How do you know when engine is happy with prop?
 

I was wondering how you know when you have the right prop for a particular engine. How do you know if an engine is a screemer or a torquer. I know that some engines are made for high rpms and some are made to create lots of power at lower rpms but how do you tell this. Is there some sort of test you can do? How do you know that you are not overpropping an engine?

The reason for the questions is that I am getting a couple of ASP 25 engines and I am thinking that they are the same as the magnum 25 xl. The bore is 17.42mm and stroke is 17.0mm. The engine specs. are all over the place depending on where you look. Now just from the posts I have read, the magnum 25 xls which has a 18 x 16, stroke x bore runs at a lot higher rpm than the xl. Will the xl be happier with a larger prop at lower rpms?

RE: How do you know when engine is happy with prop?
 

A lot of the questions you have may have to be learned from experience by running and using different engines?

Study these OS engines to get an idea of how some of the ASP/Magnum engines will respond.

The ASP XLS series is more or less a copy of the OS "FX" series

The ASP XL series is copy of the earlier OS "SF" engines before the FX series.

There were even ealier ASP engines copied after the OS "FSR" series.

RE: How do you know when engine is happy with prop?
 

Sure there is, slap on some props and give them a go;)

RE: How do you know when engine is happy with prop?
 

OK, so I"give it a go'. What am Ilooking for? When will Iknow when all is a best that it can be? Atach is useful at the bench - not the same as in the air. I can see / hear throttle response on the bench - not the same as in the air. How can I tell?

RE: How do you know when engine is happy with prop?
 

Things to look for:
If the engine is lumbering and running hot, tuning High needle is a bear, above half throttle no more extra rpm...... Prop is too large
If the acceleration is lightning quick, engine screams it's lungs out, little thrust.....prop is too small
With the right prop, the engine tunes well, increases rpm up to about 3/4 throttle, has decent acceleration and thrust. A joy to operate.

RE: How do you know when engine is happy with prop?
 

A 9x5 prop ought to be about right for the engines. Maybe even a 9x6 would work well too.

RE: How do you know when engine is happy with prop?
 

Hi!
For the sport engines you mentioned ...there is no difference!!! They are all the same, can handle both small and large props!
The difference is only noticable when it comes to racing engines (Nelson, Jett, MD,some MVVS ,some Rossi etc) compared to all sport engines.

Take your .25 engine for example...they can handle both tooth pick props, 7x4 and up to 11x5 without problems ...providing you set the engine right.The question is as usuall: Which prop for what plane!

There isn't such a thing as "a too small prop" for an engine...it's only a too small prop for certain plane! And the other way around...a too big prop for a plane! our glow engines can handle rpm without problems...it's too much heat that is our worst enemy!

Take a look at the planes below!
The blue trainer with a span of 160cm that weigh 1,8kg is powered by a .28 engine using a 11x5 APC prop ,slighly modified, with less blade area than standard. RPM with this prop is around 9000-10000. Performance is very good with near hoovering capability, but remember, this is a trainer and it has this large diameter prop to both climb well and fly slowly, not to be a fast racer!

The same engine put in a smaller model more aerobatic model with a span of, lets say 110-120cm, weighting around 1,2-1,5kg , a 8x6 or 9x5 prop would be more suitable. RPM with those props might be around 13000-14000rpm using a quiet silencer.

The same model in a Quarter midget, pylonracing type model, with a span of 110cm, weighting 1,2kg, equipped with a tuned pipe could swing a 7x5 or 7x6 prop at around 21000-25000rpm without problem...of course as always ...provided you set the engine right!

Soo what it bears down to is you as user have to set the engine correctly to not run it too lean, which means lots of heat! And you can only learn what is good for your engine by experience, by listening to how to the engine behaves and sounds.

RE: How do you know when engine is happy with prop?
 

Jan, Are you talking of the same engine, that has an operating range of WOT 9,000 to 25,000 rpm?? Think again. This is real bad advice for a novice flier.
At 9000 rpm a .25 engine is bogged down and fry the piston/cylinder set.
at 25000 rpm that same engine probably will throw the connecting rod.

Typical .25 rpm are in a 12,000 to 16,000 envelope. anything out of that range will need (very) special attention.
Your pictures show an MVVS engine. Change the range from 14,000 to 18,000 rpm in that case. Also use castor oil in the fuel. Above 20,000 rpm the engine needs to have the connecting rod clearance adapted, or it will encounter mishaps. I know because I did prepare engines for those conditions, so they survived to fight another day.

RE: How do you know when engine is happy with prop?
 

What Jan did elude to......"Which prop for what plane!" The question the OP needs to answer: Type of plane and flying style.

RE: How do you know when engine is happy with prop?
 

Flybyjohn: Follow what PE gave for operating your engines, and give them a long life. I spent time trying to get a Fox .19 engine to peak on a 10x6 prop, and finally move to a 9x6 prop which made the needle valve become effective. Had no trouble adjusting the needle valve to the 300 rpm's below peak. It is really best to operate engines a bit below full power to make them last for a number of years.

aerorich73

RE: How do you know when engine is happy with prop?
 

How do you know when an engine is happy? Listen to the engine (im going to try and explain over the inet…not easy)
An overproped engine will not unload….or unload for a short period of time then drop rpms again.
Example: put a big prop on your .25 maybe a 9x6 or 8x6, tune the engine and fly the plane at WOT (wide open throttle). Gain some altitude then put the plane in a 45 degree dive and listen to the engine…..it should increase in rpm….now once the engine maxes out….level the plane, fly it for a min or so and LISTEN…..what does the engine do? Increase rpm? Decrease rpm? Stay about the same? If the engine is overproped the engine will have a hard time maintaining rpm, it will eventually slow down. Engines that are correctly proped should not loose rpm, they normally increase just slightly. Engines that scream (hold the same rpm) during climb, dive, level flight AND on the bench are underpropped.
p.s. Correctly Propped engines normally unload without a dive, they normally unload when the airplane speeds up.

Again this is hard to explain, just go to the field, try some props, see which one sounds/feels the best and fly that…..most of the time the smaller props win

RE: How do you know when engine is happy with prop?
 

If a prop is so small it causes the engine to fall off the right side of its bhp curve producing rpm at the cost of net power, I'd argue that the prop is indeed too small. All you have then is a high frequency noise generator.

RE: How do you know when engine is happy with prop?
 

A guy put a 8 x 4 on his OS LA 40 and the engine wanted to surge in certain situations. It didn't sound like the normal LA 40 engine.

The engine was on a Diamond Dust at the flying field. It did pretty good until he caught a fire ant hill on a low pass. It was history after that!

The Diamond Dust delta was intended for .25 engines and even though the 40 was very inefficient with the small prop, it did OK on the small plane.

http://www.diamonddustrc.com/glow_mo...amond_Dust.htm

RE: How do you know when engine is happy with prop?
 

Hi!
Pe! I can assure you that the MVVS.28 I talked about can handle both a modified 11x5 APC prop at 9000-10000rpm as well as a 7x5 or 8x6 prop at 15000-23000 rpm and doing so running on just 5% nitro!
This engine unloads in the air (and on the ground) with that big prop! But it took some time to get the prop shape right before it did.
Soo...don't be afraid to try different prop sizes...and don't be afraid to "over rew a two stroke" for there isn't anything to be afraid of...you won't destroy anything just because you rew more than the recommendation says!

RE: How do you know when engine is happy with prop?
 

you are walking the edge at 20,000 rpm+ with that small prop. Believe me.

RE: How do you know when engine is happy with prop?
 

I've only heard of 'unloading' being something that happens in the air as the prop becomes more efficient by moving forward. I don't think you can 'unload' on the ground.

That Diamond Dust that Jim is talking about was gliding along the top of the grass wide open when it hit the ant hill. I saw another DD make a full throttle pass inverted when the guy pulled up to climb out. I think he had a .25. It made a perfectly formed burst of confetti in the air. The little .25 was screaming by and suddenly all became quiet with a whump.

RE: How do you know when engine is happy with prop?
 

I never dreamed a LA 40 could run that fast let alone be able to fly a plane with that small of a prop. I never saw the guy with that engine again.

RE: How do you know when engine is happy with prop?
 

Well I found the instructions for the 25 engine and the recomended prop is a 9x5 or 9x6 and the practical rpm range is 2000 to 16000 with a note stating that you should prop the engine to get between 10000 to 14000 peak.
I understand what was said about matching the prop to the airplane but I just wanted to go a step further and match the prop to the engine to get the most efficiency out of the engine and then match the engine prop combination to the right type of airplane. I guess I was just wanting to know if I picked the right engines for my airplane. With all this information, I think I will be able to tell if I am overpropped or underpropped now.

RE: How do you know when engine is happy with prop?
 

Just put a 9 X 5 or 9 X 6 prop on the engine and use it.

it will be fine that way.

I was around a OS 25FX for a few years and those where the props he used. It ran perfect.

RE: How do you know when engine is happy with prop?
 

Hi!
I agree with you w8ye! 9x5 and 9x6 will be fine for most aerobatic/sport typ planes! 10x4 or larger on slow flying planes.
Unloading is perhaps wrong word. What I meant was, what the engine is capable of turning without being too sensitive on the needle!

RE: How do you know when engine is happy with prop?
 

+1

Needle spread is a good measure of how happy the engine is.

RE: How do you know when engine is happy with prop?
 

Now ya went and confused me (again).

Needle spread? - the characteristic of an engine running at full throttle as to its sensitivity to small changes in the HS needle as measured by its RPM? Small needle spread (too sensitive) means that virtually any adjustment of the HS needle results in a noticeable RPM change? Broad needle spread (desireable) means that the RPM remains (virtually) unchanged with some adjustment of the HS needle?

Small needle spread would suggest that the engine is "just barely"able to turn this particular prop at that RPM?
Broad needle spread would suggest that the engine is "easily" able to turn this particular prop at that RPM?

Sounds like you would move up the prop scale until you find that point where needle spread is small or you cannot attain your RPM goal then back off to the previous prop (where you do have decent needle spread).

Am Iclear? Iis my head buried to my ears in confusion?

RE: How do you know when engine is happy with prop?
 

Correct . . .
I never liked it when one click lean was too lean and one click rich was too rich

RE: How do you know when engine is happy with prop?
 

I now know "when an engine is happy with a prop".

Yeaaaaa!!! I'm easily pleased (just ask my Ex).

RE: How do you know when engine is happy with prop?
 

Like when running 0% nitro. Well maybe two clicks........:)