Why 75%?

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nworthin
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Why 75%?

Post by nworthin »

While I'm sitting around waiting for my new DA62 to be delivered, I've contented myself with reading the POH and flying the sim (X-Plane 11 and Aerobask DA62 with Pilotedge).

One thing I've come across in the POH are "recommendations".

In particular: Can anyone explain why 75% power is "recommended" -- whatever that means -- without regard to range or cost versus running at max continuous?

I totally get why 100% power might be time limited but why even put language in there with regard to "recommended" power settings other than to limit to:

1. Max continuous
2. Temp or oil pressure
3. Possibly icing or rain settings

This reads like weasel words or lawyer language. Anyone have any insight?

I expect that on some flights I would prefer to opt for max continuous since the relative increase in direct cost/mile at the higher power settings looks like less expense than the hourly cost of repair running at slower speeds. And, I would get there faster. On other flights I will be running slower for range. I want the option and not to have to worry that above 75% I am increasing the likelihood of a failure.
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Gordon
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Re: Why 75%?

Post by Gordon »

I'm just guessing here but I think they are talking fuel economy.

The recommendation is "Recommended cruise power setting is 75%" rather than "Recommended MAXIMUM cruise power setting is 75%"

If you look at the cruise performance tables - for example: Cruise Performance above 1999 kg (4407 lb) up to 2300 kg (5071 lb) (P332 of 506 in my manual) and looking at say, 10,000ft ISA+10, you can see from 50% up to 75% the percentage fuel consumption increase and speed increase between the power steps 50%-60%-75% matches so 50-60% is 9.2% increase in fuel flow for a 9% increase in speed 60%-75% is 11.7% increse in FF, 11.1% increase in speed.

However, above 75% to 95% you get a 15% increase in fuel flow for only a 10% increase in speed. That's still not too bad IMHO but in these environmental times maybe that's why it's recommended.

I agree with you fuel is cheaper than maintenance with such small steps in FF so if range and reserves are not a factor - fly faster rather than slower but 75% seems a good sweet spot!
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Re: Why 75%?

Post by CFIDave »

Just to add to what Gordon said, all of Diamond's Austro-powered aircraft have a recommended cruise power of 75% load. It represents sort of the "sweet spot" between speed, fuel consumption per mile, and range. It has nothing to do with lawyer language or liability.

While you can fly all day long at the 95% maximum continuous power setting, you're only going to get to your destination a few minutes earlier than 75% power on a multi-hour flight. If you're going a long way, you may find that flying at 95% power you'll need an extra fuel stop, thus negating any speed advantage. Drag increases with the cube of airspeed, so the extra horsepower provided by 95% power doesn't get you very much additional airspeed.

Often a better way to go faster is to fly at 14-15,000 feet on O2; up to that altitude range you pick up an extra 3 knots TAS for each additional 2000 feet of altitude. (Above that altitude range, you'll be above the Austro turbo's critical altitude and thus won't go faster, but you'll consume less fuel.)

We routinely fly at 75% power in cruise unless:
- we unavoidably encounter strong headwinds (e.g., 40-50 knots) -- then it may be worth the extra fuel consumption flying at 95% power to minimize the time our aircraft is subjected to strong headwinds, or
- we really need to get somewhere sooner (although we're not going to get there much sooner).
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photoSteveZ
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Re: Why 75%?

Post by photoSteveZ »

One other factor is the higher prop rpm at higher power settings. I find running at 2170-2200rpm significantly more tiring than at 2150rpm or lower: the extra noise and vibration just beats you up that much more. That said, I routinely cruise at up to 85% power at 15-16K’.
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Re: Why 75%?

Post by ememic99 »

In addition there’s better prop efficiency in cruise at slightly lower RPM which is in DA42/62 connected to load. E.g. my average TAS at FL160 is 178 kts at 87% (2300 RPM and FF 14.6 GPH), 85% (2250 RPM and FF 14.1 GPH) and 80% (2200 RPM and FF 13.8 GPH) while sweet spot 75% gives 175 kts (2070 RPM and FF 13.0 GPH).
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