Real TAS for Planes?

WDD

WDD

Final Approach
PoA Supporter
Joined
Oct 16, 2019
Messages
6,166
Location
Atlanta / Marietta
Display Name
Display name:
Vintage Snazzy (so my adult children say)
When I finally get my check ride done, I'd like to go a few places a few hours away with the wife. What should I rent that would get me there and back in a shorter amount of time than I can do with the Sky Hawk?

I've been training in the C 172. The POH has TAS speeds that these 30 year old, well used planes will never again see in a cruise. Which is understandable - engines are old, air frames aren't as clean as they were, etc.

Where can I find an idea of what "real" speeds other 30 year old rentals might have?

For example, if the Sky Hawk I'm training in has a real TAS of 105 kts, would a 30 year old Tiger really give me 130 kts? Or would it be just as fast as the Sky Hawk?

Archer - about the same as the Sky Hawk?
 
Unless something is wrong, a plane should get close to POH speeds throughout its life. My current AA-5 maintained book speeds before and after overhaul. What the old engine couldn't do near the end of its useful life is maintain redline at higher altitudes. But set at 2550 rpm, it maintained 115 kt with either the old, tired engine, or the new zero-time engine.

So, what you see in the POH for a typical cruise power setting (say 65-70% power) is close to what you will get. Factors that might affect that by a few knots either way include installation of wheel pants, other than standard pitch propeller, engine STCs (e.g. high compression pistons), etc. You have to know these factors to estimate the potential difference between standard book and expected cruise speeds.

Going 130 kt instead of 115 kt doesn't gain that much on a 350 nm trip. About 20 minutes out of about 3 hours. I've taken my 120 kt max cruise plane from New York to Florida and back. Still beats driving.
 
My Mooney was about 5 knots slower than POH values with a 50 year old engine and accessories. After my major overhaul of everything in front of the firewall, it does pretty close to exactly what the book says.
 
Well maintained airplanes will keep to their specs. I'm not up to speed on what's out there to rent but there are quite a few planes faster than a 172.
 
Don't forget there is a significant difference between IAS and TAS. The book speeds are probably pretty close to what you are getting.

TAS is on top left, IAS is in middle left.

IMG_3162 copy.jpg
 
Last edited:
Maybe they are not always that far off and its just needing to get to know the plane better.

When I fly at 7500msl if I'm going less than 148mph ground speed i know I have a headwind component. If it's over 148mph there's a tailwind. This is for 2300rpm, 19" MP, full fuel and 2 big dudes in a 1972 182P. I just looked at the POH and it indicates exactly 148 for this scenario. I am pretty sure the poh numbers are for a full plane so we were at 350lbs under MTOW, but no wheel pants. The poh shows 11gph and I sticked tanks before and after on this very flight yesterday and show 10.9gph.

I'd say our numbers are pretty close. But back when I started it sure seemed all over the place because i was so new to it and so many trip variants.

If its just you and your wife you shouldn't be near or at MTOW and if the rental place doesn't require a top off you can get it even lighter. But it wont give you tons more speed.
 
POH performance's are always best case scenario. As Salty said, calculate TAS in flight ant it will very likely be closer to POH over IAS. My 182P POH may say 135 TAS at a given cruise setting. I'll usually see 120-125 IAS and 128-132 TAS.

Coming back from Oregon last month I had a 90 MPH tail wind. That was fun. Cut an hour off enroute flight time.
 
WDD said:
When I finally get my check ride done, I'd like to go a few places a few hours away with the wife. What should I rent that would get me there and back in a shorter amount of time than I can do with the Sky Hawk?

I've been training in the C 172. The POH has TAS speeds that these 30 year old, well used planes will never again see in a cruise. Which is understandable - engines are old, air frames aren't as clean as they were, etc.

Where can I find an idea of what "real" speeds other 30 year old rentals might have?

For example, if the Sky Hawk I'm training in has a real TAS of 105 kts, would a 30 year old Tiger really give me 130 kts? Or would it be just as fast as the Sky Hawk?

Archer - about the same as the Sky Hawk?
Click to expand...

I’ve heard this, but then I wonder...I just don’t know.

Wouldn’t most C172’s (or any older plane) say from 1980 or so, either have replaced the motor, or been repaired enough that it was the same as original. Air frames too, if kept up, repainted, etc. and not let to deteriorate, what would really hinder an airplane from 40 years ago to meet or be very close to the POH specs?
I mean If you preflight it and see no dents, nothing out of place, and know the maintenance has kept the motor i good condition and refurbished, what other factors would make a plane be different? New plane smell?
 
LongRoadBob said:
I’ve heard this, but then I wonder...I just don’t know.

Wouldn’t most C172’s (or any older plane) say from 1980 or so, either have replaced the motor, or been repaired enough that it was the same as original. Air frames too, if kept up, repainted, etc. and not let to deteriorate, what would really hinder an airplane from 40 years ago to meet or be very close to the POH specs?
I mean If you preflight it and see no dents, nothing out of place, and know the maintenance has kept the motor i good condition and refurbished, what other factors would make a plane be different? New plane smell?
Click to expand...
Cam lobes can wear, reducing the amount of fuel and air into the engine, resulting in less HP. But it's got to be pretty bad to be more than 5 or 10 knots.
 
One of the overlooked items in accurately estimating TAS vs. book is the accuracy of the tachometer. At one point, mine was reading 100 rpm high, which made all my airpseeds at defined power settings look lower than expected. Once the tach was re-calibrated, all was right in the world, and I was within a few kt of the book speeds at all settings.

As others have mentioned IAS is NOT a measure of TAS. As a rule of thumb, TAS is IAS + 2% per 1000 feet MSL altitude. Of course, you can get accurate TAS by using your trusty E6B or the tools on most panel or portable GPS units. But the rule of thumb is usually within a knot or two at light single altitudes.
 
Last edited:
For every antenna, subtract 1 knot if it’s an airfoil, 2 knots if it’s not.
Don’t forget (for those of us in the southeast), warmer temperatures and humidity degrade performance.


Tom
 
Salty said:
Cam lobes can wear, reducing the amount of fuel and air into the engine, resulting in less HP. But it's got to be pretty bad to be more than 5 or 10 knots.
Click to expand...

Don’t forget, with Lycoming 4 bangers, they intake valves are shared, so 1 bad cam means 2 weak cylinders.


Tom
 
New air is considerably more polluted than old air, so the currently climate-changed air will no longer allow for true air speeds. If you don't believe in global climate change then you will have to fly with fake air speeds, faux air speeds, alternative air speeds (not to be confused with alternate air!) or straight up fake air speeds. For further reading, see page 8 of my pamphlet where I lovingly explain the differences that indicated altitude, pressure altitude, and absolutely fake altitude have on tire pressure.
 
Also, it will depend on how you fly it. My 91 Tiger will do true 130 kts at most altitudes at somewhere between 2550 and 2600 RPM and 9.7 GPH. That’s with a fairly new top overhaul and a Powerflow exhaust and somewhere around 2300 lbs. Many Tiger owners fly as close to 2700 RPM as they can get at a given altitude and consequently go faster with greater fuel flow. Some regularly claim 140-142 kts TAS and IDK, maybe they are right. If it’s a wet rental, then it’s going to be full rental power which should be something close to POH numbers absent some significant headwind, etc.
 
My stock 1976 Tiger with a 1400 hr Lycoming factory OH engine does 135 kts at just under 9.6-9.8 gph and 2650 rpm (digital tach) at 5500 ft MSL up to about 8500 ft loaded to about 100-150 lbs below max gross weight. I haven't tracked speed when loaded more heavily, and it doesn't go materially faster (that I can discern) when loaded more lightly (say 300 lbs below max gross). I think this is a few kts below book, but close enough for me.
 
chemgeek said:
Of course, you can get accurate TAS by using your trusty E6B or the tools on most panel or portable GPS units. But the rule of thumb is usually within a knot or two at light single altitudes.
Click to expand...
I think you probably meant this otherwise just a minor correction...the portable gps will give ground speed vs TAS. A good example is shown in the earlier G5 screen image.
 
Sinistar said:
I think you probably meant this otherwise just a minor correction...the portable gps will give ground speed vs TAS. A good example is shown in the earlier G5 screen image.
Click to expand...

Many of the hand held GPS units have a TAS calculator in them. But you have to input the data just like you would with an E6B.
 
Sinistar said:
I think you probably meant this otherwise just a minor correction...the portable gps will give ground speed vs TAS. A good example is shown in the earlier G5 screen image.
Click to expand...

On my panel and portable GPS, you input altitude, temp, and IAS, and it will spit out TAS. If you also enter your heading, it will also use your GS to calculate the direction and speed of winds aloft. Your trusty E6B will have a function to enter pressure altitude and temp, which will allow a conversion between IAS and TAS. Or you can just use the 2% rule which is close enough for government work. ;)
 
WDD said:
When I finally get my check ride done, I'd like to go a few places a few hours away with the wife. What should I rent that would get me there and back in a shorter amount of time than I can do with the Sky Hawk?

I've been training in the C 172. The POH has TAS speeds that these 30 year old, well used planes will never again see in a cruise. Which is understandable - engines are old, air frames aren't as clean as they were, etc.

Where can I find an idea of what "real" speeds other 30 year old rentals might have?

For example, if the Sky Hawk I'm training in has a real TAS of 105 kts, would a 30 year old Tiger really give me 130 kts? Or would it be just as fast as the Sky Hawk?

Archer - about the same as the Sky Hawk?
Click to expand...
You should get the POH TAS, maybe less a couple of knots, but there are lots of caveats:

- you won't make book TAS unless you're following the power settings for 75% power at your current density altitude
- you won't make book TAS unless you're at the density altitude where they calculated it (typically between 7,000–8,000 ft DA, where normally-aspirated piston engines do best)
- you won't make book TAS if you're not at the same gross weight they used to calculate the number (which probably wasn't maximum gross)
- you won't make book TAS if there's any downward air movement
- you won't make book TAS if you're in turbulence (because you're losing speed waggling around)
- you won't make book TAS if your tachometer or MP gauge is miscalibrated (because you won't really be at 75%)
- you might not make book TAS if you have a forward CG (they probably went for an aft CG to get the best-looking number)
- you might not make book TAS if you don't have the same wheel fairings, antennas, etc. as when the calculated it

So realistically, expect a few knots less. Book TAS for my 1979 PA-28-161 is 127 KTAS, but I've never seen better than 124 KTAS, even under close-to-optimal conditions. Still, that's pretty good.
 
Mine does almost exactly book numbers. The old ragged out 172 rentals we had didn't make close to book...
 
chemgeek said:
On my panel and portable GPS, you input altitude, temp, and IAS, and it will spit out TAS. If you also enter your heading, it will also use your GS to calculate the direction and speed of winds aloft. Your trusty E6B will have a function to enter pressure altitude and temp, which will allow a conversion between IAS and TAS. Or you can just use the 2% rule which is close enough for government work. ;)
Click to expand...
Some airspeed indicators (like mine) have a circular sliderule built in for TAS. You turn the knob to line up the pressure altitude and OAT in the little window at the top, and the needle will be over your TAS in the bottom. It depends on you having the right OAT and pressure altitude (as well as a well-calibrated ASI), but that's equally true if you put the inputs into a full E6B (mechanical or electrical).
asi.png
 
David Megginson said:
Some airspeed indicators (like mine) have a circular sliderule built in for TAS. You turn the knob to line up the pressure altitude and OAT in the little window at the top, and the needle will be over your TAS in the bottom. It depends on you having the right OAT and pressure altitude (as well as a well-calibrated ASI), but that's equally true if you put the inputs into a full E6B (mechanical or electrical).
View attachment 81282
Click to expand...

Yup, looks very similar to the one in the Mooney. Easy peasy.
 
Bill Jennings said:
Yup, looks very similar to the one in the Mooney. Easy peasy.
Click to expand...
Exactly. One time analogue beats digital -- no need to take my eyes off the panel and switch through menus on some electronic device.

(That's comparing it to electric E6B's, not a continual TAS display on a glass instrument with digital probes, of course).
 
If the rigging is correct, the engine is sound, the gross weight and the CF are center of envelope, you should get POH values. Aft CG you should get a little more.
 
ktup-flyer said:
Mine does almost exactly book numbers. The old ragged out 172 rentals we had didn't make close to book...
Click to expand...

I wonder how close the control cables were rigged to factory specs.
 
flyer770 said:
I wonder how close the control cables were rigged to factory specs.
Click to expand...
Not at all...they wouldn't fly straight hands off no matter how much you trimmed. One of them had a bent firewall when I did my solo in it.

I can trim my 182 in smooth air and it will go straight and level until it hits an air pocket or I bump it
 
IIRC, my dads 172 with an O360, wheel pants, and powerflow exhaust would see about 117kts ktas at 2450-2500RPM. It also beat the published service ceiling by about 4000'!
 
chemgeek said:
Unless something is wrong, a plane should get close to POH speeds throughout its life. My current AA-5 maintained book speeds before and after overhaul. What the old engine couldn't do near the end of its useful life is maintain redline at higher altitudes. But set at 2550 rpm, it maintained 115 kt with either the old, tired engine, or the new zero-time engine.

So, what you see in the POH for a typical cruise power setting (say 65-70% power) is close to what you will get. Factors that might affect that by a few knots either way include installation of wheel pants, other than standard pitch propeller, engine STCs (e.g. high compression pistons), etc. You have to know these factors to estimate the potential difference between standard book and expected cruise speeds.

Going 130 kt instead of 115 kt doesn't gain that much on a 350 nm trip. About 20 minutes out of about 3 hours. I've taken my 120 kt max cruise plane from New York to Florida and back. Still beats driving.
Click to expand...

That.

Most planes I recall flying could hit book numbers or close enough to be a rounding error.
 
WDD said:
would a 30 year old Tiger really give me 130 kts? Or would it be just as fast as the Sky Hawk?
Click to expand...
Much faster than a Skyhawk.. and potentially faster than 130

But a Mooney is your best speed value
 
ktup-flyer said:
CAS is IAS corrected for instrument and position error
Click to expand...

Thanks. I know that. Googling it has given me that same definition over and over. I was a little surprised that none of the answers I found, at Bold Method or anywhere else, went into any detail with actual real-life examples.

In specific, I was wondering about the actual *numbers*. How close is Corrected/Calibrated Air Speed in knots to Indicated Air Speed in knots? Or percent? Or depending on X and Y?

And do the new glass displays, which say that they're providing IAS, actually provide a corrected/calibrated indication?

Am I making any sense? ;)
 
Tantalum said:
Much faster than a Skyhawk.. and potentially faster than 130

But a Mooney is your best speed value
Click to expand...

Depends on the Mooney. Not an M20C. Also, if you want space, the Tiger is going to dominate a short body Mooney on that and be up with a mid body. The UL will also be better.

Oh, and 140-145 is not uncommon in Tigers.
 
Stephen Poole said:
Thanks. I know that. Googling it has given me that same definition over and over. I was a little surprised that none of the answers I found, at Bold Method or anywhere else, went into any detail with actual real-life examples.

In specific, I was wondering about the actual *numbers*. How close is Corrected/Calibrated Air Speed in knots to Indicated Air Speed in knots? Or percent? Or depending on X and Y?

And do the new glass displays, which say that they're providing IAS, actually provide a corrected/calibrated indication?

Am I making any sense? ;)
Click to expand...
My POH has a chart in it that converts IAS to CAS in different flap configurations
 
N1120A said:
Depends on the Mooney. Not an M20C. Also, if you want space, the Tiger is going to dominate a short body Mooney on that and be up with a mid body. The UL will also be better.

Oh, and 140-145 is not uncommon in Tigers.
Click to expand...
That's what I get in my 182.
 
Also, are you in a 180 HP 172 or a 160 HP 172? 105 for a 160 would be pretty fast, actually. 105 for a 180 would be about 5-10 knots slow. An Archer or 180 will bear a 172 180 HP - by about 5-10 knots. A Cheetah (160 HP) will go faster than a 172 180HP, and a Tiger will torch it.
 
You must log in or register to reply here.
Back
Top