Arrow PA-28 Perf Charts - Choosing RPM

kontiki

kontiki

Cleared for Takeoff
Joined
May 30, 2011
Messages
1,121
Display Name
Display name:
Kontiki
Attached file is a fair reproduction of the performance chart found in the POH for a PA-28. The aircraft obviously has a CS prop.

Under both the 55% and 65% power columns, there are two columns for MAP, one set for operating at 2200 RPM and one set for operating at 2500 RPM.

When are the 2200 RPM settings preferred and when are the 2500 RPM settings preferred?

The only thing I could come up with is to use 2500 RPM settings if you might need a rapid increase in power. I'm really just guessing though, I have to expect there is more to it than that.

Thanks,
 

Attachments

  • Perf Charts3.pdf
    15.1 KB · Views: 89
When Lindbergh was briefing the Doolittle Raiders on how to get max range he counseled them to use the minimum rpm that the engines could deliver and still run smoothly.

If you are not going to be bombing Tokyo, decide whether speed or fuel economy is most important...look at the fuel burn and airspeed figures for each setting.

Bob Gardner
 
kontiki said:
When are the 2200 RPM settings preferred and when are the 2500 RPM settings preferred?
Click to expand...
It's so slow at 2200 that I never had patience for it (although it was -200 model, same principle applies). Why buy Arrow and run it in Lindbergh mode? Might as well buy a Cherokee then, save on insurance.
 
Sac Arrow said:
Hmm, that's odd, my power table is different.
Click to expand...

This table came from the schools, checklist. It contains a subset of the data from the POH for a PA-28R-201 SN 2844001 and up.

I can see now the answer is in the rest of the performance tables. Just following these tables are a series of graphs, one graph for each column of manifold pressure settings. Each graph lists a specific fuel flow, and with it I can find the associated TAS for pressure altitude vs temp, using those MAPs at that RPM.

I've read through this section a couple times and thought i got it, but in reality I had not.

I can see I really need to plan my next flight just like the example in the POH.

I'm really curious to see how closely I can get it all to match for my next cross country. It's possibly kind of geeky, but I think I'd consider even buying an arrow, if I can get decent performance out of it.

It seems docile and very robust (hey it's a trainer), and roomier than a Mooney, albeit slower. Sorry to bother you folks with the question before reading it all carefully.
 
Umm guys, aren't -200 and -201 different airplanes? -200 has a Hershie bar wing. Doesn't -201 have an Archer wing? No wonder tables are different.
 
zaitcev said:
Umm guys, aren't -200 and -201 different airplanes? -200 has a Hershie bar wing. Doesn't -201 have an Archer wing? No wonder tables are different.
Click to expand...

I just noticed that too. I assumed he was talking about a 200 series. But that aside, I think the -200 and the normally aspirated -201 still have the same engine? I would expect speeds to be slightly different but not power settings. For example my 65% cruise setting is 22 inches @ 2400 rpm. The 65% cruise settings in his POH would be closer to 70% for me.
 
if you want to fly slow, why'd you buy an Arrow? You could have just stuck with a Cherokee and gotten the same speeds and fuel burn from 2200rpm in the Arrow as 2500 rpm in the Cherokee . . .
 
Generally speaking, in normally aspirated engines, there is very little difference in either speed or fuel consumption between various MP/RPM combinations for the same % power. The biggest difference is noise level, which is why I tend to prefer the lowest RPM recommended for any given power setting in cruise (typically the bottom of the green arc on the tach). The limiting factor on that is altitude, as there will be an altitude above which you cannot obtain the desired power setting at any particular RPM due to the decrease in ambient air pressure, at which point you'll have full throttle and the only way to increase power further will be to advance the prop control to a higher RPM setting.

The other issue is obtaining climb power, where if you use less RPM than recommended, you either won't have enough MP available even at full throttle to maintain climb power, or the MP to obtain climb power at that RPM will be higher than the maximum recommended MP for that RPM, risking detonation problems.
 
kontiki said:
Attached file is a fair reproduction of the performance chart found in the POH for a PA-28. The aircraft obviously has a CS prop.

Under both the 55% and 65% power columns, there are two columns for MAP, one set for operating at 2200 RPM and one set for operating at 2500 RPM.

When are the 2200 RPM settings preferred and when are the 2500 RPM settings preferred?

The only thing I could come up with is to use 2500 RPM settings if you might need a rapid increase in power. I'm really just guessing though, I have to expect there is more to it than that.

Thanks,
Click to expand...

Typically the lower the manifold pressure and higher the rpm, the lower the ICPs and engine and prop efficiencies are and the noisier the cabin. Personally I only use the throttle at low altitude to keep the top of the green arc, use mixture to reduce power and RPM to increase it as well as regulate my ICPs when running higher power LOP (the more RPM, the lower your detonation risk).
 
Henning said:
Typically the lower the manifold pressure and higher the rpm, the lower the ICPs and engine and prop efficiencies are and the noisier the cabin. Personally I only use the throttle at low altitude to keep the top of the green arc, use mixture to reduce power and RPM to increase it as well as regulate my ICPs when running higher power LOP (the more RPM, the lower your detonation risk).
Click to expand...
Henning's techniques work well with the big-bore Continentals on his 310, but are not so appropriate for the small-bore Lycomings on an Arrow. See John Deakin's discussions on the topic on AvWeb (starting with the one titled "Manifold Pressure Sucks") for more on the subject.
 
Ron Levy said:
Henning's techniques work well with the big-bore Continentals on his 310, but are not so appropriate for the small-bore Lycomings on an Arrow. See John Deakin's discussions on the topic on AvWeb (starting with the one titled "Manifold Pressure Sucks") for more on the subject.
Click to expand...

I run every engine the same, the fuel does not know what engine it is in nor do the physics of combustion.
 
Henning said:
I run every engine the same, the fuel does not know what engine it is in nor do the physics of combustion.
Click to expand...
The design of the engine changes the best way to operate it, and your engines are not the same as the OP's engine. You may operate them the same, but that ignores those differences, and I prefer to operate each different engine as it was designed to operate. Deakin discusses these differences in his articles.
 
Ron Levy said:
The design of the engine changes the best way to operate it, and your engines are not the same as the OP's engine. You may operate them the same, but that ignores those differences, and I prefer to operate each different engine as it was designed to operate. Deakin discusses these differences in his articles.
Click to expand...

What part of engine design changes it?
 
Ron Levy said:
Cylinder bore-stroke and compression ratios. Read Deakin's articles for details.
Click to expand...

None of this changes the procedure, just the limits, the physics of combustion do not change except through chemistry.
 
Henning said:
None of this changes the procedure, just the limits, the physics of combustion do not change except through chemistry.
Click to expand...
Actually the chemical ratios also change things, but that's not the issue here, and things do change based on the physical parameters of the combustion chamber, including the ratios I mentioned. At least, that's what they taught me in my ME classes at Michigan.
 
Henning said:
Typically the lower the manifold pressure and higher the rpm, the lower the ICPs and engine and prop efficiencies are and the noisier the cabin. Personally I only use the throttle at low altitude to keep the top of the green arc, use mixture to reduce power and RPM to increase it as well as regulate my ICPs when running higher power LOP (the more RPM, the lower your detonation risk).
Click to expand...

Just guessing ICP is something like Internal Combustion Pressure?

I'll have to dig into the new FAA PP Handbook for a bit.
 
kontiki said:
Just guessing ICP is something like Internal Combustion Pressure?

I'll have to dig into the new FAA PP Handbook for a bit.
Click to expand...

Inner cylinder pressure but yeah, basically that. Since we have a fixed ignition timing system in our craft, we cannot vary when the maximum pressure will hosting the cylinder with that, and we want it to hit shortly after TDC; too much either side costs power, too much before TDC ends up in detonation. So, we are afforded 2 methods to operationally change the timing of the pressure event, we can either do it mechanically by changing the RPM of the crank which will put the piston in a different location at peak pressure, or we can change it chemically by altering the mixture. As we lean out, the pressure wave builds faster so to maintain the same ICP timing we increase RPM, as we enrich the mixture we reduce RPM to maintain the point of greatest mechanical advantage for the flame front.

Now I know that sounds complicated and makes you wonder, "How am I supposed to know where I am in all that?". Luckily though, you can hear it and EGT/CHT readings will confirm where within the process one is. Low CHT with high EGT about 5-15 degrees Lean of Peak tells you you are at the high power side of efficient ops. Richening from there will increase power as well as CHT as the timing event is advanced. The trick at this point is that at higher power settings this can run you into detonation. To escape detonation you have 3 options, add fuel, reduce fuel or change RPM, all three will change the timing of the ICP event.
 
Henning said:
Now I know that sounds complicated and makes you wonder, "How am I supposed to know where I am in all that?". Luckily though, you can hear it and EGT/CHT readings will confirm where within the process one is. Low CHT with high EGT about 5-15 degrees Lean of Peak tells you you are at the high power side of efficient ops. Richening from there will increase power as well as CHT as the timing event is advanced. The trick at this point is that at higher power settings this can run you into detonation. To escape detonation you have 3 options, add fuel, reduce fuel or change RPM, all three will change the timing of the ICP event.
Click to expand...

I think trackinq you. I still remember the flying W and a bit about air/fuel ratios from my A&P school days.

What I don't recall hearing about is flame front management. That is beyond staying out of detonation.

Ignition occurs shortly after mag points open. Of course go too lean (to Stoichiometric) and flame propagation is way too fast for power train to absorb.

I think you're talking about where in a fuel/air/RPM/MAP graph you get the best flame front propagation speeds.
 
You must log in or register to reply here.
Back
Top