True airspeed not what book says

[Supercop0184]

OK, yet another probably silly question by me. I am going over my owners manual with a fine tooth comb and comparing it with some numbers that I got in a flight the other day. My airport that I base at is in Fort Worth, I have a place in Durango Colorado. That is a 690 statute mile flight. My calculations based off of my speed and fuel burn always show that I need to stop because it’s too close for comfort and making it nonstop. However, when I look at the owners manual and go to the 10,000 feet page  where I compare my power settings and altitude to the power settings and altitudes listed in the table, I am not getting the true airspeed or the range in statute miles that shows in the book.  Sunday, I got my airplane up to 12,500 feet,   At about 19 inches of manifold pressure, 2400 RPM, My calculations show that I was getting 152 miles an hour true. The book at those  settings shows that I should have been getting 164 miles an hour true.  It shows that I should be burning 8.5 gallons an hour, which is accurate, and it shows my range in statute miles to be 881. I don’t feel I could get close to 881 miles. Most times, I feel like i would be lucky to get 700 miles to Durango. I just don’t Dee like I’m going as fast as I should. Anyone else have any similar experience or maybe can shed some light on this for me?

 

thanks!

 

[Aviationinfo]

On 1/7/2019 at 2:44 PM, Supercop0184 said:

Anyone else have any similar experience or maybe can shed some light on this for me?

YES!!  Pretty much everyone.  

POHs are notoriously optimistic, derived from “perfect” airframes and professional test pilots, and done under optimal conditions.

I have found my 66 E POH to be less over-optimistic than others, and my plane comes close-ish. Having said that, we have a cowl mod on ours that added a few knots.

My 66E Manual even has a note on there that indicates the figures are accurate within 3%, which could account for some of the difference  between actual and book figures.

If you think about it, your fuel flow at the given power settings matches your POH.  That’s because the engines in these airplanes haven’t changed at all (unless you have some modern day mod like electronic ignition or Powerflo exhaust or your engine is in poor shape).   However, airframes DO change: Small leading edge imperfections, slightly misrigged gear doors or flight controls, worn prop blades, etc etc all add up to rob speed and range.  

Best advice I can give is to check flight control and landing gear door rigging, and watch for trends. And... never trust your POH without some mental fudge factor when you’re trying to determine important things like takeoff or landing distances, or range for example.

Edited January 9, 2019 by Aviationinfo

[N201MKTurbo]

Lets see, the check's in the mail.

I'm from the government and I'm here to help.

A 201 goes 201 MPH.

I won't … oh never mind...

Edited January 7, 2019 by N201MKTurbo

[Supercop0184]

34 minutes ago, Aviationinfo said:

YES!!  Pretty much everyone.  

POHs are notoriously optimistic, derived from “perfect” airframes and professional test pilots, and done under optimal conditions.

I have found my 66 E POH to be less over-optimistic than others, and my plane comes close-ish. Having said that, we have a cowl mod on ours that added a few knots.

My 66E Manual even has a note on there that indicates the figures are accurate within 3%, which could account for some of the difference  between actual and book figures.

If you think about it, your fuel flow at the given power settings matches your POH.  That’s because the engines in these airplanes haven’t changed at all (unless you have some modern day mod like electronic ignition or Powerflo exhaust and your engine is in decent shape).   However, airframes DO change: Small leading edge imperfections, slightly misrigged gear doors or flight controls, worn prop blades, etc etc all add up to rob speed and range.  

Best advice I can give is to check flight control and landing gear door rigging, and watch for trends. And... never trust your POH without some mental fudge factor when you’re trying to determine important things like takeoff or landing distances, or range for example.

Thanks for your response. Makes sense - I just wanted some confirmation. It’s the first time I’ve taken the POH and compared published TAS to actual numbers. 

And yeah i never push it with take off distances! Lol 

[Supercop0184]

25 minutes ago, N201MKTurbo said:

Lets see, the check's in the mail.

I'm from the government and I'm here to help.

A 201 goes 201 MPH.

I won't … oh never mind...

Not sure I understand the reference. 

[gsxrpilot]

1 minute ago, Supercop0184 said:

Not sure I understand the reference. 

All bull shit statements...

[Supercop0184]

Just now, gsxrpilot said:

All bull shit statements...

Yeah I kind of figured. :/ some people eh? Lol

[gsxrpilot]

I would think you could get back from Durango non-stop. Have you tried flying it LOP? It's a 50/50 proposition in a carbureted C, but on the days it works, you're burning 6.5 gph.

I'd try DRO - T67 at 13,500 and see if you can get it leaned a bit more. I think you could make it. What have you got for a Fuel flow meter?

With a little luck and a tailwind.

[N201MKTurbo]

13 minutes ago, Supercop0184 said:

Not sure I understand the reference. 

There are lies, there are damn lies and then there are POH speed numbers.

[Vance Harral]

55 minutes ago, Aviationinfo said:

POHs are notoriously optimistic, derived from “perfect” airframes and professional test pilots, and done under optimal conditions. 

While there's some truth to this, as a CFI, I've frequently traced complaints about "not making book performance" to pilots who aren't actually using the charts correctly.

As an example, flying at 10,000' indicated on your altimeter is not going to give you the true airspeed from the 10,000' line in your POH unless the atmosphere is at standard conditions - not even if you're a test pilot in a brand new, perfectly rigged airplane.  If you're crossing the Rocky Mountains enroute from Ft. Worth to Durango, the temperature at 10,000' indicated is frequently much warmer than standard.  It would not be unusual for the density altitude at 10,000' indicated to be around 12,000', where the published book numbers for true airspeed are going to be slower for a normally aspirated airplane.  Given that you don't mention OAT or density altitude anywhere in your original post, maybe a misunderstanding about the POH tables themselves accounts for some of the discrepancy?

[Supercop0184]

34 minutes ago, N201MKTurbo said:

There are lies, there are damn lies and then there are POH speed numbers.

Ok - I get that. Lol 

[Supercop0184]

20 minutes ago, Vance Harral said:

While there's some truth to this, as a CFI, I've frequently traced complaints about "not making book performance" to pilots who aren't actually using the charts correctly.

As an example, flying at 10,000' indicated on your altimeter is not going to give you the true airspeed from the 10,000' line in your POH unless the atmosphere is at standard conditions - not even if you're a test pilot in a brand new, perfectly rigged airplane.  If you're crossing the Rocky Mountains enroute from Ft. Worth to Durango, the temperature at 10,000' indicated is frequently much warmer than standard.  It would not be unusual for the density altitude at 10,000' indicated to be around 12,000', where the published book numbers for true airspeed are going to be slower for a normally aspirated airplane.  Given that you don't mention OAT or density altitude anywhere in your original post, maybe a misunderstanding about the POH tables themselves accounts for some of the discrepancy?

And this most definitely could be true - OAT was 40 degrees. Didn’t think about density altitude 

[Supercop0184]

42 minutes ago, gsxrpilot said:

I would think you could get back from Durango non-stop. Have you tried flying it LOP? It's a 50/50 proposition in a carbureted C, but on the days it works, you're burning 6.5 gph.

I'd try DRO - T67 at 13,500 and see if you can get it leaned a bit more. I think you could make it. What have you got for a Fuel flow meter?

With a little luck and a tailwind.

It would probably take another 7 minutes to get to 13,500 - but I’ll definitely try. It’s so hard for me to get it LOP

[gsxrpilot]

12 minutes ago, Supercop0184 said:

It would probably take another 7 minutes to get to 13,500 - but I’ll definitely try. It’s so hard for me to get it LOP

Let's go fly together sometime. I'm not saying I can do it any better, but it would still be fun.

[Hank]

2 hours ago, Supercop0184 said:

OK, yet another probably silly question by me. I am going over my owners manual with a fine tooth comb and comparing it with some numbers that I got in a flight the other day. My airport that I base at is in Fort Worth, I have a place in Durango Colorado. That is a 690 statute mile flight. My calculations based off of my speed and fuel burn always show that I need to stop because it’s too close for comfort and making it nonstop. However, when I look at the owners manual and go to the 10,000 feet page  where I compare my power settings and altitude to the power settings and altitudes listed in the table, I am not getting the true airspeed or the range in statute miles that shows in the book.  Sunday, I got my airplane up to 12,500 feet,   At about 19 inches of manifold pressure, 2400 RPM, My calculations show that I was getting 152 miles an hour true. The book at those  settings shows that I should have been getting 164 miles an hour true.  It shows that I should be burning 8.5 gallons an hour, which is accurate, and it shows my range in statute miles to be 881. I don’t feel I could get close to 881 miles. Most times, I feel like i would be lucky to get 700 miles to Durango. I just don’t Dee like I’m going as fast as I should. Anyone else have any similar experience or maybe can shed some light on this for me?

 

thanks!

 

This is in my C over the central Appalachians in the summer. 

So if I did the math right, at 9500 msl:

144 + 19% = 171 mph

Power was WOT minus a bit and 2500, with 201 windshield and 3-blade Hartzell air brake. The OAT is out of view but well above ISA.

Start by checking two things:  are your gear doors flush when closed, and how good is the rigging?

I've flown 4:45 like this twice, landing with 12-13 gallons both times, so still had 1:15-1:20 left in fuel but not in me . . . .

[Supercop0184]

5 minutes ago, gsxrpilot said:

Let's go fly together sometime. I'm not saying I can do it any better, but it would still be fun.

Paul, I’ll fly with you any day of the week and consider it an honor. lol - let’s go!

[gsxrpilot]

We just did DRO to 84R which is 100 miles further, on 48 gal.

[Vance Harral]

1 hour ago, Supercop0184 said:

And this most definitely could be true - OAT was 40 degrees. Didn’t think about density altitude 

10K pressure altitude at 40 degrees Fahrenheit is a little over 11K density altitude.  In my airplane, the difference is about 4 knots.  Doesn't seem to account for 100% of the delta you're seeing vs. book numbers, but that's at least some of it.

[asaxet]

Didnt see you mention if you have a engine monitor...   For my E....  as purchased jpI 450 fuel flow indicator and stock single cyl cht and  egt.     Never able to get very close to any poh numbers...   Installed a new 4 cyl  cht, egt, ff monitor and amazed how much better the range is now that  better monitoring  of what is going on with the engine is posible...   and how much error there was with the originals....  unfortunatly it wont help your speed much... but definitely better range now...  (and I haven’t gone gami yet.)..   guessing it would be the same for a carb equipped  bird...  FWIW

[carusoam]

Working the WnB can really work in your favor...

• lighter is better for speed...
• the AOA of keeping a heavier plane up causes higher drag...
• Back of the envelope (not the calculation kind ) is better for speed...
• the further back the Cg is, less drag is generated by the elevator holding the nose up...

Both weight and balance were highly important in getting the book numbers to work. Unless they really are just lies...

PP thoughts only, not an aerodynamicist...

Best regards,

-a-

[Hank]

From my picture above, please remember Book performance at 10,000 msl:  20"/2500 => 160 mph at gross, 163 mph at 2200 lb (about where I was loaded).

Note that ISA = 23°F, while my non-recorded OAT was probably above freezing (maybe mid-40s F, although I've seen 59°F this high over central WV), and there's a note that "each 10°F difference above standard temperature will cause a 1% reduction in horsepower." And I still made almost 10 mph above book. Many people insist that my 3-blade Hartzell prop carries a 3-5 knot speed penalty . . . .

It can be done. If you're below book speed, find out why. Things being rigged properly are easiest to check (flight controls, main gear doors and especially nose gear doors); then induction leaks, wear / slop in the carb beat box, condition of doghouse and baffles.

Edited January 8, 2019 by Hank

[smccray]

5 hours ago, gsxrpilot said:

I would think you could get back from Durango non-stop. Have you tried flying it LOP? It's a 50/50 proposition in a carbureted C, but on the days it works, you're burning 6.5 gph.

I'd try DRO - T67 at 13,500 and see if you can get it leaned a bit more. I think you could make it. What have you got for a Fuel flow meter?

With a little luck and a tailwind.

With a naturally aspirated plane, the Advanced Pilot Seminars literature basically says that the engine efficiency isn’t that different going to LOP above 8k MSL  I suspect most of the range increase would come from traveling at a slower speed (reduced parasitic drag) rather than greater efficiency of the engine operations.  Either way the net result would be the same.

If pulling the prop back to a slower RPM is an option, that might be a better way to increase efficiency rather than traveling LOP. That said, I suspect the difference between those two strategies may be so small that it’s a mental exercise not a real world strategy.

I recall an article from years ago saying that max range in an NA airplane would come from climbing high enough that indicated airspeed is close to best glide with the engine operating WOT at peak EGT. I do wonder how the slow climb the last few thousand feet would play into that math, but I suspect there a few people around here smarter than me who could make that determination.

 

[carusoam]

We have discussed LOP climbs in the past...

For the most part, it doesn’t get real world use...too much challenge, not enough benefit...

an NA climb to 10k’ lasts about 10 minutes in the O1...

But, when every gallon counts... start leaning with best EGT method to get off the ground... then go LOP after that... converting all of the fuel to noise, and none to refrigerant... 

This could be one of the most highly engineered flights of a 252... 

Need to get that other sponsor going... FlightAware.... I’ll be watching from NJ.

Best regards,

-a-

[ArtVandelay]

The book speeds are based on ideal conditions:
Clean aircraft, rigged correctly, engine making rated power and no antennas. Many modern aircraft have: vor, elt, marker beacon, gps, adf, lightning, transponder, ADSB antennas plus OAT probe and who knows what else.
And the numbers associated with speed mods are greatly exaggerated.


Tom

[M20F]

Weight/CG impact performance heavily as well. On a laminar flow wing like a Mooney having you average layer of wing crud can shave knots as well.  I would agree with a lot of what Vance and The latex salesman have said.  The truth is in the middle.