I know that this sort of question has been asked numerous times on here before, but here I go anyway.

What prop will give me the most amount of thrust for a .46 engine?

I am building a flying 'machine', not particularly a plane or heli and need to know.

I've heard people say "11x7", but I've found a 11x4 to be more productive. Is this because the engine doesn't load up and thus the higher rpms can be acheived running a lessor pitch? I've also heard "12.5x3.5", but wouldn't that be the same as a 11.5x4.5? Just curious.

I don't want to go through all the physical scientific trials of testing all the variations.

If you don't want to perform the physical comparisons simply pick one and believe it's the best. No two engines are the same, especially with different manufacturers. OS will be different from Magnum, which will be different from Thunder Tiger, which will be different from Tower, which will be different from Irvine. Now factor in engine conditions and fuels. You have asked a question that is impossible to answer.

To do some of the math yourself go back to last months issue of Model Aviation and read the propeller article by Andy Lennon.

I'll second what Pat Roy said plus another idea for you. You need to consider not only what engine it is, but also where you want max thrust. Do you want max static thrust for 3D on a .46 AX? Maybe the APC 12.25 x 3.75. On the otherhand, if you're running a Jett .46 on a Diamond Dust, you'll want max dynamic thrust at 160 MPH, and maybe an APC 8x8 or so.

So, things to consider for the answer to your question:
1) Engine RPM range
2) Type airplane and how you want it to fly
3) Where do you want max thrust- max static for hover pullouts or dynamic at a certain speed.

If you'll tell us a bit more about what you are building, what engine, and what you want it to do we might be able to give you a better answer.

This article http://www.dubai*********.org/propeller_faq.htm has a very good write up on prop selection and discusses how to decide if you need large diameter, low pitch or smaller diameter, higher pitch.

The short answer is it depends on your application. Large diameter, low pitch for slower flight with more power. Small diameter, high pitch for high speed with less power. This is based on props that provide the same RPM.

Dave

There are 2 really popular 3D props for .46 size engines. These will give more vertical performance. The props are the APC 12.25-3.75 and the APC 12-4.

The 12.25-3.75 is a heavy, wide blade prop that will give slightly more static thrust, but it will rev up (spool up) slower due to its weight and inertia.

The 12-4 is a narrow blade prop that gives very slightly less overall thrust, but will rev up faster.

If you have a very light plane and want fine control, the APC 11.5-4 works well. It gives less overall thrust, but more top end rpm. You seem to be able to make smaller adjustments in power easier than with the 12-inch props.

If you are talking thrust for speed, the 9-7 is excellent for all except the OS .46AX. It has more torque and likes a 10-7 better.

There ya go. Not one post was able to provide the one best prop. You have to get off the butt and expend a little effort.

This is all i can give you and its THEORY so real world may and will differ.

It will be the prop with lowest pitch and highest diameter

that has a tip speed between .78 and .82 mach

Now enjoy calculating that

(ps mach is NOT a constant and should be calculated with temp, altitude and so on)

Steven

I fly a stick with a 46 OS FX, swings a 14X4 APC. Fantastic thrust.

Let's keep this simple.

GIVEN THE SAME RPM:
A larger diameter. lower pitch prop will give more thrust and less top speed.
A smaller diameter, higher pitch prop will give more top speed and less thrust.

Compare it to low gear/high gear in a car.

As for what's the best prop for your application, only you can be the judge of that. It all depends on the engine/plane combo and how you want it to fly. Do some experimenting.

Dr.1

Kind of hard to answer a question like this when the "flying machine" that it will go on is "need to know" only... I have to wonder if its because you fear being ridiculed for trying something really different, or becaue it something you know you really shouldn't be doing.....

Forgive me if this seems like an attack, but It gives me an uncomfortable feeling when someone is asking for advice but won't tell you all the factors...

First, fear is real - it is what incites the fear that is questionable. A person who suffers from delusions may fear a conceptual paranoid thought, but to the man whom has his sanity the delusional man's fear is unfounded. Much like the fear you have, it is unfounded If I had the intent (key word) to do something that is criminal, that would be my choice (another key word). Aiding and abetting a criminal action does include another (the abetter) to have full knowledge (the knowing causation of the criminal act and/or course of criminal actions). Before one can (by legal definition) be held accountable for aiding and abetting a criminal and/or act. Your safe, because I have no criminal intent.

See, I am building an aircraft that utilizes a virticle wing structure that has the power source mounted directly above them. There will be a fuse, but much like that of a heli without any rear rotor, or counter rotating rotor above. The naturally occuring gyroscopic twist will be counter reacted with the counter twisting of the two wing panels (much like an aileron set up). Forward motion will be controlled by the two wing panels. The forward tilt of the wing panels will cause the prop wash to, in turn, move the craft forward (the wing panels will be set the same way as having a mix - much the way a delta works).

I built a very similar craft in high school (physiscs class) that was electric powered;I graduated in 1997. Materials consisted of lipo battery technology and a brushless motor w/micro servos and receiver. Mind you, this was before the big boom of the stuff now and man was it costly to do then. It went on the schools tab, of course.

Oh, I almost forgot. Yes, the craft did fly very well. Imagine a plane hovering nose up, the wing panels are positioned as such, but are controlled like a delta (mixing).

Oh, I have done my own test indeed. I have an air boat and found my own calculations to be much different than what I have read. for example, the boat has a top speed of 44-48 mph with an APC 11x3. When using a 11x6 it will only travel around 36 mph. When usung a 10x6 it does around 33 mph. Used the radar gun out of my patrol vehicle - I know it's right; they have to be calabrated annually.

Even did calculations with many props using fixed numbers of effiiciency, ie 50, 60,70, and 80%. The only thing left is to get a tension scale and physically test many variations of props...

BadSplice,

I hope California's 9th District Court legal philosophies havent rubbed off on you; anyone who seems suspicious must be condoning criminal behavior[X(]. I'm surprised people can even own RC planes there.

Sounds like you're building a PBX.

http://www.rcuniverse.com/forum/m_40...tm.htm#4025483

Is that sort of close to your idea?

Thats why I hesitated to say anything, and kinda wish I hadnt. I didnt mean to imply that you were doing anything criminal, but people who dont know what theyre doing can sometimes do dangerous things. You obviously DO know what youre doing, so from this point I will keep my mouth shut, and hope you post pics of what you come up with, because it sounds interesting!

You're alright, man. I was trying to use comedic sarcasm. I know that you didn't mean anything rude.

That PBX thingy is kinda' similar to what I'm doing. Imagine having a fuse like a heli and wings located similarly in the area of that of a plane. The only difference is that the wings are pointing up, just like the PBX. Questions: does that PBX fly pointing up (nose up)? Or, does take of and land that way. My plans, just like the electric one in high school, are to have it fly like a heli. Won't be able to do 3D stuff, but the general concept is the same of a basic heli; up, dowm, forward, reverse, swing left, swing right.

So low speed? Go with large diameter, low pitch. This will give highest static thrust and lower top speed. Be sure that the prop you select allows your engine to turn in the correct RPM range to be in it's power band.

Dave

The PBX flys similarly to a PBF, mostly with the nose up, hovering and such. They aren't too good at forward flight at speeds much faster than a jog, and the PBX can land and takeoff vertically. I've only seen videos of the PBX, but I built and flew several PBFs, and they are very good at hovering, hariers, and other 3D flight, but lack in precision forward flight.

Think Osprey. Very large diameter with a flat pitch. Don't know how much blade room you have though. The problem with all the calcs is that different props unload differently, screwing up all the equations. I don't think you'll be able to escape experimenting some.

Very similar to the V22 indeed, but with one prop and the engine will not rotate to dictate flight characteristics.

You're right. Actual trials do prove differently than the math concludes. With the array of props available, it is almost null and void to use the formulas for calculating thrust. The math does, however, give a general ball park figure.

I cannot get over how my airboat is so much faster with a 11x3 and 11x4 sized props; it came with an 11x8 - it was a real turd with that thing.

That particular 11-8 was likely a little too much load for the engine. I was never able to get much more than a 12-5 to work all that well with a .46. Just stepping down the diameter and increasing the pitch rarely works the same as using a larger diameter, higher pitched propeller. The dynamics of the diameter has different effects from the pitch. Now you have to toss in engine manufacturer performance and prop quality variables, which have their own notable effects. Don't forget the effects of the exhaust systems that could be used.

One thing's for certain, you have some interesting torque and P factors to deal with.

This is all great information, thanks. I have question and curious to get opinios.
I have a Tiger 60 with an OS FS 91. I started with a 13X8 APC and the plane was a
sled, very sluggish, not too repsonsive. I changed the prop to a Zinger (wood) 14X6
and it flew better. I have installed another APC prop at 15X8. I would like it if someone
could comment. I'm using Omega 15% fuel too.

Any feedback would greatly be appreciated!!

Thanks!

Hey there! Try one of the hovering props listed above first, they're likely to work best for you in this application. When you choose your props, try to keep the load on the engine such that static rpm maxes below about 13500 or so, or the engine life will suffer.

I thought you might be interested in some insights on your airboat speed experiments. There's actually a simple explanation of what you observed. Here's the results you obtained, and I have no doubt they're accurate.

44-48 mph on 11x3
36mph on 11x6
33mph on 10x6

Remember that a prop is basically a wing, same as on any plane. More speed, or more angle of attack gives more lift, until the angle of attack exceeds critical and the wing stalls and produces virtually no lift. It's speed and aoa that are playing with your airboat results, for the most part. Also, the area of the prop disc has a very large effect on static thrust.

I'll start by looking at your 11x6. That's really the best matched prop to your engine that you used (in my experience). With it, a healthy .46 will tach about 13k rpm, and produce a pitch speed of 74mph and about 7 pounds of static thrust (using ThrustHP to calculate). Since the airboat is a draggy vehicle, it's not likely to achieve a speed anywhere near 74mph, so the effective aoa of the prop will be fairly high. A portion of the prop blade will be stalled because of this.

The 11x3 prop is too small for longterm service on a .46. It must have tached in the 15-16k range, maybe even higher. Musta been a beautiful noise! It's high rpm results in 10+ lbs of thrust, and a pitch speed of 45mph at 16krpm (so you were likely turning even quicker to achieve the speed you did!). Since the boat is likely to be able to approach the pitch speed (and in fact did!), the effective aoa is very low and the prop is not stalled at all. A good pitch choice for the boat, but it would have likely been better with a 12x4 or so, to bring the rpm's down without sacrificing pitch speed.

The 10x6 had the same trouble as the 11x6, but at higher rpm, and with a smaller prop disc, so less thrust. Probably tached about 14000 static, pitch speed near 80mph, and only 5.5 lbs static thrust. Again, the vehicle can't achieve that speed with the drag it has, and so the prop is out of it's element.

Your hovering machine (cool idea, btw, what's it for? Camera ship?) will have a very low flight speed, essentially 0 mph most of the time. So your goal should be to generate as much thrust as possible (big prop disc, ie big diameter) with a low pitch to avoid stalling any part of the blades. Look to those 3d props and you'll do well. APC props are about the stiffest sport props available (without visiting the exotic cf props, etc.) and are very efficient. A favorite among sport and 3d fliers in this size class. Definately more efficient than the same size zinger wood or master airscrew glass filled nylon which is much flexier. The master airscrews, however, are more likely to survive the enevitable nose over.

Hope this helped.

J