prop question

what effect would switching a 10/6 prop to a 11/7 have on flight characterisitcs? my guess is none!(keeping engine the same,, say..46

2c)

in theory, you should get better climb, and higher top end speed, however, since RPM would fall, all benefits would be neutralized and you simply get a slower running engine. agree?/disagree?

Reply to
seany1124
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| what effect would switching a 10/6 prop to a 11/7 have on flight | characterisitcs? my guess is none!(keeping engine the same,, say..46 | 2c) | | in theory, you should get better climb, and higher top end speed, | however, since RPM would fall, all benefits would be neutralized and | you simply get a slower running engine. agree?/disagree?

It depends on many factors.

Your engines have a `sweet spot' where the run the best and produce the most power. Give it a smaller prop, and it'll run faster, but will produce less thrust (Note that this may, or may not, give you a higher top speed.). Give it a larger prop, and it'll run slower and produce less thrust. (Or, it may give you a bit more thrust, but a lower top speed.)

There's lots of other factors too -- the `dragginess' of the airframe, the altitude you fly at, what kind of flying you do, the fuel you use, and of course the exact engine you've got.

Actually, if things did equal out like you said (and for some combinations, they sort of do) a slower running engine may be a good thing -- it may last longer, and it'll be quieter.

You may also find that your climb improves, but your high end speed actually decreases. Or vice versa. It all depends.

Electrics make this sort of `what if' a whole lot simpler, since things like motocalc can calculate all of this with good accuracy.

Reply to
Doug McLaren

We have found that on sport planes, going from 10-6 to 11-4 gives generally better performance. 11-7 on a .46 might be loading it a bit.

Red S.

Reply to
Red Scholefield

There are some calculators available online too...

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and another that is downloadable...

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But what the others guys have said I totally agree with : ).

CJ

Reply to
CJ

a 46 usually runs best with a 10 x 6 prop. A simple rule of thumb is : if you go up 1 in diameter, you must also go down 1 on pitch. An example 10 x

6 = 11 x 4. It works in reverse also, if you go down 1 in diameter, you must go up 1 in pitch.
Reply to
zara

Considering your question as asked, one must compare the characteristics of your given engine with the prop change. Changing from 10-6 to 11-7 will REDUCE the RPM peak. For a typical .46 glow plugged 2 cycle engine the top air-speed, with a same pitch at a lower RPM, will also be REDUCED. Loading the engine with a larger prop will also REDUCE RPM, thus decreasing performance. Depending on the engine'e porting, you may have a low rpm torque curve or for the racing engines a shallow torque curve with maximum HP at a very high RPM. Loading a glow engine with a large prop can cause over-heating and poor operation. Cooling is also a function of the oil-fuel transfer through the case which carries away a significant amount of heat. Low flow and longer power times (lower RPM) can induce considerable metal heating. For performance such as speed, shorter props with higher pitch and for climb performance, longer props at low pitch will provide the 'gear ratio' to obtain whatever you are looking for. You still have to find the needed RPM for your engine and the job you want to perform.

Reply to
cainhd

Shouldn't that be 10X6 = 11X5?

Generally, most modern 46 sport engines live quite happily on 11X6 props.

Reply to
Paul McIntosh

the question asked was- what will be the difference in performance by switching a 10/6 prop to a 11/7 on a 2stroke, 46 engine? RPM will definately go down, but how does that translate to climb or top end speed? again, my answer is none, you just get a slower running engine. can anybody verifiy that?

most answers received so far a fluff !, but i do appreciate the input.

Reply to
seany1124

I think you are right. A bigger prop will give you more power and better climb but rpm will fall. I know this because today I tried to fly my electric powered SIG RASCAL after just changing to a bigger prop, and I lost RPM to the point that I was not able to take off.

SV

Reply to
Vallone

Most answers so far were right on the mark. Without specifying the particular engine or model, it is impossible to predict with any accuracy.

Some engine tolerate larger props and performance doesn't suffer that much. Others will fall flat and performance will drop considerably.

For the "average" 46 sport engine, going from a 10X6 to an 11X7 will result in slower top speed and reduced climb.

Reply to
Paul McIntosh

To put Paul's answer another way, two-stroke glow engines are akin to musical wind instruments. The combination of port timing, shaft window timing and compression ratio cause the engine(s) to favor certain operating rpm/handling characteristics.

Operating said engines out of their tuning range decreases performance (power production) substantially. Just as a sousaphone is not expected to produce a low E, although it can be pulled there by a talented musician, IIRC. Model two-stroke engines were designed to perform in a certain rpm range. Operating them outside of this range usually leads to lower performance (a weak E).

Of course, depending upon the type of model, sometimes the added benefit of a larger prop can benefit a particular model more than the extra horsepower gained by using the correct size prop.

This is why propeller recommendations are only starting points. Each model/engine combo will have its own "perfect prop".

Being an ex model Diesel engine enthusiast (can't tolerate the fumes these days), varying the compression ratio (normal for a model Diesel) to suit the load is also applicable to glow two-stroke engines.

While glow engines do not have variable compression ratio facilities built into them, with a given prop load, we are able to retard the timing by varying the nitromethane content of the fuel, choosing a different glow plug, or both. If those tuning opportunities are insufficient, we can add a head shim to decrease the compression ratio.

Ed Cregger

Reply to
Ed Cregger

AND, just so you are completely clear, suppose your vehicle is geared so that it reaches its top speed right at the HP peak in top gear. Adding another gear or decreasing the ratios farther will result in slower acceleration and a lower top speed.

Many cars capable of 130+ mph will usually go faster in 4th gear than the overdrive 5th because the HP available isn't sufficient to overcome drag with the higher gearing.

Reply to
Paul McIntosh

It definitely depends on the aircraft...

If its a very "clean" design, that doesn't require full throttle fo take-off, then its possible that the 11X7 will unload enough in fligh that it doesn't over-load the engine and you can see a speed increas and very little rpm loss.

If its a typical .40 size trainer you might overheat the engine possibly damaging it with the 11X7 on a .46

-- fhhuber50677

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Reply to
fhhuber506771

Assuming all other factors are equal, this rule is "general". The rule doesn't apply to some changes.

10x6 and 11x5 are similar, 12x4 give less than both of those at similar RPMs.
Reply to
The Raven

I concur.

I have a 46AX which I did some gentle running in on my trainer, I only had a

10x6 prop and could tell that was undersized.

Now fitted to the intended airplane an 11x6 is ideal for everything but landing (plane is very hot on landings) so, I changed to an 11x5. 11x5 works well for landing but really screams at full throttle. Tomorrow I'll take some ground RPM readings and see what I'm turning with 11x5.

Reply to
The Raven

Agreed, and lets not forget that not all brands of props are created equal.....

Reply to
The Raven

Assuming you could still get the same RPMS (which we know you won't), you're looking at approx 41% more thrust.

Engine may take longer to wind up, you'll see less revs but get more thrust (watch you don't overload the engine), and the prop won't help wash off speed on landing (something I need on my aircraft).

Reply to
The Raven
8>) That was good!

Ed Cregger

Reply to
Ed Cregger

| I think you are right. A bigger prop will give you more power and better | climb but rpm will fall. | I know this because today I tried to fly my electric powered SIG | RASCAL after just changing to a bigger prop, and I lost RPM to the | point that I was not able to take off.

... except that in this case, electrics and glow engines will behave totally differently. You can't really extrapolate what will happen to one based on what happens to the other.

For an electric, adding a larger prop (but keeping everything else the same) will generally increase static thrust, decrease RPM and therefore pitch speed, increase current draw, motor heating and decrease motor efficiency (the last one assumes that you were starting from an appropriate prop, of course.)

Really, under normal conditions static thrust is not a terribly important factor by itself (though you can make it into a factor of why your plane won't fly by making some silly choices) unless you've got a 3D plane or are looking for something to take off very quickly.

Correct prop selection is somewhat of an art, with oodles of variables to consider, and I'll bet somebody's written good guidelines on it somewhere on the 'net. But trial and error works pretty well too -- and with a 0.46 engine (or a small electric) it's relatively cheap to try lots of different props.

Reply to
Doug McLaren

A bigger prop wil reduce RPM.

Inspection of the engines torque and power curves will reveal this equates to less power out of it.

However, if the airframe is large, and slow, the propellor itself may actually work better, so compensating for this. It is normal to run 40 2 strokes at sub 10K RPM when their peak power is deliverd at 16K plus, simply because the props needed to allow them to rev that high are too small to be efficient (for slow models anyway), and the motor has plenty of power, so throwing some away on a gross mismatch is not a problem, and they run quieter and don't wear out so much.

As to whether they run hotter? well if efficiency is relatively similar, the less power they are delivering, the cooler they will run, so over propping is likely to make em cooler, not hotter.

Reply to
The Natural Philosopher

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