Propeler size

[hlondono111]

Does the properler size affect how the engine works? can you put a 10x8 on a os 40 to make it go faster? and what is 10x8 mean?

[mcflyboy]

10x8 means that the propellers diameter(length) is 10 inches, and it's pitch(how much it pulls through the sky in turn) is 8 inches.
Putting a 10x8 on an OS40 would make it slower as this is too much prop for that engine, a 9x7 might make it go faster, but you also have to account for the type of airplane. if its a general purpose trainer, then you don't want to be going fast anyways, the slower things happen the easier it is to learn.

[Dead Beat]

10x6 is the standard prop for a 40 size engine the rule of thumb is if you go up one inch in diameter you go down in pitch

[DaveSawers]

High pitch for high speed, low pitch for good acceleration or thrust (for vertical manoevres).

Reduce the diameter, and the engine turns faster. As long as there is enough prop blade area to drag the airframe through the sky, you will go faster.

Optimizing prop size for the performance you want is a process of experimentation. A prop that works well on one airframe may not be so good on another even if you use the same engine.

[fhhuber506771]

You have to consider "propeller load" on the engine... With the correct loading, and engine will produce best HP and best tourqe at close to the same RPM.

The 10X6 typically recommended for a .40 engine gives a certain amount of load. If you reduce pitch without changing diameter (10X4) you have reduced the load.. the engine will produce peak torque AFTER it produces peak HP.. it will also produce excessive rpm. If you increase pitch without reducing dia.. the peak torque is reduced because of the load being higher than the HP of the engine at an rpm which is lower than the rated peak torque speed.

So.. for a .40 engine, you "trade" 1 inch pitch for 1 inch dia to maintain close to the same propeller load, thus maintaining the maximum efficiency of the engine.

Note: the engine manufacturers will "fudge" the HP rating of the engines. By putting a low torque load on the engine, you can rev it to a higher rpm, getting a higher measured HP out of it.. but at that high rpm, the torque rating may be SIGNIFICANTLY reduced. (very ineficient and darn near useless for swinging a prop.) Don't work for maximum rpm when selecting a prop. Sure i can stick a 8X8 on a OS .46 FX and turn it about 17,000 rpm... but that won't do a heck of a lot of good for pulling an airplane. The same engien with a 11X5 is going to produce MUCH more thrust at 12,000 rpm, and with a 12.25X3.75.. it will take a 7 lb model unlimited vertical.

****
:p had a small error that made it say something VERY wrong..

[mcflyboy]

The same engien with a 11X5 is going to produce MUCH more thrust at 12,000 rpm, and with a 12.25X3.75.. it will take a 7 lb model unlimited vertical.


You sure man, an 46fx in a laser 3d at 5 1/4lbs would onlyt just go vertical. I know you're just proving a point but I couldn't help but notice that.

No offence

[fhhuber506771]

I had a Magnum .46 (90% to 95% the power of the OS .46 FX on a good day) with the APC 12.25X3.75 on a 7 lb 4 oz "Tamecat" (looks like F-14 .40 to 60 size trainer) and the thing would go unlimited vertical...

Been there.. done it.

[Trogdor]

I guess it depends on your definition. My H9 Twist with 46FX (~5.5 lbs) & APC 12x4 prop will pull out from a hover slowly but accelerate a bit on the way up on a good day. It does almost seem to get stuck at a certain altitude when doing this but that just may be some optical illusion when looking at the plane at that high altitude moving slowly straight up! If flying at normal speed then pulling to vertical and giving full throttle, it will go as high as I want it to! But maybe the fat wing will hold it back (more drag) whereas the tamecat (even being heavier) can keep going up easier when starting at a relatively high level flight speed.

I guess more info on the plane is really needed, maybe not necessary for me to elaborate but I can't seem to get to sleep! :)

[fhhuber506771]

Yes.. starting from level flight at speed was needed... but it never slowed down as it went up.

and depending on which 12X4... its a bad comarison to an invalid one when comparing to the APC 12.25X3.75. It calculates out to a higher base engine load for the 12X4... so you can expect hither rpm in flight with the 12.25X3.75 and the lower pitch longer blades are more efficient for low airspeed thrust (not much with that small change... but its there)

The drag of the model has less to do with it than the prop. the climb will hit a given airspeed after rotation from level and then its all pull power fighting gravity. (thrust = drag+gravity = constant speed vertical ascent.. just as if its a helicopter.)

Pulling out of a hover into vertical takes more power than constant speed ascent.. Constant speed ascent can be done at P:W =1.1:1 to 1.2:1 Its generally going to take 1.5:1 to pull out straight up from hover and not fall to a side, "top" or "bottom" (whats a side top or bottom when the nose is straight up?.. is up really up?.. Ah.. this belongs in philosophy :p)trying.

[Desertraptor]

My 2nd post here so be gentle as I am new to planes. I have a lot of reading to do.

I have purchased an Ultrafly Extra 300. Which I have test flown and landed safely once. I am an experienced rc heli flyer so I didnt think an Extra would be beyond me. I may proove myself wrong yet? :D

Anyway my question is this. For the sake of scale the real Extra has a three blade prop. The current 2 blade on the model is an APC 9 x 6. What size three blade would be suitable?

Thanks

Peter

[ivanc]

8x6 3-blade

[fhhuber506771]

Always ask how they intend to fly the airplane. ;)

The basic rule of thumb when going from 2 blade to 3 blade at .40 size is to drop 1 inch diameter OR one inch pitch. (you drop one inch of BOTH from .91 ci size to 1.5 ci.)

So the question comes up:: Do you need the static thrust for 3D? If you do... then 9X5 might be more appropriate. (maybe a 9X4 if the engine has been heavilly loaded on the 9X6 2-blade..)

What engine? 9X6 sounds like the correct prop for a strong .25 to a weak .35.

[Desertraptor]

Thanks guys.

The plane is an electric (I guess I'm in the wrong place?). I guess I'll mostly be just sport flying for now. The motor is the Ultrafly Brushless A/30/29

[fhhuber506771]

as for saying what prop is right for that motor.. yes, the elctric forum will have a better idea.

However.. the size to power-load on the motor rules are still going to be close. So if the motor has been getting hot, you reduce the prop load a bit for better motor life. (9X4 3 blade) If its been running cool and happy.. and you want the 3D performance.. 9X5 3 blade. If you want to maintain the same top airspeed and keep the same appx load then the 8X6.

The rules of prop-loading don't change with the type of engine/motor ... just the recommendation for what to try in your specific case changes when you change what turns it.

An electric will TRY to turn the wrong size prop its "kV" rpm.. even if you have a 19-12 prop where a 7X4 prop belongs. It will heat up like crazy (and let the smoke out) TRYING to put out the needed power... and only have heat + poor speed to show as symptoms.
A glow engine with that much oversize would simply refuse to run much above idle speed without severe backfiring telling you very plainly... no way will this work.

[Desertraptor]

Thanks for your help. Appreciated

[ClydeM]

Since we are discussing props, which is a better to use?
Wood or composite type??

Thx

[ivanc]

Good pilot (smooth landings ;) ) - wood.
Not that good on landings (prop ocasionally hitting ground :eek: ) - composite.

[fhhuber506771]

Which material depends on a lot...

The better composite propellers are more consistant than the wood props for having the same shape every time. The GOOD composites can handle higher rpm than any wood prop. CF props are very expensive and sometimes shatter more easilly on striking something than a wood prop. No flex... APC's grey props chip just about as easilly as a wood prop... Bolly Clubman series seem more resistant to breakage than the Master airscrew black glass-filled nylon.

Sometimes appearance is the compelling reason for choosing the prop's material. A WWI model looks better with a wood prop.

If you break a lot of props.. wood props are often cheaper than the composites. Though the black Master Airsccrew glass-filled nylon is more reistant to breaking from a minor strike with the ground..... they are still breakable.

I now usually choose the APC grey composite props due to consistancy of shape at a reasonable price... until you get over 12 inch dia where the "Dynathrust" props are so much less expensive that I can't reasonably choose anything else. (because I still break too many to use the $12 + APC "Pattern" props...) A $3.50 14X6 that does the job is just better for me.

Every prop material and shape has bennefits and weaknesses... Find what gives you the best results while staying within your budget.

[ivanc]

Hitting the dirt (landing on the grass) with a wood prop breaks it in 95% of the cases. Same situation with APC grey prop - 5% (or less). MA black - 1% (or less).

Hitting the paved runway with a wood prop breaks it 100%. APC grey - breaks it 5%, shortens it - 80%. MA black - breaks it 25%, shortens it 0% (it'll be either be OK or will break).