Quote:

ORIGINAL: Juice

...I have some RPM numbers at various pitch settings. Can anything be determined from these numbers?...

Juice,


I will, using the Pé Reivers Prop-Power, give the the output on each of these props and find the RPM with the highest torque...

Output (in HP) times 5,252.1 and divided by RPM, equals torque (in lbs.ft.).
So, to simplify the calculation and since were not actually interested in the exact numerical value of the torque, I will only divide all HP numbers by the RPM at which it was achieved, to get a 'torque factor'... I used APC's and Bolly's 1.2 constant...

18x13, 3,840 RPM = 0.48 HP; torque factor .000125 (This is definitely too large...)
18x11, 4,400 RPM = 0.62 HP; torque factor .000141 (HP and torque)
18x10, 4,450 RPM = 0.58 HP; torque factor .000130

16x13, 4,760 RPM = 0.57 HP; torque factor .000120
16x12, 4,960 RPM = 0.59 HP; torque factor .000119
16x11, 5,200 RPM = 0.63 HP; torque factor .000121(HP only)

Peak torque seems to be somewhat below 4,400 RPM... because just over this number there is a rather sharp drop in both torque and in HP, despite the rise in RPM, but another HP peak is apparent (local absolute...) at 5,200 RPM.

It would be interesting to see what an 18x12 would show, since the gain from the 18x13 to the 18x11 is very pronounced.


This engine has an HP curve with very a pronounced 'bog' and a torque curve with a very pronounced 'peak'...

Guessing HP curves from prop values is useless unless we know exactly how much power the prop requires. Take Dar's example above. The peak HP value appears to be the same however it's 800RPM(really 1600RPM) different with different props. The engine is not changing it's torque curve. The above only proves that the prop constants or pitches are not accurate. This engine's peak torque should occur near 2500-3000 prop RPM. It should not be at 4,400 RPM like this indicates.

For example the Graupner 12x6 is well below Pe's estimates for power absorption. This is due to the fact that it's really closer to 5.5" pitch not the marked 6".

Since you have the ability to adjust he pitch to whatever you want I would try flying it at many different setting and see what works best. That engine should not be prone to detonation based on the design, so you shouldn't have to worry about that.

Quote:

ORIGINAL: gkamysz

Guessing HP curves from prop values is useless unless we know exactly how much power the prop requires. Take Dar's example above. The peak HP value appears to be the same however it's 800 RPM(really 1,600 RPM) different with different props. The engine is not changing it's torque curve. The above only proves that the prop constants, or pitches are not accurate. This engine's peak torque should occur near 2,500-3,000 prop RPM. It should not be at 4,400 RPM like this indicates.

Greg,


While it may be more accurate to measure an engine's torque using torque beams, Pé Reivers calculator is being made more accurate, as it is updated by him.

The torque curve of nearly all engines, is not as 'smooth' as some full-size car manufacturers want us to think...

Torque is affected by many variables and the outcome is a rather bumpy curve...Not only are there global peaks and dips, but there are changes on a smaller scale, which may bump the torque up, or down, by 10% and more, over a span as small as 100 RPM.


Please check out David Gierke's large .60 engines shootout in MAN 05/03, for torque curves plotted using 'torque beams'.

Smooth is a rather inaccurate description of all of them.


And you are right. The torque curve does not change. At the same RPM (at full throttle), the engine will consistently make the same torque...
But if you change the RPM, the value could change.

None of the props showed the same RPM as another... as this would have indicated a change in the torque curve...


But even a rather small RPM change; from 4,400 to 4,450, caused a drop of 8% in the value of the torque... It can happen.


And, I am totally not sure this engine really shows its peak torque at 5,000-6,000 crankshaft RPM.
...And Juice did not supply a prop and an RPM that would substantiate your bold claim...

Almost all engines make their peak HP WELL beyond the torque peak. Maybe you can show us one that doesn't. You could see such a dramatic drop in torque in a very poorly designed engine. But considering that RCV designs engines for purposes greater than model aircraft I think they know what a good torque curve looks like. The intake and exhaust systems are not so critical in model aircraft engines that one would see such strange curve.

The biggest problem I see with your assumptions is that the prop pitches are accurate. Remember he is setting the pitch by eye on a scale. Using a pitch gauge would be more accurate but most modelers don't have one.

Plot the data on a chart and look at it to see if it's realistic. The torque output at 4400 RPM is so far out of line that it's not realistic. The RCV-91CD reviewed by AJ Coholic in Fly RC showed none of this sort of behavior, and even using prop constants to back calculate the HP and torque curves show that torque is highest at the lowest RPM tested. And HP to be highest at the highest RPM tested.

One last comment. You've neglected to use the three blade constants in you HP calculation.

Greg,)


You are right. I did not use the 3-blade constant because I originally did not link between the previous posts (and videos) that discussed the 3-blade prop; and the post in which the RPM numbers appeared...

However, because only comparative figures were needed, to see what RPM displays the greatest torque, any constant I would use would give me the same result...

The HP numbers, however, were all factored down as a result...


I will post them again...


As to peak torque... The relationship of horsepower, RPM and torque is a mathmatical function...

Torque (in lbs.ft.) times RPM and divided by 5,252.1, equals Output (in HP)).

So, peak torque will ALWAYS be at a lower RPM than HP peak. There is no mathmatical possibility for it to be otherwise (unless the engine blows up during the test...[:@]).

The second special relationship between these graphs, is that if they are displayed on a single, two-axis Cartesian Plane, with the above units of measurement; they will always intersect at 5,252.1 RPM. Torque and HP will have the same number.

At that RPM, the numerical value of the torque will locally become lower than that of HP, as RPM rises.

But even at this point, the torque curve could still be rising, as RPM is increased (HP will still be rising, due to the mathmatical function that determines it).

Where the torque curve is level (same value over an RPM range), HP will increase as a direct function of RPM (and this part of its curve will be on a straight line from the root of the axes).


The graphs are not plotted together, but one is calculated from the values of the other, that are actually measured...


And, even in the light of Andrew Coholic's review of this engine, I only used the RPM numbers Juice supplied to plot this torque graph here.

These results do suggest a peak torque value at (or a bit below) 4,400 RPM...

There's nothing I can do to change that... (although some full-size car manufacturers do 'tailor' their graphs to smooth-out 'unattractive irregularities'... I 'caught' one such brochure, where the torque and HP did not 'compute'...Gotcha!!!).

HP numbers; as calculated with the correct 3-blade constant of 1.45...

18x13, 3,840 RPM = 0.58 HP
18x11, 4,400 RPM = 0.75 HP
18x10, 4,450 RPM = 0.71 HP

16x13, 4,760 RPM = 0.69 HP
16x12, 4,960 RPM = 0.72 HP
16x11, 5,200 RPM = 0.77 HP

...

And as to power output, my Saito .72 could actually spin an APC 4-blade 11x6 (constant = 2.0) at 10,860 RPM, which are 1.20 HP...

This RCV engine is not very powerful...

Dar, thank you for the education about how hp figures come about. It's really the sort of response I expect in a automotive forum.

Explain this torque curve. Edit: Oops!! see revised plot below.

Greg,


These curves appear in a Cartesian plane with two vertical axes.
You had used oz.in. units in the right side axis and spaced them so both curves would appear within the same area.

It is, however, a bit twisted...

The horizontal spacing between 3,840 and 4,400 RPM should have been much greater than between 4,400 and 4,450 RPM...
The former should have been nearly 11 times larger than the later... But they are equally close together, as portrayed in the graphs...

Also, the absolute HP numbers are higher all over the range, compared with those computed with the generic 3-blade constant (1.45) from the Pé Reivers Prop-Power.

I did not compute if the lbs.ft. to oz.in. numbers are proper, but since they are a bit less than double the numbers of .61 size two-stroke engines and this engine is geared 2:1, I guess that you did do your homework...

And yes, in automotive fora I also participated, when I did a bit of hot-rodding (over 15 years ago).
It is very difficult in this country...

What else would you want to know about it?

Oh something got screwy with the plot. I looked at the curves more than I paid attention to the axes. The plot looks more reasonable. I still question such a high peak in HP and what looks like another of similar magnitude at near maximum RPM. This is not a typical torque curve for a model engine as the intakes and exhausts are not of dimensions that would allow this sort of "tuned" breathing.

Compare this to the much more common curve of the RCV-91CD. I doubt there is an significant design difference between the two engines. These figures are calculated from the Fly RC review I mentioned. The curve is very flat and and the greatest torque was obtained at the lowest RPM of 6720, slowly dropping as RPM increases.

Greg,


I guess most model engine manufacturers do not have a PhD in rocket science...
So, as engineers, they would usually just 'slap together' combos that work.

Occasionally; especially with an engine like no other, like this RCV; some variables do act together, through resonance, flow-speed, orifice sizes, Etc., to cause a nearly vertical, marked drop (or a peak) in torque, over a very narrow RPM span.


I would urge Juice to set that 18" prop to pitch numbers around 12", in 0.5" (or smaller) increments; and to take RPM measurements, to see where this drop actually happens. It is in the engine's internals, so we might not know exactly why it happens, however.

Ye hear, Juice???


As you can see from your graphs, since the left side and right side axes are now 'synchronized'; the torque (now in lbs.ft.) and the HP curves will intersect at 5,252.1 RPM, as I wrote in the previous posts.


"I love it when a plan comes together" (Now, who said that???)

As to the latest post, Greg, the numbers just don't coincide with Juice's observations and RPM readings...

Also, would that RPM represent actual crankshaft RPM, or prop RPM???


This engine is not supposed to run at these prop RPM numbers, although its rotary valve actually does not have the RPM limitations of spring-loaded poppet valves.


As to absolute HP; I believe the generic prop factor Pé supplied in his PropPower, may be too low, for the wide-blade 'Hamilton Standard' look-alike prop Juice is using and I already remarked in regards to this in post #31.

But as to the torque, as portrayed by Juices numbers, I can only call them as I see them...

I don't know if the engines actually differ, but they do show different numbers.


Juice, 'Need more input'...

Yes, Dar they do intersect when using these units. It doesn't prove anything, it's simply a fact of the torque/hp relationship. That doesn't change the fact that the curve is not reasonable. I stand by my interpretation of the data presented that the torque figures near 4400RPM are erroneous. To truly learn anything we would need first to verify that the marked pitch on those prop is accurate.

See, the people at RCV are engineers. This is why I doubt such an odd curve is real.

Quote:

ORIGINAL: gkamysz

See, the people at RCV are engineers. This is why I doubt such an odd curve is real.

OK, Greg.

Let's now see if Juice can produce RPM numbers for fractional pitches, between 11" and 13" for that 18" diameter prop...


The numbers don't lie and the fact that the RCV people are engineers, does not immunize them from creating bumpy torque curves...
It is not an engineering error.

What does seem to be extreme in that drop, that not only is it a drop in torque, but that even the rise in RPM did not prevent the HP from falling also.
Actual dips in the HP curve are rather rare (except after peak HP), while torque dips are all over the place.


If we could only ask RCV about it... We'll see what they say.

Wow... Good discussion Dar and Greg! I just got home from a long day (which, by the way, included flying with the 18x10 3 blade prop again) so I'll have to read everything carefully tomorrow. Some of the discussion is over my head at the moment. But that's why I posted the question... so I can learn new things from people like you guys.

I'll try to get those RPM numbers... If not this weekend, next weekend for sure.

Dar... One thing I'm wondering is if you are taking into account the 2:1 gear reduction in your calculations. The RPM numbers I'm posting are the prop RPMs. The piston RPMs are 2x the prop RPMs. Again, I have to read everything carefully still, so if you have already taken it into account, please forgive my ignorance.

(Hopefully, at the end of it all, I'd like to be able to fully understand everything that was talked about here, and be able to make these types of calculations myself. I hope you don't mind me asking you dumb questions. )

Juice

Quote:

ORIGINAL: Juice

Dar... One thing I'm wondering is if you are taking into account the 2:1 gear reduction in your calculations. The RPM numbers I'm posting are the prop RPM. The crankshaft RPM is 2x the prop RPM. Again, I have to read everything carefully still, so if you have already taken it into account, please forgive my ignorance.

Juice,


Horsepower calculations do not take into account reduction drive ratios, since work is not affected...

Applying a force over a distance per time unit is equivalent to applying twice the force over half the distance.


Even though the torque is roughly doubled (I take gears at 100% efficiency), the HP output produced is unaffected, so the reduction ratio does not need to be taken into account.


When the power absorption of the prop is used, the engine is regarded as a 'black box' which produces this output... The RPM is what the prop is spinning (crankshaft RPM cannot be measured in the engine...).

I only mentioned these facts in my posts, since Greg posted graphs that got to 10,000 RPM (which is the actual crankshaft speed) and in some graphs he posted torque figures, that are roughly double those of typical .91 four-stroke engines... I felt an urge to explain...


I did not actually state any torque values; just comparative numbers, to show if torque is higher at one point than it is at another.


We will be waiting for your results... Do the larger and smaller props too, please, so we can have all the results together, at the same measuring sequence.


No need to apologize... People don't know this until they learn it.

Dar, this is ridiculous. If you are stating HP values, there are torque values to go along with them since we know what RPM they were produced at. You just neglected to convert the number to a common unit.

Quote:

ORIGINAL: gkamysz

Dar, this is ridiculous. If you are stating HP values, there are torque values to go along with them since we know what RPM they were produced at. You just neglected to convert the number to a common unit.

True, Greg.


...But since I entered that whole mumbo jumbo regarding the 5,252.1 (...), I did not want to get into fractional torque values in lbs.ft., or to use oz.in...

So, yes, I did neglect to convert, but since I looked for the highest value, A>B is just as good as 1.31>1.19...

The thing is, 90% of the prople who fly RC are not going to bother calculating anything, they are going to stick what they feel is an appropriate engine on the plane and fly and only hard core "experts" are going to notice any difference.

All well and good. But that prop looks totally SWEET!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Sorry, I gots a "thing" for 3 bladed props.

Yep, me too, especially Graupners, Dar will have to fix me up with some Bolly 3 blades one day.