Running props over square?
- Thread starter kiloalpha
- Start date
tgrayson
New Member
Here are two charts that I adapted from "Theory of Flight" by Richard von Mises. I left the numbers off because they would only clutter the chart. I calculated the slope of each line. The rule of thumb that shdw is proposing is derived from the ratio of the two slopes. The slope of the left chart is the torque of the engine, which is relatively constant as you reduce RPM, keeping the manifold pressure the same.
The slope of the right chart is determined (I think) by the rate at which the mean effective pressure (MEP) in the cylinder changes with changes in manifold pressure.
The numbers support shdw's thesis, on an engine very different from the ones in the airplanes he flies. I don't consider the issue proven, though, unless you can make an argument as to why the slopes of the two graphs should always bear the same relationship to each other.
Steve, as for the "O-320" chart you posted a link to (the actual chart says "O-360"), it really doesn't have the range of data to make a comparison to the IO-360 chart you posted; rounding errors could easily throw the data into the same range as the numbers shdw and I generated.
Hey man, I have never claimed to be a genius.:crazy: But I do know that I don't know all that much.
I think that's why we are all here, we all want to argue about what we don't know.
Tgray: does that graph on the left take into account engine propeller efficiency?
If so we will have to find some sort of rule or unwritten rule of thumb that designers bench off of to give the RPM graph. Meaning if they build the aircraft and equip it with a prop that will give it that similar graph. I think the RPM graph would be the only graph in question as it has more than one factor involved in performance.
The MP graph should remain fairly consistent as long as engine efficiency is the same I think, right?
If so we will have to find some sort of rule or unwritten rule of thumb that designers bench off of to give the RPM graph. Meaning if they build the aircraft and equip it with a prop that will give it that similar graph. I think the RPM graph would be the only graph in question as it has more than one factor involved in performance.
The MP graph should remain fairly consistent as long as engine efficiency is the same I think, right?
Man, you guys are throwing around different efficiencies here. Volumetric Efficiency is a very very different story than Thermal Efficiency. Now I could be completely wrong in this, but I'm not exactly sure that thermal efficiency is pertinent to the over square question at hand. The reason I say that is because the real concern about operating at a high MP isn't necessarily the pressure in the manifold, but instead the cylinder pressure.
The quick and dirty, in my view, is that the argument revolves around operating a high MP with low RPM which results in a higher cylinder pressure working against a greater resistance. The reason I say volumetric efficiency is pertinent is because with higher VE, more air can be introduced to the cylinder. With more air, more fuel can be added to create a larger burn, and therefore more cylinder pressure. Thermal efficiency doesn't really come much into play for building cylinder pressure, or at least in my understanding.
Oh, and by the way, automotive VEs greater than 100 can be reached N/A.
Yes it can. But it is through such a narrow operating range, that it is negligable. You may have an engine that has a 4500rpm power band, but will only achieve better than 100% through 50-100 rpm. We are talking about extream max performance engines (think NHRA or NASCAR), and not in an everyday street driven vehicle. Most of those engines are not even streetable.
As for the greater cyl pressure debate, I have to just shake my head at that one. It takes so much pressure to bend metal in an engine, that running oversquare values of 1900rpm and 50" of MP still will not bend metal. The problem more or less lies in the direct corilation of pressure=heat. Your typical light GA engine will not be hurt by running oversquare within resonable limits. Also, as RPM changes, so does VE(lower RPM's = lower VE). The engines are desgined to be at their optimum VE at max RPM. So running an engine that has a redline of 2700rpm at 2200rpm, the BE would decrease by a large enough amount to reduce VE low enough to not hurt things. It is eaiser to bend a con rod by over speeding the engine than by running lowRPM/highMP. But it is eaiser to overheat an engine with lowRPM/highMP's. I would be more worried about detonating the thing than bending something. And properly set up, and monitered closely, you will have nothing to worry about.
tgrayson
New Member
No. BHP never takes into account propeller efficiency, only THP does that. I would expect that propeller efficiency would be variable all along the x-axis on both charts. For the right one, as you reduce MP, you're reducing torque; how else can the governor maintain RPM other than by reducing the blade angle and hence changing efficiency?
Wow I just want to smack myself for not thinking that, and I will shoot an e-mail off to my flight dynamics professor if we don't come up with anything for this. I am about tapped out, this is the limit of my knowledge in this area and as you can see I'm still working out the kinks on this level. I wouldn't even know where to start looking for how that graph was created, my dole book might have it I will see tomorrow.
I have been under the impression that the intake manifold is what limits the difference in MP/RPM because it isn't made as strong as things like the cylinders. Is this wrong, if so what is the limiting factor for that chart I have been posting if it isn't the intake, is it really cylinder pressure?
For the bold I assume you mean follow the engine limitations and just like any other limitation if you stick to it you wont' break things?
tgrayson
New Member
Propeller, engine, and aerodynamic interaction is, in my view, the most complicated subject matter in basic flight theory, and I certainly haven't mastered the subject. You need to hold about a half dozen charts in your head at the same time and that's at the limit of the human short-term memory capacity. Any conclusions I draw are tentative.
USMCmech
Well-Known Member
The stress on the intake manifold is highest a IDLE, not full power.
Think of your intake manifold like a straw. Sucking in with nothing blocking the end prdouces no stress, block the end with your finger (or a throttle vlave) causes it to collapse.
Ok I see where my confusion was there, I was taking that thought from the manifold pressure sucks article and just applying it where ever the hell I saw fit.
Can anyone explain the limiting factor in the sea level and altitude pressure chart for continuous operation that I linked earlier. What are they concerned with keeping safe as MP/RPM spread increases and how does this link with different engine powers having different MP/RPM spread limits?
Thanks.
tgrayson
New Member
Detonation limits, I believe. Reducing RPM increases BMEP (brake mean effective pressure) in the cylinders. As for why it varies from engine to engine, displacement does appear in the formulas, with larger engines tending to reduce the increase in BMEP produced by RPM decreases. Also, compression ratios probably have a role, with higher compression engines having a greater propensity to detonate.
LatitudeDancer
Well-Known Member
They're not doing it right if that's the outcome.
I'm not too smart so I'll just insert this excerpt from a DA40 POH. The first time I went for about a 2000/25" combo on an XC with an ATP instructor, he about pooped. I just wanted to see what the engine sounded like.
So thats the culprit?!?!? Those ATP guys teach you that you must be squared?
Bingo! The higher the compression ratio, the more likely it is to detonate. That is one reason why so many are afraid to get away from 100LL, because the lead can act like a cushion for detonation. I know this is talking cars, but the therotical limit of premium mogas compression is around 11to1 for street use(and most consider that a ragged edge). The car I pit crew on (my brothers car) runs something like 130 (10to1 but 32lbs boost, HUGE CYL PRESSURE). More pressure (think compression ratio) equals more likelyhood of exploding the fuel rather than burning it. More pressure also equals more heat. Basically, there is less space in the cyl when the piston is at TDC on a higher compression engine. And lower compression also equals less power (as a general rule). The fuel burnt is still capible of makeing the same pressure, even if burnt in a smaller area. But, as RPM's decrease, so do VE, because the engine is desgined to operate at certian RPM's (think power band). So running an engine at a lower RPM is going to generate less HP. Do you think a 172 SP makes 180hp at 2300rpm. Nope. Mabey it makes about 145-150 at WOT and 2300rpm. 180hp is only made at WOT/sea level and 2700RPM. At 6500'/2700rpm/WOT it's still not makeing 180hp. And just because you have a constant speed prop, doesn't mean that because you are getting max rpm, does not mean you are getting full power at WOT either.
I went to ATP, and they teach a standardized procedure to get you to the airlines. Thats why they are the way they are. I did well there, and their porgram works. But most of their instructors just regergitate what they have been tought, and it's not always right. Running a Seminole at 25/25 in a climb is not the best way to do things, so when it was my turn to be PF on x-countries, I would set the airplane up as I saw fit (full power in a climb, and whatever I could get MP/RPM wise per perf. charts). It almost got me the boot because one person I flew with told dispatch, and I got a talking to. My argument was why would Piper put it in the perf. charts if it was going to hurt the airplane. Never the less, I lost, but didn't get the boot. They basically had no leg to stand on other than "thats how we want it." So I never flew with her again.
kiloalpha
Well-Known Member
Okay here is the info that you all been waiting for.
4,000 feet
MP / RPM TAS FUEL
22 / 24 154 92 lb/hr
23 / 23 156 89
24 / 22 158 83
25 / 21 155 80
26 / 20 145 75
6,000 feet
MP / RPM TAS FUEL
22 / 24 166 90 lb/hr
23 / 23 167 87
24 / 22 161 81
25 / 21 156 77
26 / 20 --- --
4,000 feet
MP / RPM TAS FUEL
22 / 24 154 92 lb/hr
23 / 23 156 89
24 / 22 158 83
25 / 21 155 80
26 / 20 145 75
6,000 feet
MP / RPM TAS FUEL
22 / 24 166 90 lb/hr
23 / 23 167 87
24 / 22 161 81
25 / 21 156 77
26 / 20 --- --
trafficinsight
Well-Known Member
remind me again, but those are book power settings too right?
nice data
I think it was all taken off of page 45/46 from http://www.lycoming.textron.com/support/tips-advice/key-reprints/pdfs/Key Operations.pdf which is the engine manual for his aircrafts engines. I don't know if his book gives the same information or not.
Great data, unfortunately it disproves the units of power at least as they are described. I guess still ok for a baseline, not perfect calculations, but probably still fine to make benchmark power settings at altitude.
Hey you might want to play around with some other settings, maybe even explore the 75 power range and see if you can get the fuel burn lower while keeping that speed up. I don't know if you noticed, but that link from that I linked again here has the chart for the engine as well to help you.
23 / 23 156 89
24 / 22 158 83
25 / 21 155 80
They got 6 gallons an hour and 2 knots speed increase, you can save them another 3 and loose 1 knot from the original for a total of 9 ghp saved and 1 knot lost. IMO that is a number maybe worth mentioning, good luck.