True Air Speed

[Apache123]

I'm in the process of researching "The Next Plane"™

When looking at a Cherokee Six 260 recently I was asking the owner about performance and he said he gets about 120kts true at 23"/24 regardless of altitude.

This didn't make sense to me as in the Apache at 23/23 and about 7,500' I'll true at roughly 135kts but down around sight-seeing altitudes (1,500') I'll see about 120kts true.

Am I missing something or was the guy's head in the clouds?

 

[Clark1961]

Am I missing something or was the guy's head in the clouds?

Head was in clouds (thinking indicated) or he misunderstood the question. The Sixes walk away from me so I'm guessing he was talking indicated.

 

[Tony_Scarpelli]

She is seeing the indicated speed stay the same but is not calculating or worrying about true.

I don't know if there is enough difference between speeds for you to tell the difference. Only less fuel and more load carrying capacity.

My 250hp burns 14.3 at 75% so I doubt she burns ore than 15. But it costs me less than 3 knots to burn 12 gph. I suspect with that box you will see even more fuel advantage if you actually do fly about 125knots.

I'm in the process of researching "The Next Plane"™

When looking at a Cherokee Six 260 recently I was asking the owner about performance and he said he gets about 120kts true at 23"/24 regardless of altitude.

This didn't make sense to me as in the Apache at 23/23 and about 7,500' I'll true at roughly 135kts but down around sight-seeing altitudes (1,500') I'll see about 120kts true.

Am I missing something or was the guy's head in the clouds?

 

[ChitDisturber]

It's always been a pet peeve of mine when folks trying to sell their planes advertise TAS not IAS.

When I buy I as what does she cruse at in IAS, as in what does the ASI show in cruse not calculating temp or altitude.

 

[poadeleted20]

It's always been a pet peeve of mine when folks trying to sell their planes advertise TAS not IAS.

When I buy I as what does she cruse at in IAS, as in what does the ASI show in cruse not calculating temp or altitude.

Why? IAS is essentially meaningless for any navigational or other planning purposes.

In any event, as to the OP's question, power increases at the same MP/RPM setting as you climb -- about 5% more between 2000 and 7000 feet. As a result, IAS can be expected to go up about 1.6% on the same power setting, or about 2 knots in the IAS ranges we're discussing -- not much at all. OTOH, TAS goes up dramatically -- about 12 knots in this speed range. So, I think Tony's right -- this person is seeing about the same IAS, not TAS, at the same power setting as altitude goes up/down.

 

[coma24]

I have to agree, it's ALL about the TAS.

My plane goes '163kts' at cruise. Well, so can a Mooney 201 on the right day. The difference is I'm doing that at 10,000ft, where my TAS is actually around 195.

If you'd like a more dramatic example, speak to an SR-71 pilot

 

[Apache123]

Why? IAS is essentially meaningless for any navigational or other planning purposes.

In any event, as to the OP's question, power increases at the same MP/RPM setting as you climb -- about 5% more between 2000 and 7000 feet. As a result, IAS can be expected to go up about 1.6% on the same power setting, or about 2 knots in the IAS ranges we're discussing -- not much at all. OTOH, TAS goes up dramatically -- about 12 knots in this speed range. So, I think Tony's right -- this person is seeing about the same IAS, not TAS, at the same power setting as altitude goes up/down.

This is what I figured, but they were convinced their True was the same up and down.

 

[Apache123]

It's always been a pet peeve of mine when folks trying to sell their planes advertise TAS not IAS.

When I buy I as what does she cruse at in IAS, as in what does the ASI show in cruse not calculating temp or altitude.

An example (and a bird that I considered until I woke up) why I wouldn't do this is the Beech P-Baron. At cruise you'll be pulling 235kts true, but indicated airspeed will only be 170kts.

 

[ChitDisturber]

Why? IAS is essentially meaningless for any navigational or other planning purposes.

In any event, as to the OP's question, power increases at the same MP/RPM setting as you climb -- about 5% more between 2000 and 7000 feet. As a result, IAS can be expected to go up about 1.6% on the same power setting, or about 2 knots in the IAS ranges we're discussing -- not much at all. OTOH, TAS goes up dramatically -- about 12 knots in this speed range. So, I think Tony's right -- this person is seeing about the same IAS, not TAS, at the same power setting as altitude goes up/down.

For planning its fine, for aircraft performance I rather them give me IAS, it's harder for them to F' the numbers up.

 

[kenjr]

I'm in the process of researching "The Next Plane"™

When looking at a Cherokee Six 260 recently I was asking the owner about performance and he said he gets about 120kts true at 23"/24 regardless of altitude.

This didn't make sense to me as in the Apache at 23/23 and about 7,500' I'll true at roughly 135kts but down around sight-seeing altitudes (1,500') I'll see about 120kts true.

Am I missing something or was the guy's head in the clouds?

My Cherokee 235 can do that. Six is faster for sure. That said, I wouldn't get into a six unless it was the 300, but that's me...

 

[Captain]

For planning its fine, for aircraft performance I rather them give me IAS, it's harder for them to F' the numbers up.

Plan to be disappointed. Nobody advertises their performance in IAS. Not private sellers or OEMs. First, TAS is a higher number so right there sellers would rather go with that. Second, and more importantly, TAS means something.

IAS goes down with altitude. So which altitude did you want that IAS? Sea Level? Not much flying there so again, worthless. Might as well ask for a planes ground speed.

 

[35 AoA]

There have been plenty of times when I have felt a little bump in the stick as I have pushed through the "number" in the low/mid 30's at like 330-350 KIAS. That is the difference between KIAS and KTAS. Down at 5k, 350 KIAS is like mach 0.70 ish (that might not be totally accurate, but it's in the ballpark)

 

[Velocity173]

Always advertise in TAS and in MPH. Looks much more appealing.

 

[Velocity173]

I have to agree, it's ALL about the TAS.

My plane goes '163kts' at cruise. Well, so can a Mooney 201 on the right day. The difference is I'm doing that at 10,000ft, where my TAS is actually around 195.

If you'd like a more dramatic example, speak to an SR-71 pilot

Or the U-2; 10kts separate stalling or going through VNE.

 

[Sac Arrow]

For planning its fine, for aircraft performance I rather them give me IAS, it's harder for them to F' the numbers up.

I'm not sure I understand your point, seems a little backwards. TAS is a real measure of the plane's performance. IAS are the numbers by which you fly it.

 

[Captain]

Or the U-2; 10kts separate stalling or going through VNE MMO.

FTFY

 

[ChitDisturber]

I'm not sure I understand your point, seems a little backwards. TAS is a real measure of the plane's performance. IAS are the numbers by which you fly it.

Beacuse some old 30hr a year pilot is not going to F' up simply reading me his IAS and cruse settings over the phone, but as the OP noticed sometimes they will screw up figuring TAS out.

I'd rather have solid IAS numbers on a plane and extrapolate the TAS, versus have some guy that doesn't know how to calculate properly or use the ring right, give me some messed up numbers.

 

[Sac Arrow]

Beacuse some old 30hr a year pilot is not going to F' up simply reading me his IAS and cruse settings over the phone, but as the OP noticed sometimes they will screw up figuring TAS out.

I'd rather have solid IAS numbers on a plane and extrapolate the TAS, versus have some guy that doesn't know how to calculate properly or use the ring right, give me some messed up numbers.

I see.