Sorry if this question has been answered a million times already.

Would a smaller diameter larger pitch or larger dia lower pitch prop give better vertical? I've heard arguments both ways. Does the story change for torquey 4-stroke vs. 2 stroke engines?

I'm trying to get better vertical performance out of an 8.5lb chipmunk (see pic at left) with an OS 91 4-stroke.

Thanks,

Jim

I dont know about the OS, but you need a huge diameter, fine pithc. this will give you a slow speed but the acceleration and pull will be emence, go the outhe way and your looking at speed and crappy vertical pull.

Ian

To get the best "vertical" you really need big diametter and high pitch. This then gives you speed as well as thrust. If you are just after static thrust, then big diameter, small pitch is best. This also gives acceleration.

But such a prop is not suitable for a chipmunk, where you do need some speed, it is not a funfly. Basically your problem is you need a bigger engine.

Props to try are 14x7, 14x8, 15x6. Give them each a go, and see which gives you the best performance.

Sprink: big dia, big pitch??????????

That will load the engine up way too much.

but a 15x6 should do, give it a try

Ian

I think for an 8.5lb model to get good vertical you'll need more power! Try a medium pitch (6,7) with a bigger engine, say 120 stroker........

Andy

Edible - I know that a big dia/big pitch would load up the engine too much, hence my comment that Jimmy really needs a bigger engine

A 90 four stroke will never give a 8.5lb model real vertical performance. I have an 8.5 lb chipmunk myself, so I know how they go. I have a 91 FX in mine, with an APC 15x7. At that weight the wing loading is about 28 oz/sqft.

Per CG, its weight should be between 6-8lb. If it did come out at 7lb, then the 90 4stroke would probably be fine. Wing loading would be lower, so it wouldn't need so much speed either.

For 3D You need a minimum thrust to weight ratio of 1.5:1
You need a large diamater propeller to give you this. You also need to have enough speed at that thrust to get good velocity with the above thrust. Pick a Large diameter to get you the thrust, but you need pitch to give you the ability to generate speed with that amount of thrust.

Thanks everyone for the advice. Looks like big dia lower pitch to increase thrust. I've tried an APC 14-6 and Master Airscrew 13-8. APC was slightly better. I will try the 15-6 and see how it goes.

I also just 'borrowed' a tank of 25% Nitro off a friend. Holy S*^# what a difference from the 10% I've been burning. Vertical is about double what it was before it peters out. That was a pleasant surprise, altho my wallet will suffer. Like everything else about this hobby, spend more, get more!

Sprink, you have a 91FX in yours?? Wow, that thing must scream! Mine with the 91 4-stroke is REALLY fast. I have the throws about double the book high rates recommendation and snap rolls are blinding. I'm waiting for the wing to rip off one day, already cracked both wing mounts, luckily not on the same day. Its beefed with tristock now so if it breaks it will be somewhere else. If I put a bigger engine in I'm sure I'd kill the airplane. Also, can you get yours to flat spin? Mine wont come close yet but the CG could safely come back a little more.

LOLOLOLOLOL

Look at this

LOLOLOLOLOL

And again

I can see you explaining to your wife why you are taxing your plane with your son in the cockpit!

priceless. Dont forget to hang a large diameter low pitch prop to get enough thrust to lift that cargo off. Or maybe a couple JATO rockets.

Geistwere: how do you mean? If your saying its mine, well, LOL, Im 14, LOL

Jimmy, I don't do any 3D with mine. Throws are near the recommend amounts. Good for smooth pattern type aerobatics. It does go well with the 91 FX, but still not "unlimited" vertical, but near enough for my needs (I can get two or three rolls before starting to slow too much)

Must admit, not tried a flat spin. Planning on flying it again tomorrow, it hasn't been out for a while (getting dusty! LOL) so may try one.

For my innovation pro, i am trying to get better vertical out of it,
and i think that if i increase the nitro level to increase torque and then go to a higher pitch prop(from 13x4 to 13x6). I should theoretically get quite a bit more thrust.

NO, high pitch gives you speed, large dia gives you thrust.

So, if youve got a large dia fine pitch prop, youll have loads of thrust, increased throttling and long verticals. Get a smaller dia, large pitch you'll have loads of speed, lousy throttling and crappy verticals

The nitro wont give you more torque, all nitro does is increases throttling and rpm.

i would stick with the prop your using

What fuel are you using?

ian

5% part synthetic.
I thought the reason why nitro gives you better throttleing and higher rpm was because of the extra torque it gives!
So maybe i should try to find an Apc 13x4 W prop to try.

An irvine 53 wont spin that prop. so dont waste 8.50 (pounds) on somthing only a west will turn.

A 12.25x3.75 will be perfect.

I use model tehnics becra 10% heli fuel. Synthetic, amazing cooling blah blah.............. all at a discount price.

Ian

Quick (or maybe not) discussion on the amount of thrust that a prop generates:

Three main things affect thrust, in no particular order: rpm, diameter, pitch.

Given the same rpm, increasing the pitch or diameter will increase thrust. However, increasing diameter has a far bigger effect proportionally than pitch. Hence the generally addage about going up in diameter, but down in pitch to generate more thrust. Basically what you gain from the diameter is more than what you lose from the pitch.

The other issue is whether just increasing the diameter or pitch will increase thrust, given that this will reduce rpm. Well again in most cases, diameter gains will generally outweigh rpm losses, but pitch gains is a close run thing, depending on whether you are improving the efficiency of the engine or not, ie whether losing rpm means you are increasing or decreasing torque.

Another thing to consider is dynamic v static thrust, and this is where pitch is important (dynamic thrust is where the propellor is already moving through air). Big dia/low pitch props generate lots of static thrust, but less dynamic thrust. Thats fine for slow flying funflys, but not much good for fast flying pattern ships.

Finally there is accelleration v speed. low pitch props generate give great accelleration, but obviously far less speed, while high pitch props arn't good at acceleration, but do give speed. A bit like low and high gears on a car or push bike.

Other things to consider are:
efficiency of the prop design (eg stiff APC passes on more power to the air, and absorbs less in flexing),
chord of the blade (wide blade props creat more thrust than narrow),
number of blades (2 blades are more efficient than 3 etc).

Ultimately if you want to make a big difference to thrust, you need a bigger engine as I said. Changing props is generally just fine tuning.

Kinga like gear ratios, huh? :stupid:


Ian

Yeah, exactly like gear ratios.

Top gear it is difficult to pull away, but you go very fast when you do. Bottom gear, you hardly have to push to move, but you don't move very fast.

Oh i get it now

edible,

you said nitro doesn't give you more torque - thats wrong, it does! Nitro makes the fuel air mix in the cylinder burn hotter therefore forcing the piston down with more force, which is converted into rotational torque by the crankshaft. The only reason you don't see much of a difference is that the amount of nitro ie 10% gives a corresponding torque increase ie 10%, which is bugger all really! End of lesson!

Andy

Whether torque, rpm or what I dont know, but 25% nitro gave me about twice the amount of vertical as 10% nitro. All other things being equal (entry speed, prop, weather, etc.). I should have paid closer attention to amount of RPM increase with 25% but I did not.