Fastest Altitude for a Cessna..?

[scifi]

Hi Guys, it's been a long time since I studied the PPL air density theories, but I seem to remember that as you fly higher your Speed increases.
However I took a C152, two-up, to 10,000ft and found that it was very difficult to climb much further, almost on the edge of a stall. Also the Indicated Airspeed was hovering around 60-65 knots. So, without converting IAS to Ground Speed, it certainly seemed to be a lot slower.

As we intend to fly in convoy with some other Cessnas, I was hoping to choose the best altitude, to give us a bit of a speed advantage.

[Jonzarno]

I suggest reading up on the difference between Indicated Air Speed and True Air Speed.

Here is a link (https://www.dauntless-soft.com/products/Freebies/TrueAirspeedCalculator/) to a calculator that converts IAS to TAS based on altitude and temperature.

The other thing you need to take into account is wind which tends to strengthen and veer as you get higher and which will obviously affect your ground speed.

[evansb]

Have you consulted the POH? Table 4.3 indicates 8,000 feet at 75% power for maximum cruise speed, 107 kts. If wheel fairings are installed, an additional 2 kts of speed may be realised.

[Jonnyknoxville]

The fact that you took a 152 up to 10000 feet , two up , and tried to climb higher says to me that you shouldn't be flying at all . You're a danger to yourself .

[Pilot DAR]

Firstly, assure that the airspace you're in welcomes you at higher altitudes, it gets more and more controlled up there. Also, consider the requirements for having oxygen available above 10,000 feet. Consider your, and your passenger's health in terms of breathing thin air.

Yes, as you fly higher, your speed may increase due to less dense air, but then in a 152, your power decreases, so the net effect is poor, and easily overcome by small unfavourable winds.

The service ceiling of the plane is affected by many factors, but, you may have found it that day, for that plane, with that load. I have tested a few planes to their absolute ceiling for engine certification testing. Personal record: 20,800 feet in a normally aspirated Cessna 185 configured with a carburetor, running on auto gas (we were testing for vapour lock and fuel flow at high altitude on the auto gas). Manifold pressure at full open throttle was 12", stall horn on, 72 MIAS. I had the plane just over 21,000 feet, but it would not stay up there under power. Oxygen was obviously used. Personal record in a C 150, two up, and carrying traveling kit in the back was 13,200 feet, which we reached at greatly reduced power, being carried aloft on a sever mountain wave over the Appalachian Mountains in the US. We were quite afraid that we might be carried higher, as we had no intention to be that high, and had no oxygen! Later in the flight we encountered settling air, and were taken back down a few thousand feet.

Correct, you'll get to the point where at full power, you're flying just above stall speed, and not climbing - that's it. Note that in part, the normally aspirated C 152 engine will be developing pitiful power up there. Also be aware that it may begin to run rough in thinner air, if the mag leads aren't in tip top condition. It's not a problem, as the problem will resolve itself as you descend - though if you're going to go high regularly, have new mag leads installed. Cracked insulation in old leads allows the spark to go to ground on the shield before it gets to the plug. Thinner air allows sparks to travel more easily.

Unless you have immensely favourable winds way up high, there's otherwise not much reason to go way up. The fuel burned getting up there, and slower climb, won't generally be recovered in added cruise speed. Consult the POH performance charts, and the winds aloft, an go only as high as you need to. If you're crossing a body of water, or other un landable terrain, higher may be better, if all other things are equal.

[visibility3miles]

You don't think as clearly in thin air, so even if you aren't high enough to be required to use oxygen, it's unwise to push it.
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Don't do formation flying near MCA.
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we intend to fly in convoy with some other Cessnas

I've flown in formation before (five planes total), with everyone at an angle to the right and behind the lead aircraft. It gives each pilot a good view forward and to their left of the plane they are following. We were at a healthy distance from each other, but close enough that the lead pilot did all the communications and informed the tower and other controllers how many of us there were.

We picked an unused frequency to talk to each other enroute.

1
...2
......3
.........4
............5

If some engines are more powerful than others, you have to adjust your rate of climb to accommodate the slowest plane.

[MarcK]

The fact that you took a 152 up to 10000 feet , two up , and tried to climb higher says to me that you shouldn't be flying at all . You're a danger to yourself .
You've got to be kidding. What's the problem with 10K feet? Oxygen levels start (in the US) at 12,500. Finding your aircraft's service ceiling isn't a big deal.

[Tarq57]

Have you consulted the POH? Table 4.3 indicates 8,000 feet at 75% power for maximum cruise speed, 107 kts. If wheel fairings are installed, an additional 2 kts of speed may be realised.
Really? Is 75% available at that altitude?

[Winemaker]

You've got to be kidding. What's the problem with 10K feet? Oxygen levels start (in the US) at 12,500. Finding your aircraft's service ceiling isn't a big deal.
Yes, when climbing I start to feel the altitude at about 9,000 feet and get slower and slower the further up I go (I live at 1200 feet ASL). I don't notice any cognitive effect at the top of Rainier, 14,400 feet but I do start puffing getting there.... Loveland Pass in Colorado is a paved road at 11,992 feet; no oxygen required.

[scifi]

Hi Guys, thanks for the answers so far. I looked in the POH in the Cruise Performance section. Fig 5-7 indicates that 8000ft is the highest the engine can still produce 75%.
It becomes 68% at 10k and then 62% at 12k.

So it looks as if 8000ft is the optimum cruise level for those Cessnas….The TAS increases from 101 kts to 107 kts.

https://www.cpaviation.com/images/downloads/Cessna%20152.pdf.

I was thinking it had something to do with the L/D ratio remaining constant whilst the !AS- TAS improved. It does now look as if the engine's performance is the limiting factor.
.

[MarcK]

Really? Is 75% available at that altitude?
https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/617x973/c152_cruise_ad5e189efe8a7c8c6b2aed302daf155c35244735.jpg

[scifi]

Thanks Marck, that was what I wanted to Cut and Paste..

[B-757]

The fact that you took a 152 up to 10000 feet , two up , and tried to climb higher says to me that you shouldn't be flying at all . You're a danger to yourself ... Why so..??.. Please explain..

Fly safe,
B-757

[BackPacker]

So it looks as if 8000ft is the optimum cruise level for those Cessnas….The TAS increases from 101 kts to 107 kts.

Only in still air. Which it rarely is at that altitude. And only under ISA conditions. Which are rare as well.

I would recommend spending some time with SkyDemon, or maybe another flight planning tool that has the same capabilities. In SD you can input performance data for your aircraft at various power settings, and at various altitudes - essentially the table that MarcK included. SD also gets live weather information from the internet, including forecasted wind at any altitude, at any location. Based on that you can ask SD to simulate your flight, at a particular time, at various altitudes and at various power settings. That allows you to pick the best altitude and best power setting for your flight. (And "best" can then refer to lowest fuel burn, or shortest time enroute.) And it will give you terrain/airspace warnings and an idea about clouds/freezing level so you know which altitudes cannot be used for those reasons.

Oh, and if you haven't flown for a while and are going to attempt a high-altitude, max performance flight, read up on the leaning procedures for your aircraft. You'll only get book performance if you follow book procedures.

Having said all that, something else. You mentioned you're going to fly "in convoy". I'm assuming this does not mean "in formation", in which case you would have a whole other set of challenges to deal with. But if "in convoy" means some sort of fly-out where a number of aircraft all fly to the same destination at more or less the same time, my advice would be: Fly your own flight. Plan and execute the flight as if you were going on your own, and just try to coincide your take-off or landing time with the other aircraft in your party. I did a fly-out "in convoy" once in a 120 HP DR400, while the rest of the aircraft all had 160 HP or more. So my cruise speed was easily 10-15 knots slower than the rest. I was trying to keep up with them, trying to cut airspace corners all the time, reducing time-on-ground for lunch and refuelling to the minimum, and it wasn't fun and didn't work anyway. On the way back I flew my own flight independent of the others and it was much more enjoyable.

[scifi]

Thanks Backpacker for your insights...
The convoy is just going to start at random times up to about 10:00, so it won't really be Formation Flying as such., The route will be the same, so ATC will see a few of us all doing the same thing that morning.
I think I could manage 8000ft for most of the route, until we meet airspace restrictions. Hopefully we will have a favourable wind, there and back..!

[Victorian]

Hello. I took a 152 across the Canyon more than once at 11,500’. Solo. No problem at all with airspeed, stalling etc and still a little bit of climb left in the bag. But it did take a long time getting up there. A tour pilot responded to one of my calls with “how’d you get up there?”.

More recently I took a different 152 to the Southern Alps in New Zealand and struggled to get above 8,500’, also solo. Different horses for different courses I suppose. The first one lived at 5000’ and was tuned for it (climb prop, anyway). The NZ one ... wasn’t.

[A and C]

Physics says that a normally aspirated engine will make 75% power up to 8000 ft ( assuming ISA ) so as long as the density altitude calculation is done you can pick the best altitude or flight level for the cruise.

As the temperature rises the TAS increases ( ref ISA ) so in general the higher you go the better TAS you get so it becomes a balance between wind speed & direction and TAS to get the optimal cruise altitude.

The final part of cruise altitude selection is turbulence, an aircraft that is fighting turbulence with lots of flying control inputs will be less efficient than one cruising in smooth air and this factor may result in the selection of cruise altitude above the theoretical best cruise altitude for airframe efficiency and passenger comfort.