Headhunter70

Gents, I am having problem where I would need your expertise.

I am using a Webra 91 P5 with a 12x10 prop which turns 12600 RPM. In theory gives that a speed of 118 mph.

The question here If I would reduce the prop size to 10x10 and maybe gain another 1400 RPM that would give in theory a speed of 132 mph. Does it make sense ? Should I expect a significant increase in speed ?

What would be the limit of the prop diameter. I do not believe it is possible to reduce the diameter without taking the airplane into consideration....how to find the best setup without experiementing with a lot of props (which I do not have yet).

The plane is about 4 kg and has a wingspan of 1,70 m

rgds
Gordon

bob27s

My Feedback: (19)

The short answer is NO but give it a try never hurts to try.

The setup you described will not likely see a 10mph increase with the prop change. But you may see some improvement.

First, forget the prop math. Props are designed differently. A Zinger 11x6 performs a great deal differently than an APC 11x6 or Vess 11x6. It can be a reference, but it does not take into account airfoil, blade design that the prop does work.

There are a few threads on this, and we have documented (measured) some aircraft flying far beyond the math-calculated pitch X rpm. In most cases, you end up far slower than what you calculate.

Prop selection is a balance between prop/engine/aircraft They work as a system. Sometimes you have to try different props to learn which is best.

In most cases, you want to keep the engine turning up. RPM = HP. On that Webra keep it in the 13,000 rpm range. So you are close now. Looks like you have a pipe setup which gives you the flexibility to tune the power curve for your prop.

Smaller prop diameter creates a smaller "disk". That prop disk creates drag. So smaller diameter will generally be quicker - but you trade off for other aspects of aircraft performance such as take-off roll and acceleration.

Bob

Headhunter70

Thx Bob for your answer...

and here the million Dollar question...is there a formular/guideline available that desribes that system ? I am sure it is not easy as we have a lot of paramter to consider but I am looking for something that gives me the best prop for a certain engine on an aircraft ...Hope it doesn't sound to much rocket sience...but I believe on full scale planes they do the math and do not try hunderds of props..

rgds
Gordon

Von Ohain

You can calculate the optimum propeller diameter for your aircraft, BUT R/C rated gear seldom comes with much of a datasheet.
You need a lot of parameters which simply isn't avaible for R/C rated equippment, and finding these parameters yourselftrial and is more work than just funding optimal prop by trial and error.

But for the sake of discussion:
If you know the drag coeficcients of the fuselage, the weight of the aircraft, and airfoil data for the wing, which you need to predict AOA, and drag for a given weight and speed.
Then you will know how much thrust you need for a given speed (top speed, the most demanding speed).
Thrust times speed is power, power of the engine is known, if you knew the efficiency of your airscrew (wich you don't, its r/c graded equippment with no data avaible at all) then you would know what kind of a prop you need.
But forget about it, R/C planes never comes with any of the data you need. the closest you get is some high end speedplanes, wich does let you know what airfoil they use, and data is avaible for some airfoils.

speedracerntrixie

My Feedback: (29)

I think with that engine, you are well below the power curve for that engine. If not mistaken, that engine has the same port timing as the heli version. You need to prop it to get it into the 15,000 rpm range. Experimenting with props will give you the answer. You can go down in Dia but at some point you will get too small to work with that airplane. An 11X9 or even a 11X8 may be worth a try.

ger56

Hi ,

You should give a try with a 12x9 or 11x9 . I believe your engine is taking all the rpm in the air .
Calculation Yes but Life test are much better .

Ger56 ( KGW)

Headhunter70

Well I guess I will give it a try, I have 11x 8 APC in stock... we have measured the plane with a 163 km/h last week on level flight and with a 208 km/h in a dive
( well it was on old fart with bad eyes who hold the radar gun ) at least I will have a reference to see if there is an improvement....but I have my doubts

diggs_74

One other thing to consider is that increasing speed by increasing pitch x RPM is not a linear increase, it's exponential.. As the plane increases in speed, the drag produced increases as well, that in turn eats up power and reduces the increase you've calculated.. It's all a bit over my head but that's as I understand it.

HighPlains

My Feedback: (1)

Time to wade into a few subjects on this thread.

The OP talks about the speed with his Webra based on the prop pitch and RPM.

First off, the static RPM is usually quite a bit lower than the actual RPM in the air. This is known as "unloading" and is dependent upon a number of factors, such as the engine timing, compression, exhaust system, and finally the drag of the aircraft. While some combination's might only unload 500 to 1000 rpm, others may unload about 5K. These are in level un-accelerated flight. Your goal is to prop the engine so when it is unloaded it is at the rpm that produces the maximum power. Fortunately, the rpm for peak power is usually 500-1000 rpm with a minimal variation in output power.

An airplanes speed is directly proportional to the power of the engine when matched to the optimized prop. Double the power and the speed will increase by 26% (cube root of the ratio of the power)

Bob27 gives practical advise, but I will amplify a few points on props.

All props are compromises, but if your goal is strictly speed then the options become much narrower. As a rotating wing, the design of the prop is to generate the greatest thrust with the least drag. So an optimized prop must consider the shape and speed of the airflow over the nose of the airplane. What works on one model will not work on another quite as well due to way the air moves past the fuselage. Even spinner size effects this. So while you might think a constant pitch prop would be the best, it is not because the air closest to the fuselage is moving slower than the air further away from the surface of the fuselage. So a prop designed for speed may have the pitch at the hub 5 to 10% lower than the pitch at the tips. Most sport props have pitch curves that do not make great speed props. If at all possible stick with racing props, though the selection is somewhat limited to certain engine sizes.

When you are close to the right prop, changes in diameter or pitch are in tenths of an inch, not one or two inches. You can learn a lot from carving wood props (changing pitch, diameter, and airfoils), or working for a prop manufacturer.

js3

My Feedback: (3)

Gordon,

What do you think of the Webra .91 P5? I have one that I'm about to break in. So far it's never seen fuel.

Thanks,

Headhunter70

John,

I think its one of the most powerful engines in it class, not necessarily the best choice for high RPM (16 K +) but I am very happy so far. I used WEBRA's break in procedure.. 20 min with 20% castor oil and a 14x7 prop and than switched to my normal fuel. With that prop I can pull my 4kg+ plane vertical to the moon

rgds
Gordon

js3

My Feedback: (3)

Thanks very much for your thoughts. I'm looking forward to running mine.

ger56

Hello Headhunter70.

Did you have any chance to test the prop I recommand .

I have heard last week was not as good for you as the ones before and also have heard
you were having NZT clearpills to try to challenge the same pilot level as your competitors .

Seems you are running out of the pills looking at your last flight .

Well even if this is most unfair , I do understand it's difficult for for you to be laways behind those KGW's planes .

take care

Ger56

Headhunter70

Hi Ger56 ,

You are telling the truth , NZT helped me a lot to pilot at my best and to be one of the fastest on the field .

Unfortunately I am running out NZT so I may ask one of the guy on the field ( Bruno ? ) to pilot my plane as i can't
be ridiculous and go back to my poor airplane piloting ,after 2 fantastic weeks where I was almost able to compete the KGW's .

I know it is unfair but my only goal is to be fast and to beat the KGW planes .

Headhunter70

Headhunter70

I have tried a 11x9 last sunday, which gave me a promising 14600 on the ground, but already during take off the engine started to make problems, being to fast for aborting the take off I made one round and landed, put the engine more rich and flew again....speed was the same like with the 12x10...so I guess I will stay with the 12x10 which has proven to be a reliable setup...

these plane is anyhow not really made for SPEED...look forward to fly my Jackal with Jett engine 0.56 in two weeks and hopefully soon the Nemesis NXT

rgds
Gordon

rmenke

My Feedback: (13)

HH:

Now you are talking real engines and power, the Jett. Webera 90 is a good 3D power machine, but far from a good speed engine. If you find a Webra 55 speed around, that thing will push the Jett hard if you break it in carefully, more than a gallon. If you run it slightly lean for a second, its toast pushing a 10 X 9 apc with the Jett 60 pipe. Gotta stay on the rich side. Don't even try unless you have a lot of experience on the needle. ENJOY

airraptor

My Feedback: (66)

high plains dont assume all engines will unload in the air. If you have a plane in the 100 mph range and have a high pitched prop that prop can be stalled on the ground indicating a higher rpm then in the air. The prop will load up in the air and bog the engine down.
To get this plane faster you will need to get more power. try a YS115 40% nitro and a 14x12 prop. Also get rid of the wheel pants also.

freakingfast

My Feedback: (3)

Just slapping different props wont show what's the ultimate setup when using a tuned pipe. Header length must be experimented with to achieve that goal. When using a tuned muffler, you are more locked-in, then prop swapping is your main tool along with plugs & fuel (10% nitro max on Webras).

I will have to agree with Bob and now your current assessment, that you are very close to the best prop with the 12X10.

As for Remenke's suggestion of using a Webra 55 (I'm assuming he's referring to the round black head version), I'm concerned that it is incapable of swinging enough disk area prop needed for that size of Nemesis plane.

Tach numbers for the Webra 55 GT (black head) with red Jett muffler on 10% fuel with APC props 10X8 15400, 10X7 17100, 9X9 17600, 9X10 16600.

If you want to go faster than the current 91 in use, I'd be leaning towards airraptor's idea of a YS 115....but that much nitro.... optional

Headhunter70

Gents,

just to give you an update...was running last Saturday a new prop 11x11N...gave me 13600 RPM on the ground...now the Nemesis is flying like a racer

rgds
Gordon

robbinsp

My Feedback: (10)

i have a 40 size patriot with an enya 60 2 stroke 7033 on a pitts muffler.

i am not concerned on longevity of this engine, however id like to some flight time out of it...
question is...

what prop for max speed given my setup. I plan on running 30% nitro heli fuel full synth oil 20%

thanks