vasek

My Feedback: (4)

hi,

i have no idea how to compare two different engines as to their performance in a given plane

i have a .40 Partiot (prop jet) and would like to know which of the two engines would be faster:

.49 MVVS or .60 Super Tiger bluehead (from the 70's)... both engines are new, the MVVS is a ABC and the ST is ringed.

the MVVS is a speed engine, the ST is bigger but may be not have as much RPMs but can swing a bigger prop;

HOW to evaluate their comparative performance?

is there a formula for this?

i need some advice,
thanks, Vasek

Not24

My Feedback: (1)

I know of no formula that can make that determination. One would think the larger displacement would have the advantage, but there are more variables than that. The only sure fire way to tell is to run them and see what they will do. If the 60 is capable of swinging a bigger prop, then it is capable of swinging one with more pitch, which will increase the top speed. I like to run 11-7 on 60's and 10-7's on 46's. If your 60 is a hot one, maybe you can run an 11-8 or a 10-10.

DarZeelon

Vasec,


In general, engine performance can be compared by the RPM they can spin a given prop...
But this obviously has its limitations.

If you compare an A-brand .50 to a B-brand .50, you would normally take an RPM reading, with an 11x6 prop and another, with a 10x6, to see how much each engine unloads in high speed flight.

The MVVS .49 is probably for lower RPM than some other engines of its size bracket.
It accentuated high torque at mid-high RPM.

If you use it with the #3248 tuned silencer and high speed is your goal, you would want to prop it so it will spin around 14,500 RPM, on the ground. With the tuned silencer tucked-in to minimum length, it would unload to about 16,000 RPM, in a high-speed pass.

If you try propping it for more RPM, its narrow passages will not allow more HP.


The ST Blue Head was a baffle-piston, loop charged engine, with even lower RPM capabilities.

It was used at some point for F3A. I think Ed Moorman used one.
But it will not make the same HP as the smaller MVVS.

It could, however, spin larger props, without getting out of the 'meaty part' of its HP curve.

piper_chuck

My Feedback: (12)

While I'm far from a speed expert, I've learned if you want to go fast you need high RPMs and high pitch. Also, it's best to keep weight to a minimum. Since replacement parts are hard to come by, it's best to not push older engines to their limits. Given these things, and assuming the MVVS is lighter, I'd go with it.

buzzingb

My Feedback: (2)

I have a Patriot 40 with an os50sx in the nose. It is bad nose heavy with the 50. If you use a 60 engine you might want to consider shortening the nose some. I tried to add lead to the tail but it was quite a chunk so I just fly it nose heavy. The Patrio will need all the power you can get but it likes high rpms the most. I fly mine with 9X9 and I think the rpms are around the 15+ range. The 49 may be the best engine for this plane.

Flyboy Dave

My Feedback: (21)

I don't pretend to know anything about the MVVS....but I do know the S.T.
would be a poor choice for it....too much weight, not enough power. []

FBD.

jessiej

I have two ST Bluehead 60s with no - baffle pistons and PDP porting.

jess

w8ye

My Feedback: (16)

Jess,

Welcome back

There was a day when the Blue Head 60 PDP with a 11-7 1/2 prop was king?

There are still some of these around this area left over from the good ole days. They never seem to wear out. I had one for a while.

Enjoy,

Jim

Cyclic Hardover

My Feedback: (3)

Ask Will Robison here to send you his RPM/Pitch/Prop chart in Excel. Although its not perfect , it has come very darn close for me to start with when picking a first prop.


If it were me and those were my options, I'd take the MVVS and try a 10/6 (APC) for starters and just see how it goes.

DarZeelon

Jess,


PDP is a feature of loop-scavenged engines, originally introduced by John Perry on the K&B .61 engines and later on the HB engines (both obviously with baffled pistons).

It consists of two louvers on the exhaust side of the piston-baffle, angled toward the baffle on either side, which introduce a cool, fresh charge into 'hell's kitchen', the piston's hot-spot.
This gives these old-tech, loop-scavenged engines the power of Schneurle engines, and extends their lives as well.

I am not sure if it is possible to have PDP ports, with TST (Travasi Super Tiger), some ST baffle-less engines have.


Can you post any good photos of the inside of those Blueheads?

w8ye

My Feedback: (16)

There were several variations in the "Blue head" Super Tiger G60. Including the flat top piston.

Anymore with their age, it's sometimes hard to tell if a person is running a plain G60, or one of the Blue heads? The blue fades away with use.

The current ST GS40 has, what one might call, PDP ports added in addition to the usual Schnurle ports. Some people would tend to call these PDP ports but it's really just a 6 port GS40?

Enjoy,

Jim

vasek

My Feedback: (4)

thanks to all for their input,

will go with the MVVS

i was hoping there would be some kind of mathematical formula that would look something like this (for the same aircraft):

comparative Speed = prop size\step x RPMs

so one could mesure max RPMs for two diff. engines having diff. prop sizes and still be able to "estimate" the faster setup... guess not

cng

I had a patriot with an os46fx plenty of motor ran a 10/7 apc on it dont know the rpm but it flew great. With retracts you could get to 80+ mph on the straight line. I have seen this same plane with retracts and a jett 50 with 10/7 apc prop doing over 120mph verified with a radar gun. have fun but don't take your eye off it. I lost mine when I lost the muffler on a down wind turn low from the ground. have fun

50+AirYears

I have seen two different formulas that predict the engine HP using pitch, diameter, rpm, and a divisor. Checked them both out against the power/torque curves in published engine reviews, and found that both are within 2 or 3% of the measured HP around the middle of the RPM band that was measured, while one drifted to almost 10% low at the low end, and almost 18% low at the high end, and the other one was just the opposite.
There is a formula to predict the maximum speed of a plane from prop pitch and rpm, but whether or not a given plane will even come close depends on a lot of other factors, including weight and total drag.
In general, the engine that turns a given prop at the highest rpm is developing the highest HP. That doesn't mean the highest power engine is best for a given plane. A high performance .91 might turn a 9-7 prop at 25000 rpm, but you wouldn't replace a milder .40 turning that prop at maybe 15000 if the plane were a 4 pound 500 sq" Quickie 500. The 91 would be too large and heavy to be practical.
As far as unloading of the prop in flight, one of the previous reviewers for MAN was set up to instrument his engines and monitor them through telemetry. He said people seem to get upset with him when he reports that very few engines unload more than maybe 500 rpm in flight, and even most tuned pipes don't increase RPM as much as people like to think.

piper_chuck

My Feedback: (12)

I don't know what most people think, but the most recent one I've measured gave me an increase of around 2,000 RPMs. Of course this was on an engine that was designed for a tuned pipe.

DarZeelon

Spinning a 9x7 APC prop at 25,000 RPM would, according to the PropPower calculator, require 4.26 HP.

Can you please name the high performance .91 engines that you were referring to, which can actually make this HP AND be capable of spinning at this RPM?


Also, using telemetry some years ago, most engines, when appropriately propped, would unload around 1,500 RPM in high speed flight.

However, when you prop a muffler equipped, sport .61 engine with a puny 11x7 and it spins very close to its HP peak in static conditions, it will be 'over the hill' when air-speed takes some of the load off the prop and it will unload significantly less... making less HP.


If anyone wants the PropPower (MS Excel file), E-mail me.

50+AirYears

Just kind of making up a generic example. And the comment about the tuned pipe also refered to the articles on unloading in the air. I don't believe the particular reviewer did anything with any performance engines, just the normal run-of-the-mill sport engines most people would use. I also never read a full fledged review on any of the full race engines used for competition free flight or control line speed like .15s that top 40000 rpm, or one control line .65 that had been tacked on dynos at over 28000 rpm. High power, but totally unuseable for any sport RC flying.

vasek

My Feedback: (4)

Thank you Dar, i got the file [sm=thumbup.gif]

downunder

I've never read an engine test on a speed .15 (the full on race .15's) probably because I'd say it's near impossible to do on a bench. The nearest I've ever read was on the development of a .15 by Luis Parramon of Spain who set the current world speed record in a competition at 302.5Km/h (188mph). It took him 2 years working in an engineering laboratory at a university to sort the engine/pipe out and it gave 2.7HP at around the 40K mark (that's 1080 HP/litre!).

But that record was in a competition where you only have a limited number of attempts. The absolute speed record (where you can try all day if you want) is 335Km/h (208mph). If the first engine was doing its best at 2.7HP then this one would be producing around 3.3HP (the models are virtually identical so the drag is too).

BTW...this is all done on zero nitro fuel.

Kweasel

My Feedback: (29)

That old ST60 is a light engine and is at its best with light prop loads, 11X6-7 because of the large carb. There were many different variants and they all look the same. The MVVS 49 will be better for a propjet where smaller props turning higher RPMs usually go fastest.

Red B.

Not that I have actually tried it, but a 1.05 cu.in. ROSSI 93R105 ducted fan engine is supposed to deliver 6.97 bhp at 23.000 rpm.
Rossi usually delivers what they promise, so it does not seem unreasonable that 4.26 bhp from a similarly built .91 engine should be possible.

/Red B.

DarZeelon

Red B.


Rossi also promised the .65 makes 12,000 RPM on a 12x12 prop... But actually got 10,500...

I would accept calculated HP numbers, for propellers with blade-tip-speeds that are Mach 0.7, or lower, using a tested prop calculator, like the PropPower.


If we can assume torque not to be dependent on RPM... If a very high power, piped .91 can make torque, which is calculated to an output of 2.8 HP, at 14,000 RPM; then if the same torque could be retained to 28,000 RPM, the output would be double that, or 5.6 HP, at that RPM.

If I estimate the 1.05 to make 16% more torque than the .91, due to its increased displacement, its output at 28,000 RPM would theoretically be about 6.5 HP...

But you say it is rated at 6.97 HP @23,000 RPM; 7.3% more horsepower at 18% lower RPM... The BMEP (a direct descendant of which is torque) and thus, VE must be 27.3% higher than that .91...
You could make the exact calculation from below, as well...

Impossible, I would say.

downunder

Hmmm..ok...well given that the .15 I mentioned above has 2.7HP then the .91 should develop 16.38HP but if you cut the revs down to 28K with that hypothetical flat torque curve then it'll only be about 11.5HP. These things aren't scaleable in the real world though.

DarZeelon

Brian,


True, they are not completely scalable.

I was wondering about that .15.... Even the best, most efficient tuned pipe cannot 'cram' more than 0.25-0.3 Bar of boost into the cylinder. You need a positive displacement supercharger, or a turbocharger, for higher boost pressures.

The best F1 pylon engines mange (reputedly) 3.6 HP from 6.5 cc, around 35,000 RPM.
This shows that the torque they make is very similar to that of a premium, tuned sport .40 engine (Jett?), that actually makes about 1.6 HP around 16,000.

So these engines work at very similar BMEP and VE numbers...

And so, I believe, must this F2D .15.... So at what RPM must this engine spin, for this 'glass ceiling' torque to make 2.7 HP?

...And I believe torque is very scalable. And engine of twice the displacement will make twice the torque. Half the displacement - half the torque...

If the RPM this engine was running is 40-42,000 RPM, it would entail a boost of over 1.5 Bar, from the tuned pipe... No boost from nitro, as this is an FAI event.

I say it again; Impossible.

downunder

Torque is scaleable...HP isn't. F2D is combat and while the high end engines are extremely powerful they're not in the same class as F2A (which are the engines I'm talking about). An "over the counter" F2A engine (like an Irvine 15R or Profi) will produce around 2HP at 40K and scaled up this would mean an F1 pylon engine making 5.3HP.

Not impossible.