STC's and updated performance charts

jasc15

jasc15

Pre-takeoff checklist
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Joe
After joining a club something like 2 months ago, I finally had my checkout yesterday in one of their 172s. I learned that it is a Penn Yan 180hp conversion. I asked about updated performance charts and learned that these aren't typically created for STC'd aircraft. At the very least, I'd like to have fuel burn figures for planning purposes. The instructor said that the Penn Yan site may have info on this, but I haven't found anything. Can someone point me in the right direction?
 
Ask the other people what they burn. That will give you more accurate numbers than any chart.
 
I agree it would be the most accurate, and the instructor told me what it was. So I guess my question is more general:
How useful are performance charts if they arent necessarily available, or even accurate for an aircraft?
 
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jasc15 said:
I agree it would be the most accurate, and the instructor told me what it was. So I guess my question is more general:
How useful are performance charts if they arent necessarily available, or even accurate for an aircraft?
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I don't know much about the turbine world, but I suspect that they're more accurate there, in large part since you have less control over the engine operation so there are fewer inputs in the input -> output algorithm.

In the piston world, I think the idea was to give you a reasonable idea of what your aircraft would likely do. For example, if the performance charts say that you need 6,000 ft of runway to take off, you probably won't be able to use a 2,000 ft runway. If they say you need 2,000 ft, then you're probably good on a 3,000 ft runway, etc. That way, someone who comes to an aircraft for the first time has some form of reference of what to expect from it.

While what they give you will typically work and is usually not a bad way to operate, it can be far from the best in certain applications, typically the higher power ones. The Beechcraft Duke comes to mind as one, for instance, that basically advocated running at peak TIT, and I think maximum performance cruise was recommended at 79% power. No wonder people thought the TIO-541s were problem children...
 
In the general sense, STC's like this one do not require updated performance data as long as the performance is demonstrated to be at least as good as the unmodified aircraft. Since it takes a lot of time and money to generate all that new data, they usually don't do it if they aren't required to do it.

In the specific case, for a 180HP STC'd 172, get a copy of the Cessna 172Q POH -- it has essentially the same engine/prop as the STC-modified 172's (both Air Plains and Penn Yan), and the performance data are directly applicable.
 
For a 172M with a Penn Yan conversion these guys

http://www.bordenflyingclub.com/cmqperformance.shtml

have this on their website:

Fuel consumption ranges from 7 to 9 gallons per hour. Typical cruise performance is 100 KIAS burning 8 gph providing an endurance of 4 hours from the 38 gallons of usable fuel and still leaving a 30 minute reserve.

Can't vouch for the accuracy of those numbers, but they seem reasonable. My 177B with basically the same engine ran around 9 gpm in cruise.
 
I suppose I should have mentioned it's a 172N model.

By the way, it seems like most of the 172's I've come across are N models. Was this a significantly larger production run than the other models?
 
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Joe, I've got those charts at home. I'm not there now, but I'll see if I can dig them up later today. The plane I rent most often is a 172N with the Penn Yan 180 hp engine. You are correct; the documents are not routinely provided. The plane's owner doesn't even have them. I got them from another pilot, who obtained them from the local DPE.

As for fuel burn, it averages out to about 10 gph, maybe a bit less. But Cap'n Ron is correct - the 172Q manual will give you the same data.
 
Our club has a 172N with the Penn Yan 180 hp conversion. I typically fly at 65% power (2410 rpm, IIRC). The graph provided with the STC shows that it should burn a little over 8 gph at that setting and that's about what it does. The 50 gallon long range tanks give it 6 hours endurance, about twice my personal endurance. :D
 
Sooo... If you got a graph with the STC... Where's Joe's graph? I'd expect that kind of stuff to be in an AFM supplement that comes with the STC.
 
TMetzinger said:
Sooo... If you got a graph with the STC... Where's Joe's graph? I'd expect that kind of stuff to be in an AFM supplement that comes with the STC.
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Don't be too disappointed if your expectations aren't met. Engine upgrade STC developers rarely spend the time and money to do all the testing needed to produce that data, and usually settle for "aircraft performance no less than unmodified configuration" (or whatever the exact phrase is).
 
Ron Levy said:
Don't be too disappointed if your expectations aren't met. Engine upgrade STC developers rarely spend the time and money to do all the testing needed to produce that data, and usually settle for "aircraft performance no less than unmodified configuration" (or whatever the exact phrase is).
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Yeah, but Ghery says he GOT a performance graph with the same STC for the same type airplane. That implies to me that the graph came with the STC. So I'm wondering if either Joe's club has lost/filed away the graph, or if Penn Yan started creating it later.
 
TMetzinger said:
Yeah, but Ghery says he GOT a performance graph with the same STC for the same type airplane. That implies to me that the graph came with the STC. So I'm wondering if either Joe's club has lost/filed away the graph, or if Penn Yan started creating it later.
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I've seen the Penn Yan package, and it did not have cruise performance graphs (altitude/RPM/speed/fuel flow).
 
Mid 8s if you're easy on it, up to high 9s if you're hard on it.
 
Next time I fly the plane I'll take a picture of the graph. It is simply fuel flow vs. % power (or rpm?). Looks hand drawn, as well. I don't know if it came with the STC or not, but it's in the plane, I've been using it for 11 years and it corresponds fairly well with reality.

Aircraft performance? All they say is that it will meet Cessna's claims. Well, the flaps are limited to 30 degrees and the max gross weight is now 2550 pounds.
 
Ghery said:
Next time I fly the plane I'll take a picture of the graph. It is simply fuel flow vs. % power (or rpm?). Looks hand drawn, as well. I don't know if it came with the STC or not, but it's in the plane, I've been using it for 11 years and it corresponds fairly well with reality.
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It can't be RPM, since with a f/p prop, RPM for a given %power varies with density altitude. However, there is a direct correlation between horsepower and fuel flow (when properly leaned), so a) I can believe such a chart exists, and b) if shown as FF vs HP, it would be equally accurate for any engine, not just this one.

Aircraft performance? All they say is that it will meet Cessna's claims.
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Yup -- the old "meets or exceeds" verbiage discussed above.

Well, the flaps are limited to 30 degrees and the max gross weight is now 2550 pounds.
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...but it will meet or exceed the takeoff and climb performance of the original airplane even with that extra weight. They just don't tell you by how much, because they didn't do the testing to determine that.

So if you want complete, reliable performance charts for all phases of flight, get the 172Q POH.
 
jasc15 said:
By the way, it seems like most of the 172's I've come across are N models. Was this a significantly larger production run than the other models?
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The 172N had the H2AD engine which proved troublesome, which is why it's the one that most commonly gets converted to a 180hp engine.
 
So I went to the tiedown yesterday to get a key that was left for me, and I decided to look through the POH. I was glad to see a supplement for the Penn Yan conversion, which had a power table showing RPM as a function of press. alt. as well as a fuel flow curve as a function of power. This is good.

My next nit to pick is weight and balance. I don't think there is an updated W&B envelope for this conversion (unless it's equivalent to the one in the 172Q POH). I have the basic empty weight with CG, but not arms for fuel, pax and baggage (I would guess the arms are unchanged, but the envelope should be different). I would imagine that this is something that should be required in an STC.
 
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the arms for fuel pax and baggage would not change because of an engine switch
 
jasc15 said:
My next nit to pick is weight and balance. I don't think there is an updated W&B envelope for this conversion (unless it's equivalent to the one in the 172Q POH). I have the basic empty weight with CG, but not arms for fuel, pax and baggage (I would guess the arms are unchanged, but the envelope should be different). I would imagine that this is something that should be required in an STC.
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Updated empty weight & arm sounds like things are completed correctly. The envelope doesn't change.
 
If gross weight changes, doesnt it make sense that the envlope changes? Especially since Penn Yan claims that it can be operated with 4 pax and full tanks.
 
sure the envelope changes with a gross weight increase but the location of the seats and fuel tanks doesn't change
 
jasc15 said:
So I went to the tiedown yesterday to get a key that was left for me, and I decided to look through the POH. I was glad to see a supplement for the Penn Yan conversion, which had a power table showing RPM as a function of press. alt. as well as a fuel flow curve as a function of power. This is good.
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...but it still doesn't give you takeoff, climb, or speed performance data.

My next nit to pick is weight and balance. I don't think there is an updated W&B envelope for this conversion (unless it's equivalent to the one in the 172Q POH). I have the basic empty weight with CG, but not arms for fuel, pax and baggage (I would guess the arms are unchanged, but the envelope should be different).
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Correct on both counts.

I would imagine that this is something that should be required in an STC.
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It is if you have the increased gross weight from the STC, and there should be a new W&B envelope in the STC package. I suspect it's the same as the 172Q's, but I'm not positive.
 
OK I know this sounds super conservative and I am probably the least experienced pilot on here, but:

In answer to your question, for the two planes I rent, I did the following:

1. Look at all charts, graphs, listings (for different RPM settings / altitudes etc) of GPH

2. Use the highest fuel burn numbers

3. Allow 1 hour "extra" fuel in tanks

4. Use the above numbers to plan my flight and fuel stops (plus for startup and taxi in the 152, for example, you need 0.8 gallons)

Again, not very exact or scientific but I do not want to run out of fuel and this seems like a pretty good way to avoid doing that.
 
Ted DuPuis said:
Ask the other people what they burn. That will give you more accurate numbers than any chart.
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This. Exactly what I had to do when I bought into the STC'd 150 (150 HP conversion). 8 GPH is alot different than the 6.5 GPH given on the charts.
 
jasc15 said:
If gross weight changes, doesnt it make sense that the envlope changes? Especially since Penn Yan claims that it can be operated with 4 pax and full tanks.
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If the gross weight changes then the envelope will be taller for some area but the fore and aft limits shouldn't change.
 
Clark1961 said:
If the gross weight changes then the envelope will be taller for some area but the fore and aft limits shouldn't change.
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Not necessarily. Typically, the aft limit stays the same, but the forward limit moves aft as weight increases.
 
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