Performance Issue...

killbilly

Vocals, Lyrics, Triangle, Washboard, Kittens
Flying from TKI to AUS last night, I rented a 1976 C-172N. It was a relatively cool night, and I was flying at 6500 MSL.

Now, the POH and the owner of the airplane both tell me that it cruises around 114 TAS, and would indicate about 110.

However, I was lucky to get it to break 99 KIAS. Okay...fine, there was wind.

Here's the problem, though, and this is where I'm getting confused...

Winds aloft around 6500 last night were 120 @ 7, give or take a couple knots, and my course was around 180-185. I wasn't drifting much, and was able to maintain that heading without a whole lot of correction - maybe just a couple degrees.

But the math says that for me to be going that slow I'd have a much higher wind (around 30 kts) to be fighting with a much greater wind correction. And my GPS confirmed this...it showed about 95-99kts ground speed, but my track, gyro and compass all had me on a pretty steady 180-185 bearing.

I know winds aloft forecasts are usually a bit wrong, but to be THAT wrong is not normal, and the fact that I was able to maintain heading tells me that the wind wasn't a 30kt wind, y'know?

I puzzled over this the whole way back. I should have been INDICATING at least 105-110.

I experimented with trim, trying to stay level but get a better pitch attitude, and experimented with power settings, too, running it anywhere from 2300 to 2450 RPM, trying to get the right combination to get the best airspeed. Finally, I made darn sure the ball was centered and that I wasn't flying a little sideways and dragging. Made visually sure the flaps were up, made sure carb heat wasn't on and sapping RPMs. Even tried a couple different altitudes, too.

So what am I missing? Shouldn't I have been flying faster? At least, shouldn't I have been INDICATING faster? Am I just overthinking this? Because I routinely get 105 KIAS out of the 152 we have. I'm stuck on the idea that the 172 shoulda been faster.

They were right - the PPL is definitely a license to learn. I'm learning a million different things the more I go out by myself....
 

killbilly

Vocals, Lyrics, Triangle, Washboard, Kittens
You do understand that steady wind has no effect on the airspeed indicator?
Yes, I do and should have made that clearer in the post, but was typing fast. I think I said it that way trying to say that the KIAS and Ground speed from the GPS were matching most of the time. Sorry...this has me confused and a little annoyed.
 

tgrayson

New Member
Yes, I do and should have made that clearer in the post, but was typing fast. I think I said it that way trying to say that the KIAS and Ground speed from the GPS were matching most of the time. Sorry...this has me confused and a little annoyed.
I still think you're mixing up two different problems:

1) Why was my airspeed lower than published, and
2) Why was my groundspeed different from predicted.

Your airspeed will be controlled only by two factors: thrust and drag. I've never had a C172N model that would get over 105 KIAS. Why? For one, the airplane is old and beat up, meaning that it's draggy and probably out of rig.

Second, is the airplane generating as much thrust as it should? Is the tachometer really accurate?

As for groundspeed, sounds like it was compatible with small wind. Your IAS was 99 knots, your GS was 99 knots? Your TAS was obviously a bit higher, so you had a small headwind of a couple knots. That's what you'd expect with 120@7.
 

SteveC

Really?
Staff member
...Am I just overthinking this? Because I routinely get 105 KIAS out of the 152 we have...
105 KIAS or 105 MPH?

(edit to add, found my own answer...maybe:

Q:
1. How fast do Cessna 150-152's fly?

A:
According to Cessna's specifications the cruising airspeed for a Cessna 150-152 is 105 KTS (nautical miles per hour), which equals 120 MPH (miles per hour). Like other airplanes, this speed is optimistic. When manufacturers test airplanes for specifications they set them up for the best possible performance, but production airplanes rarely meet the "book" numbers. Most Cessna 150-152 pilots agree that the airplane's average airspeed is closer to 95 KTS (about 110 MPH).
 

meritflyer

Well-Known Member
Random thought -

I flew a T182 at 17,000 the other day and pushed 220 over the ground.

ATC asked me twice what kind of bird I was flying.
 

unclenobby

Well-Known Member
Relatively cool? Not knowing the exact details but I looked at the metars for both locations and they were way above standard (+15 or so), while altimeter setting was pretty close to standard (29.92).
This puts your density altitude 2000 ft above indicated. For density altitude 8500 feet 99kts CAS = 110 KTAS approx. I'd like to know what the temp at 6500 feet was to get a better idea but sounds right to me.
Performance figures in the POH don't use KIAS but RPM, Pressure altitude and temps relative to standard for this reason .
The wind may have been more of a stronger headwind then forecast hence the small heading correction and lower GS.

I think from re-reading your post above you based your KIAS calc based on Density Altitude at Airport elevation and not at cruising altitude since at DA 2000ft 110 KIAS = 114 KTAS
 

killbilly

Vocals, Lyrics, Triangle, Washboard, Kittens
Good stuff, thanks, all.

Altimeter ranged from 29.97 to 30.00 throughout the entire trip.

Can't remember what the temp was. I just looked at my notes and didn't write it down. Dumbass. :mad:

Y'know, thinking about this, I'm relying too much on the GPS lately. Getting a bit spoiled/dependent on it. Not that the perf. issue was related to navigation, but I'm getting lazy.

Time to leave it in the bag and fly chart/e6b for a while.
 

Polar742

All the responsibility none of the authority
Y'know, thinking about this, I'm relying too much on the GPS lately. Getting a bit spoiled/dependent on it. Not that the perf. issue was related to navigation, but I'm getting lazy.

Time to leave it in the bag and fly chart/e6b for a while.
+2 for billy....

I'm proud of ya....:rawk:
 

killbilly

Vocals, Lyrics, Triangle, Washboard, Kittens
+2 for billy....

I'm proud of ya....:rawk:
Thanks.

I was just talking to a friend about this on the phone, and I think that's the major lesson I learned last night. I bought it because I wanted a cool toy which was supplemental to navigation, and God knows I do like having Wx in the cockpit.

But my CFI, a guy who's been flying for 30+ years, doesn't have one. Doesn't want one. Hell, he doesn't even have an ANR headset. It's one thing to be a luddite, I suppose, but quite another to have the skillset he's got. I'd like to have the skillset. Charts don't break and, unless you're really hell-bent on its destruction, E6Bs generally don't break either.

Was thinking this last night as I passed over Waco...which is quite pretty at night, by the way.
 

HeyEng

NAHB Doesn't Give a Crap
Good thoughts on the GPS, but it is a really good tool. Nothing like having the direct-to when the engine fails and of course the XM weather. That has come in REAL handy for me on several occasions already. Of course you want to keep up with the E6B and charts and VORs and hell maybe NDBs, but don't discount the value of GPS.

REALLY helps with G. Bush flying all over hell's half acre with the TFRs that are popping up all over the place!
 

Polar742

All the responsibility none of the authority
Charts don't break and, unless you're really hell-bent on its destruction, E6Bs generally don't break either.
I like how you think. I just wish I was still proficient on the CR2 thing.....

Once you learn the old-school, then new school makes you feel like you're cheating.

What's even cooler is when you do the math, and you either 1) do it before the box does 2) your headwork catches a mistake you made programming it....

Like GalaxyIFE said, GPS is a great tool, just like the chart and E6B in your bag. When you're highly proficient in the old and new schools, you'll see alot of things differently.
 

killbilly

Vocals, Lyrics, Triangle, Washboard, Kittens
1) Why was my airspeed lower than published, and
2) Why was my groundspeed different from predicted.

Your airspeed will be controlled only by two factors: thrust and drag. I've never had a C172N model that would get over 105 KIAS. Why? For one, the airplane is old and beat up, meaning that it's draggy and probably out of rig.
Fair enough. The airplane is in fairly decent shape. The owner recently had a cowl repair completed and a nose strut replaced. Also, they took the wheel pants off the plane. Do they make much difference?

Second, is the airplane generating as much thrust as it should? Is the tachometer really accurate?

As for groundspeed, sounds like it was compatible with small wind. Your IAS was 99 knots, your GS was 99 knots? Your TAS was obviously a bit higher, so you had a small headwind of a couple knots. That's what you'd expect with 120@7.
I don't know about the tach being accurate. It feels right and sounds right, but I don't have a lot of time in 172s, so it's hard for me to even make an educated guess.

I don't know where the owner got 114kts from. The POH, probably, but he flies (and instructs) in that plane all the time...you'd think he'd have told me different.

Oh well. Onward and upward.
 

tgrayson

New Member
Also, they took the wheel pants off the plane. Do they make much difference?
The number of "3 knots" comes to mind, but that's probably from an Archer. Might be a bit more effective on the C172, since they're not so close to the wing.

Take that and the possible misconversion of TAS to CAS as suggested by Uncle Nobby, and that would go a long way to explain the IAS discrepancy.
 
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