Cessna 150 performance charts

[jlvandem]

Does anybody have the cruise performance charts for a Cessna 150? I'm flying a '75 model I believe. I recently went on a cross country and the crappy version of the chart that I had must have given me an incorrect TAS because my times between checkpoints were so far off. I know I did the wind calculations right because I used an electronic E6B...pretty hard to screw that up. Also when I get the TAS off the chart shouldn't there be some way to correct it for like temperature and such so I can get a more accurate reading? Basically I'm just looking at why my TAS value was so off causing my groundspeed to be way different. Any kind of help would be greatly appreciated.

Justin

[Mobius]

I would say look in the PIC, but you probably already did.  It should have all of that, otherwise, I'm sure someone around here knowns.  You TAS shouldn't really be that different based on temperature and altitude unless your climbing a couple of thousand feet.  Are you sure you had the most current winds aloft?  I've done cross country flights where the winds have changed dramatically in the couple of hours between when I left and when I landed, that might have happened, otherwise, I don't know.

[jlvandem]

Winds didn't change...we checked when we got back. You said it shouldn't change that much unless you climb a couple thousand feet...we were cruising at 4500 and I used the value on the chart for 5000 which was like 114 mph or something which works out to like 90 some knots. And then with the 17 knot tail wind I figured us to have a ground speed of like 115 or something but we only got 108 the whole way causing my times to be way off...my instructor is insane about times...he thinks they're gonna hit exactly which definately just isn't gonna happen but this was a big difference in times. Most were off by almost a minute. I think what I may have done wrong is not try to calculate the density altitude we did have a warm muggy night last night. I dunno. My instructor has no explination either and yet he tells me they need to be more accurate. I can't fix the problem with out knowing what it is. I'm just frustrated. All help is appreciated

[Mobius]

Well, a minute is pretty close for something like that in my book.  My instructor only ever calculated times to the nearest minute because there was really no chance of getting to each point exactly on time.  Even for my check-ride the examiner was impressed that I got to my check-points within two or three minutes of when I had written down, so I wouldn't worry too much, it's one of those things that really can't be controlled all that much without super accurate weather forcasts and perfect instruments.

And about climbing the couple of thousand feet, I meant if you started at sea level and climbed up to 12,000 feet, then airspeed would really change, but I figured you wouldn't be doing that in a 1975 Cessna 150, so I said it probably wouldn't change that much.  And winds can really change, even in a matter of 500 feet, so you never know, there could have been a change in wind direction or wind speed between 4,500 and 5,000 feet, that just one of those things you can't really predict all too well, and you just have to account for it while you're flying.

Just my 2 cents.

[jlvandem]

So basically my instructor is crazy...thanks for your comments.

Justin

[Mobius]

No problem, as long as your not flying to a checkpoint that's five miles away, a minute should be a pretty good time.  That's my opinion at least, others may feel differently though.

[jlvandem]

Checkpoints ranged from 10-15 nm.

[jlvandem]

Checkpoints ranged from 10-15 nm.

[Mobius]

Oh, well, then a minute is rather large.  Now that I think about it, it would take between 5 and 8 minutes to fly between checkpoints.  Maybe I didn't remember quite right, maybe it was within a minute between checkpoints.  Errr, I really can't remember now.  You'll have to excuse me, I've been studying for finals for almost a week and a half straight, and my brain is mush right now.  For some reason I was thinking there was 40-50 miles between checkpoints, but that was just ignorant of me. But still, I'd stick with the winds probably being a little different at different altitudes.  My instructor also told me that even though the cruise speed of a C172 was 115-120 kts in the POH, I should use 110 kts whenever doing cross country planning, and it worked for me.  So you could try subtracting a bit off your cruise speed to account for anything that might happen during the flight.

[beaky]

Hmmm...
My guess is the same as yours: that you miscalculated your TAS. Winds figure into it, of course, but it sounds like you're not sure where you get TAS from, or what you should have indicated on your ASI and your tachometer for your target TAS.
I use a whiz-wheel, so I don't know where you went wrong, but I do know that TAS is IAS corrected for pitot-static installation(which is hardly a factor at such altitudes and speeds),pressure altitude and temperature. Humidity doesn't figure into it very much, either,apparently... temperature is what makes the biggest difference in density for a given pressure altitude.
 You should also fudge the book numbers a bit, unless that plane just finished its break-in period right out of the factory... those old trainers are always slower than the book. Always. Hell, they were probably not as fast as the book says when they were new.

For what it's worth, my 1968 C150 (Continental O200A) Pilot's Operating Handbook shows that the "sweet spot" (closest margin between fuel burn and airspeed) for 5000 feet at 2600 RPM, which should yield 113 MPH TAS at 5.3 gph at standard temps for that altitude. That's 98 knots.
That old 150 I used to fly did not quite deliver as advertised... I usually planned (at altitudes up to about 5000) for 90 kts indicated; for typical flying in good weather , winter or summer, that would get me 95 to maybe (maybe!) 99 knots TAS at 5000. Just shy of the book, generally.  Never worried about a minute on checkpoints during my training (you can't constantly re-calculate GS on such short legs- the air is unruly), unless of course I got so far off-course that I was wandering around looking for it...

[jlvandem]

Thanks. Next time I'll be sure to shave off a few knots on the TAS for my calculations. The 150 I'm flying just recently had overhall on the engine but you're right the older they are the less performance they will likely get. Thanks again.

Justin