport" catagory of 11k-16K RPM's I haven't seen any difference what-so-ever!

I can imagine though that at the much higher RPM's you are talking about, the interblade wake turbulence could be a BIG issue though...

However since this is a beginners thread and we are not discussing DF or racing planes, for all intents and purposes the answer is..

No, NO difference between a 2 and 3 blade prop...

RE: 3 blades - easier?
 

The H9 RTF and PTS 3 blades look like paddle wheels... they don't taper toward the tips. So I guess they act sort of like more subtle flaps but without the change in lift.

RE: 3 blades - easier?
 

Yeap, most importantly they are almost flat with very little pitch.

That means the motor spins with higher RPM's will pulling the plane through the air slowly.

However the diameter gives the plane enough "low gear" power to pull it through the air...

So when a newbie guns the engine, the plane does speed up, albeit slowly.

When they throttle it back beyond the pitch speed, the plane slows down quickly to the pitch speed of the blade.

It makes things much easier for the newbie to deal with...

- BUT - to prevent the plane from falling out of the sky in a stall, you need ot have the other training aids on... especially the wing extensions...

On the PTS P-51 the airbrakes cause the nose to pitch down as they slow the plane even more, while the flaps cause the plane to pitch up... so the two sort of cancel each other out.

More experienced people fly the PTS and immediately remove the brakes, turn off the flaps and change the prop...

This changes it's characteristics as a "training" aircraft... so they then complain that it is too fast or unsuitable for a newbie...

Not so, it is just NOT being flown as designed.

RE: 3 blades - easier?
 

A 3blade is usually shorter..so on hard landings when the nose wheel kind of flex the prop won't touch the ground.

The most obviouse..more blades more wind(same pitch/dia as 2 blades) . But there's other factors involve.
Such as a bulky fuselage designs blocking the wind...that'll slow it down.lol

lets say you want a scale cesna (fater fuse) and a 3 blade prop to make it look more scale.
using the same engine from your trainer.....I'll go with longer 2 blades.lol

More blades to spin will require more power to trun the darn thing of course.
I mean if you ever rode a multy speed bicycle you'll know the camplex theory behind all this pitch and diameter stuff.lol
i think my leg hurts more if i go to higher gears from me having to push down on the pedles harder.lol

Anyway, there's a thing call a threashold...Just becuase your engine spin at a higher and higher rmp dosn't necessary mean
it'll produce more HP. At max rpm your engine will actaully produce less HP.

Example if your engine Max RPM is 16000. It's max HP output threashold might be at 13500 rpm.

RE: 3 blades - easier?
 

Not quite...

Typically when going to 3 blades you step DOWN diameter or pitch to produce the equivalent demand on the motor.

e.g. instead of using an 11x6 2 blade prop, you can get the same power output and performance by going to a 10x6 3 blade prop.


Which is why when moving to 3 bladed props we adjust for the difference.

However if you apply the proper rule of thumb, there is no net difference between 2 and 3 bladed props....

At the least for this "beginners" discussion.

RE: 3 blades - easier?
 

It just occured to me that certain prop driven transport or reconnaissance planes (real airplanes) that are designed to land on carriers have those "paddle wheel" type props as well (if my visual memory serves me correctly). Reducing throttle with props like that I guess is almost akin to down-shifting in a car... add to that, that they have turbo engines with variable pitch.

RE: 3 blades - easier?
 

Some one put up a web page not long ago on this subject, any one know what it was ??

RE: 3 blades - easier?
 

I did mentioned (same size dia and ptich)...but basically adding another blade.


that's why i just started off with the only different typically is the darn thing won't be mowing the grass
if it's not as long.:)

something about shifting gears on a bike or rahter, rather.lol

But to keep it in the realm of RC..
It's the same thing if i wanna pop a wheelie on me savage.
I put on a smaller Clutch bell (less teeth) it give me torque and excellerations, but I loose top speed.

So...is it easier to fly at a slower speed ??. Nope, becuase there's less wind blowning over the control surfaces.
Your controls become sluggish.
it's common mistake made by newbies, thinking just becuase they can fly WOT that they're good and toss thier trainner away.
but they can't land or fly worth a darn at slower speed.

RE: 3 blades - easier?
 

Beginners, keep in mind that props differ greatly EVEN WHEN THEY'RE THE SAME DIAMETER AND PITCH. I just collected my notes on the props I'd tried on the new OS55AX to email to a buddy who was thinking about trying a 3blade on his 55. I noticed when rewiting the notes into one place that I'd recorded more tests for 12x6 props than any other. And that the largest range of performance had come with that bunch.

So get over the "sound byte wisdom" that surrounds our model props. It might all be true, but it's just words.

There is significant difference between individual props that the detail of each prop means nothing. And that includes 2 vs 3 bladers.

There is significant difference between a single manufacturers props of the same type. And if you've ever measured many props' pitch with an accurate pitch gauge, you'd know that the pitch number a mfg chooses to stamp on his products seem to be a suggestion of how he thinks it might perform more than what actual pitch scheme his machines produce.

As you gain experience with props you find out the only reliable "wisdom" is to simply try whatever fits the prop shaft and has ground clearance. And the theoretical differences don't mean spit when one mfg's 6" pitch doesn't even match up theoretically to his 6" pitch on the next diameter of his same style prop. Sometimes the pitch increases out the blades different as well. And differs not just by that mfg's style.

You can talk theory all day long, but it won't put the theory into the prop blades you buy from the LHS. The only thing that seems to be "true" about the props we buy is the diameter. And if you've ever measured pitch it might amaze you the mfg's bother to produce 12" diameters that actually are 12".

Don't ever decide not to test a prop because of some conventional wisdom about props. If the prop fits, try it. You will be surprised.