Deceasing Vy with increasing density altitude

[Zonkor]

I'm currently flying in areas with high temperature differential to standard atmosphere and I am struggling somewhat with a solid climbout at high density altitude airports (7000 - 10000 ft), especially since they are often in challenging topology. I thus want to max out the performance of my Cherokee.

I'm aware that Vy decreases with density altitude, and Vx increases, until they meet at the absolute ceiling. Of course, I also lean for best performance during runup and accelerate in ground effect before climbing. But the initial climb rates I'm getting are a bit of a crap shoot before finding the right IAS for Vy -- and this in the critical initial climb phase.

So how much do Vy and Vx vary with altitude? The Cherokee POH doesn't include data about that. Any theoretical pointer, rule of thumb, or actual performance data would be helpful.

From my own experiments, for the Cherokee Warrior II at medium weight, the Vy changes from 78 kias below 3000 ft DA to about 75 kias at about 5000 ft, and to about 72 kias at 8000 ft. Does that sound about right?

[marioair]

Isn't it a straight TAS to IAS calculation on a whiz wheel? Eg if POH states 75kts at ISA condition calculate what this would be at your density altitude and that's your IAS target.

[thing]

I agree with the above. By the way, where are you flying from with an altitude of 10,000'? That's higher than I've actually managed to get a PA28 to, no wonder your take offs are a bit dodgy.

[Tinstaafl]

If you care to spend a bit of time flying a series of test climbs & descents to gather data you would be able to calculate what you need (and a lot more). It's called the bootstrap approach.

Info here: The Bootstrap Approach to Aircraft Performance<br>(Part One — Fixed-Pitch Propeller Airplanes) (http://www.avweb.com/news/airman/182410-1.html)

[custardpsc]

Is it as simple as just ISA conversion for TAS/IAS ? I don't pretend to know the answer but a bit of quick research suggests its more complicated. A quick look at a 172 poh gave sea level and 10000 ft figures for Vx and Vy and those would be already in KIAS but I'm guessing that its a curve not a straight line between those two points and this is what the OP is looking to figure out.

Bit more here that helped me understand the question, if not the answer.

Vx, Vy and Absolute Ceiling | Jetcareers (http://forums.jetcareers.com/threads/vx-vy-and-absolute-ceiling.31068/)

[Flyingmac]

The thread title contains a fatal mistake.:eek:

[citabria06g]

This is what the Gleim books teach, with Vy decreasing by 0.8 kts per 1000ft

http://joeclarksblog.com/wp-content/uploads/2011/06/chart_absolute_ceiling.jpg

The C172N manual suggests a lower rate:

http://imageshack.us/scaled/landing/41/uzdw.jpg

[24Carrot]

I think it has to be type specific.

Vy corresponds to the maximum "excess power", i.e. the airspeed at which the power produced by the engine is furthest above the power required to overcome drag. Both powers vary with airspeed and so Vy depends on the exact shape of the two curves.

Generally speaking, maximum engine power decreases with density altitude as the cylinders get filled with less dense air, i.e. less fuel can be burned with the air available. There are propeller effects too. Obviously this changes with turbo charging and wobbly props.

Power required is an aerodynamic force, (whole aircraft Drag), times the true airspeed. For any given Indicated airspeed and weight, Drag should not change, but the True airspeed rises with density altitude, and so the Power required does too.

This is covered (with diagrams) in PPL ground school books.

[Big Pistons Forever]

The C 172 Vy decreases 1 kt per 2000 feet. The Warrior has a similar wing platform, power, and weights so I would suggest that following the same 1 kt per 2000 feet would be pretty close to the optimum number.

[Zonkor]

This bootstrap approach is highly interesting! It might be somewhat overkill for most cases, but it seems very powerful since it allows to measure one performance curve and derive (from a theoretical underlying) variations in performance data for changed conditions (density altitude, weight, ...).

Although I'm flying a variety of slightly different Cherokees, and the performance would strictly only be valid for the measured plane, I'll try to obtain a some measurements and play around with the spreadsheet.

The only thing that worried me a little bit is the modelling of Vx in the article, which seems to not vary with density altitude (as does the best-glide velocity Vbg, by the way). I guess I'll have to buy the author's book (apparently long out of print) or obtain research papers to learn more about the background.

Thanks for the hint!

[Zonkor]

citabria, Big Pistons:

Thanks for the data! I wish Piper would have released such detailed performance information.

Seems like my rough data points are not too far off from your data. The 1 kias / 2000 ft drop in Vy will do nicely for now.

By the way, I also found another article [1] on here which cites a Vy decrease of roughly 1 kias / 1000 ft.

[1] http://www.pprune.org/tech-log/343259-vx-vy.html

[Zonkor]

thing:

I'm currently in AZ. Many airports around here are already above 6000', and the hot summer temperature even at altitude results in a "density altitude differential" (i.e., density altitude - pressure altitude) of 2500-3000' every day.

For instance, Flagstaff (KFLG) is on a high plain at elevation 7014'. In the late mornings and afternoons density altitude approaches 10.000' nowadays. The climbout is over forests and in some sectors slightly rising.

Much more critical however are some airports in the AZ mountains, where you have to climb out over hostile, sometimes rising terrain, and out of a valley (meaning also no radio or radar coverage, of course).

I often had climb rates of about 2-300 fpm. Even shallow turns to clear terrain (or slight downdrafts) can annihilate the climbing and cause some nervosity until the VSI needle goes North again. :eek:

That's why I want to max out the climbout right from rotation. To minimize excitement. :)