i know this is a dumb question, but everyone ive talked to around my flying area and local hobby store all have different answers lol
for more speed out of a prop. do you want more or less pitch?

....you want smaller diameter, and more pitch for speed.

k cool. if recommended was say.. 10 x 7 i would want like a 9x8. 9x7? to get alittle more out of it?

Yup - You are headed in the right direction

....yep....higher RPM and more pitch gets you the speed.

you can get to the point the prop is too small for the airframe and you start to loose allot of speed because you're prop washing the airframe and not pulling through the air. my Patriot 40 does great with a 8.75 and 9 inch props but looses allot of speed when dropped to an 8" prop and takes a good 50' more runway to get off.

kc

cool. so a 46 2 stroke.. a 9x8 or 9x7? would be a good choice? opposed to a 10x7?
thanks for all info guys.

i'd suggest the 9x8 or an 8.75x9

kc

Quite possibly but not always true - which .46 two stroke? Some are timed for more power at lower rpm, the OS .46AX is an example. In some cases a 10-7 may give more top speed than a 9-7 or 9-8 and there are folks here that have seen that consitently in certain combinations of airframes and engines. Depends so-o-o-o much on your aircraft and engine. However, the specific recommendations made by the other folks here are absolutely where you would go to experiment.

Think of increasing pitch and reducing diameter as shifting up a gear on a car or bike. You have more theoretical top speed but less pull, so if the drag is so much that you can't get there, then you won't. So often some experimentation is in order, to find the optimum combination.

But yes the general rule is, as already pointed out, reduce diameter and increase pitch. You reduce the diameter to account for the additional load from increased pitch - this is on the assumption that your baseline prop is reasonable, i.e. you're not lugging the engine down and it is in an area of it's powerband where the hp output is decent. Sometime just reducing the diameter one inch but staying the same can do the job, as the engine will unload and in most cases produce more horepower - this depends on the motor, and where you started from of course. For example, suppose you fly your Purple Whatzit with a 10-6 prop and the engine tachs at 13,500k on the ground. Now you switch to a 9-6, and the engine tachs at 14,500. The engine is probably [not definitely, but probably] producing more horsepower in this case based on typical hp/rpm curves for .46 size engines. The higher rpm gives you higher "pitch speed", in other words pitch times rpm, meaning that potential top speed is higher. If this choice works for the airframe involved, top speed will go up. And there may be more yet, you might at that point try say, a 9-7.

Small, clean airframes, i.e. low drag, respond best to higher pitch props and reduced diameter. Draggier airframes will hit a "wall" sooner, and often the best performance comes from a middle of the road choice.

Remember - more speed requires more power. To reach the best speed you need to let the engine develop as much horsepower as it can (look at reviews for performance numbers) and then, your job is to find a prop that best utilizes that power. Not to beat the gear analogy to death (but it does work farily well), your goal is to find the gear that gives the highest top speed - go too high, and you may not get there and "shifting down" might give you higher top speed.

MJD

thanks a ton. ordering a new engine tomorrow(rossi or jett..still not quite sure heh) for my prop jet. atm all i had laying around to experiement with was a magnum 46 xls and a magnum 52 xls. went with the 46 since it was already broke in and i was in a hurry. after messing with the props i gained a noticable amount in speed after starting with the stock recommended prop and dropping diameter and increasing pitch. with a apc 10x6 was 84 mph.. while not to fast. but working with a magnum 46. im not complaining. in a few days the engine will be back on the shelf and a nice new engine under the cowl.

like some of the people you mentioned you're missing the forest because you think the trees are blocking your view. the timing and porting of the engine have nothing at all to do with getting more speed from the same said engine, when you drop prop diameter you increase pitch to match the load on the engine so you keep the engine in its power band producing max performance. the AX torquer engines do not respond well to tuned pipes/ultra thrust/Jett stream systems but they respond to propping the same as any other engine and as mentioned above the airframe is the limitation. don't confusing propping with trying to get more RPM.

when propping always use the sum of Diameter x Diameter x Diameter x Pitch and keeping that as close as possible you'll keep the same load on the engine, and the same peak RPM. propping for RPM is a different ballgame and is engine specific.

kc

All engines will respond to prop changes similarly, but what MJD was saying is that trying to get big results one way or the other from an engine that wasn't designed for that purpose can be a waste of time and an unwise use of that particular engine. The engine designers use different timing events and port sizes for the intended rpm range or in a competition engines' case, for its' maximum output. I guarantee that you will get more rpm from any sport engine by changing the porting and the increase will be in effect for all props that this sport engine would normally turn.

i think you are missing my point, the 46 AX is just as happy turning a 8.75x9 as it is a 10x6, and gives more speed in most 40 sized sport airframes. the AX is a torquer engine, it'll turn big props just fine, but it'll also turn small props with allot of pitch just fine too as long as you're propping it to stay in the AXs power band (RPM range) as previously mentioned they do not respond well when you try to hop them up with a tuned pipe and smaller prop attempting to get speed from RPM but if you keep them propped to stay in their desired RPM range you can get some speed out of the torque ported engine none the less.

other engines like the OS SX versions or the YS/Jett/Rossi respond great to tuned pipes and smaller props turning a butt load more RPM, the AX does not so not all engines respond to propping the same.

kc

I can see the forest and the treess quite well, thank you.

Timing and porting of the engine define the performance curve of the engine. There are numerous .46 engines out there and his original message did not say which he had. Nor did I suggest changing any of these things or adding tuned exhaust systems or the like. Nor did he say he is already propping the engine for best pwoer output. If you are assuming he is already propping that engine for best power output you may be mistaken. Or you may not. Irrelevant in any case.

Yeah, if you are trying to match the load of the baseline prop then the general rule is drop diamter and increase pitch. Most of us know that already.

I suggested that in many cases the baseline from which people start when trying to increase speed is not at or near the peak horsepower regime of the engine. This depends on the engine and the prop you start with - that's obvious. All he said was what do you do to increase speed, increase or reduce pitch. Apart from the scenario you are discussing above which assumes the engine is at the best area in it's power curve already, or that you don't want to run it in a different rpm range for some reason, I am saying that he may want to look at the horsepower curve of the engine he's using (which we didn't know yet) because it may be advantageous to unload the engine. These two considerations go hand in hand when you are figuring out what to do to increase performance.

- I never argued that the general rule of dropping diameter and increasing pitch was not valid. It assumes you are content with where you are in the pwerband of the engine involved. Keeping the same load on the engine is also engine specific if top speed is the goal, because it assumes the rpm is where you want to begin with.

- The rule about diameter cubed times pitch is somewhat crude and doesn't match a lot of empirical data. But it is a rough guideline, yes. Some argue that the load formula works better using diameter to the 4th power. And as soon as the prop blade layout, airfoil etc. changes at all the rule goes out to lunch in many cases. In the end the tach tells the tale.

- I did not suggest that all engines respond by jumping up rpm. I said that some will, some will not, depending on the engine and the prop you start with. I specifically referred to the AX as an example where that might not help due to it's peak horsepower being at lower rpm than many other .46's.

That's all - you're reading stuff into my post that was not there.

Every engine that I've ever checked would lug if overpropped and revup if underpropped.....yes they all do respond the same. When propping for maximum speed, the plane doesn't care what the powerband of the engine is, it is only looking for maximum rpm from whatever prop is matched to that airframe. I guarantee if you are only propping for the advertised powerband, your engines might last 30 years but you will never see what that engine can really do.

the prop is matched to the engine and it's power band not the airframe, the airframe only limits how small of a prop you can us and effectively generate thrust out of it. same engine turning equally loaded props the smaller diameter higher pitched will be faster then the larger less pitched at the same RPM. not only is the pitch of the prop pulling through more air the diameter of the prop is reduced and that reduces prop breaking drag. under propping an engine that is timed and ported for torque to get a much higher RPM is simply going to result in overheating and leaning out so you're dead sticking after a couple min of WOT, it's not all about the most RPM.


kc

It would be interesting to apply our individual beliefs to a challenge set up, like AX 46s on a pair of stock Diamond Dusts, then get some clockings. Remember what you stated about timing and porting not having anything to do with getting more speed. I'd love to wager $10 for every mph that you're wrong.

you can't even read and comprehend what's been written. not once did i ever say porting and timing didn't have anything to do with it, porting and timing have to do what RPM RANGE THE ENGINE PRODUCES IT'S PEAK PERFORMANCE. I'm sorry that you can't grasp that and understand that taking an engine that is ported/timed for torque and under propping it for higher RPM is simply going to cause it to overheat and go lean in the air and die, thereby you'll never get your top end speed because by the time we get half the race finished, you're dead stick and landing...............you also can't stick a pipe on them and expect a significant gain in RPM from doing so, the timing just doesn't allow for it.

kc

[quote]ORIGINAL: KC36330

the timing and porting of the engine have nothing at all to do with getting more speed from the same said engine,

kc

Sorry dude, I guess I need a lesson in reading comprehension.

maybe I'm typing to fast for your reading ability so I'll type this one really slow.........do i need to explain what 'In Same Said Engine' means too????

timing and porting set an engines power band, the range of RPM where it reaches it peak performance.

under propping to get higher RPM is fruitless in an effort to gain speed, it simply overheats the engine and causes it to lean out loosing even more power. over propping does the same thing, they overheat and lean out loosing power. IF your philosophy worked everyone would buy a cheap engine and put a 6" prop on it and go 200mph unfortunately it just doesn't work that way in the real would outside your imagination.

you mention diamond dusts, mine has a Jett 60LX w/ 8.8x9 prop @ 21K static, another club member has a Rossi 21 w/ 6x9 prop @ 28K, in your theory his same pitched prop turning 7 grand more RPM should have a higher top end speed then mine but again in the real world he's lacking by 15-20mph and is very lacking in launch power.

kc

[quote]ORIGINAL: KC36330

the timing and porting of the engine have nothing at all to do with getting more speed from the same said engine,

kc
Hate to tell you this but guys have been modifying and running engines well past their designed rpm limits for many years. If YOU are having trouble doing it, then you are doing something wrong, try mixing your own fuel with quality ingredients. No matter how hard you try to manuever around your infamous quote, it isn't gonna work. There are only so many guys as gifted as Bill Clinton at doing that sort of thing. Your analogy of the higher revving .21 being slower than the .60 on the same ship isn't much news to anyone here, I'm afraid.

amazing how they had to modify it to run it past heir designated limits isn't it..............

have a wonderful day.
kc

No, you don't have to modify an engine to see real world gains above the advertised red line. The FOX .36 Combat Special is an excellent case in point. They were rated for 18,000 rpm......only losers ran them this way, most competitors ran them above 20,000 rpm, no mods were needed, just drop down in prop size and fly models capable of exploiting the bonus rpm. If you fly nothing but lead sleds, you will never realize the gains..

I wonder if there is any issue more contentious on RCU forums than prop selection and engine powerbands!

Well, perhaps - bring torque versus power into the discussion, then stand back and let the arguments fly!

In summary all I was trying to suggest to BBJ is that he might have some more horsepower available if he considers bringing up the rpm versus loading the engine to the same general level by matching load factors. Which to most is hardly startling news.

MJD

I wonder if there is any issue more contentious on RCU forums than prop selection and engine powerbands!

Well, perhaps - bring torque versus power into the discussion, then stand back and let the arguments fly!



At least nobody brought up castor vs. synthetic...or is after run oil necessary...or are O.S. engines junk because they peel...or...or...