I think I'm going to max out my allowed noob questions for the week.

OK here it goes, what is the benefits of a 3 blade prop? I read a site (prop one) that claimed it allowed for slower flight. This would be ideal for me once I get my trainer going. I just wondered why no other trainers had 3 blade props if it was that great of a benefit.

thx again

mj147

3 bladed props.
About the only advantage is you get more ground clearance.
Another possible advantage is slower tip speed making your engine quieter.

However, you don't get as much power out of your engine. If you already have a surplus of power, go for it and maybe not break a prop on a bad landing. If your plane is slightly underpowered already, you can try it, but it's probably safer to stick with the 2 blade.

If you want to go slower, try a prop with less pitch.

A good way to estimate what prop to buy is to multiply the diameter and pitch of the one you have. Then divide this by the diameter or pitch you want to go to. The number you get is either the diameter or pitch to use (depending on what you divided by).

FOR EXAMPLE:
I'm running a 12x6 prop and I want to go to an 11 inch.
12X6=72
72/11=6.54
Therefore I want to go to an 11x7

Or if I'm using a 10x6 now and want to go t a 7 inch pitch:
10X6=60
60/7=8.57
Thus I'd buy a 9x7 prop.

Hope this isn't confuzing and actually helps you.

Yup... that's about it. At our small scale, three bladed props aren't too efficient. Better off with two blades.

Most efficient of all is a single blade prop but that comes with a couple of other problems.

Probably one of the most dumb looking things you will ever see is a really elegant scale ship with a tiny little three or four blader on it in a vain attempt to look more scale.

John

Yep, that's why the scale guys use display props .

The Alpha trainer uses the Evolution engine system which comes with a 3-bladed prop with some huge flat blades. The prop does do a really good job of slowing the plane down, while still providing a reasonable amount of thrust. But when guys replace that prop with a normal 2 bladed prop, they are requently surprised at just how much more thrust and speed they get out of that engine.

As Aaron said, multi-blade props are good for quieting things down and for ground clearence. (WW2 fighers used them almost purely because of ground clearance).

There are several "prop size forumlas" like the one that Aaron gave. The fact that there are so many makes it pretty clear they are all rather rough guides. You can (and maybe should, if you aren't happy with the planes performance) experiment with a fairly wide variety of prop sizes to find the right balance of thrust, speed, noise, etc. Also, differnet brand props can act very differently in the same size.

it won't let you fly slower. the benifit is you will have more thrust at lower rpms if you happen to need it. the downsides are that they are less efficient at higher rpms (the kind that your .40 size 2 stroke will crank out) and always seem to be in the way when you try and work on your engine.

Also, a Master Airscrew 10x6 isn't the same as an APC 10x6. Basically prop selelection is trial-by-error. Try different sizes of different brands and see which one suits you best.

I would have to disagree with that a bit Spaceclam. True it won,t allow you to fly slower than the stall speed for that airplane but will provide more drag to allow for better decelleration in the approach to allow more control of the approach path even though the stall will come at the same speed.

The reason is a multibladed prop or for that matter a paddle bladed two blade is they increase the propellor disc solidity ratio. A high solidity ratio will produce more drag when windmilling than one that has a low ratio. Sorry for tech stuff but it is a fact.

John

i didn't way anything about drag. i do understand that though. the same reason why helicopters can't fly faster than usually 300 even with jet engines on the sides.

Actually, that's wrong again. The reason full scale helos can't go faster than about 300mph in level flight is that the leading blade is approaching high mach numbers at that point, and compressablity issues start to show up in bad ways. And if you pick up a shock wave in the rotor blades, your helo is now an elevator minus the cables.

But in general, a low pitch large diameter prop, or a prop with large chord blades will act as a brake and slow the plane down. Also, since the theoretical airspeed of a prop can be calculated to be pitch*rpm, if you lower the pitch, the plane will fly slower. In high speed flight, the difference isn't that noticeable, but in a landing approach, a few mph slower at idle can make it easier to land. Espeically since you have a definate min RPM most engines will idle at. Add on to that fact that our engines idle better with longer, low pitch props or props with heavier blades due to the increased rotational intertia, and the overall effect all adds up to make it easier to slow down a plane with a larger, lower pitch prop.

not always. there is an enormous drag penalty on those blades. it is like attatching a streamer to your prop. yes, your tips begin to hit mach 1, but at that speed there is a lot more drag than there is below, causing that drag penalty. for instance, a two bladed heli will have more problems with that because the blades are bigger and longer. For instance, the commanche (not yet in service) has 5 blades that are quite long. so, they turn at slower rpms.

Quick word about the helo: not only does the advancing rotor experience a near supersonic speed, the retreating rotor experiences almost no airspeed and stalls.

Helo's have all sorts of issue that arise as forward flight speeds increase. You have localized mach issue on part of the rotor disk and zero air speed issues on other parts of the disk. If you do a quick analysis of vector lift you will see the heli will totally lose lift on one side and/or probably destroy blades on the other side as they mach tuck. Regardless, the result is a sudden roll and loss of lift. That exact thing has happened before to some military helis during speed trials.

As for larger props allowing slower flight, I'd have to agree under certain circumstances. A high pitch prop can become stalled at lower forward speeds. I can imagine a scenario where a high pich prop could not easily sustain the proper thrust required for slow speed flight because of stalled blades. This would be especially true around the stall point because thrust would vary greatly as the prop stalled and then unstalled and then stalled again, etc. The low pitch prop on the other hand would not be stalled and could easily maintain a constant low thrust level.

As for breaking, there is no doubt that larger diameter props and multi blade props brake well. Problem with the two blades is they produce more noise than three blades because the tip speed is higher on a two blade. This is why in pattern and IMAC, where both breaking and low noise is important, you will see 3 bladed props. Both pattern and IMAC ships are grossly overpowered so a little loss from a 3 bladed prop is no big deal. I personally opt for a large 2 blade and just control throttle to keep noise down.

Cheers

. . and the trailing blade is approaching being stationary!.

-DC

you could always get an in-flight adjustable prop, cost about £30 ($45) and you just need an additional servo to control the pitch, oyu can get blisttering speed or even have reverse thrust for landing in the space of 2m

most common to find is an 11 inch adjust pitch prop, i'm gonna get one when money allows, if it ever breaks though then thats a lotta' money gone down the drain

Wow u guys know your stuffs :thumbup:

Actually I'm gonna try a few diff props, I'm planning on putting a TT pro 46 on my Superstar. So even with a 3 blade I wont be under powered as far as I've read.

I'm not much of a "hotdogger", nor I'm a fan of loud screaming engines, so I would like less noise if all possible


thx for all this info, I gotta start printing all this info and make a reference book of sorts

mj147

mj147,
If you're not a fan of "loud screaming engines", then you should really consider a 4 stroke engine. There are a number of manufacturers producing 4 strokes in the 52 size range which would work well for a 40 size plane.

A nice throaty growl instead of that mosquito "whine" of a 2 stroke. Quieter too.

The one problem I can see, once you fly a 4 stroke, you might not be happy with 2's again.
Dennis-

A variable-pitch would be fun to try

i want one of those new fangled wattchamacallit's