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Speed Question

Allison

I'm New Here
Good Evening Everyone,

I have a problem in explaining why my RV6a O320 150hp FP flies at 159 TAS full throttle at sea level, while at 5000 feet amsl it only achieves 154 TAS ????

I always thought TAS increased with an increase in altitude, up to a point of course, but I can?t explain this....


Thanks, in advance.
 
Good Evening Everyone,

I have a problem in explaining why my RV6a O320 150hp FP flies at 159 TAS full throttle at sea level, while at 5000 feet amsl it only achieves 154 TAS ????
Thanks, in advance.

You are going to be able to make more power down low in the thicker air. What's your ground speed doing?

-Marc
 
The correlation I notice is that with a fixed pitch prop I get the same true airspeed at the same RPMs regardless of altitude. But the fuel burn goes down with altitude.
 
Good Evening Everyone,

I have a problem in explaining why my RV6a O320 150hp FP flies at 159 TAS full throttle at sea level, while at 5000 feet amsl it only achieves 154 TAS ????

I always thought TAS increased with an increase in altitude, up to a point of course, but I can’t explain this....


Thanks, in advance.

Good question... so I looked up the speeds at altitude in my Tiger's (fixed pitch prop) POH.

All at 2700 rpm and TAS at "recommended lean mixture"

2,000 ft - 141 kts - 13.3 gph
4,000 ft - 140 kts - 12.5 gph
6,000 ft - 139 kts - 11.2 gph
9,000 ft - 138 kts - 10.6 gph

All following Standard Atmosphere pressure/temperatures. As you see it does seem to drop slightly with altitude.

UPDATED
After reading post #4 I added the fuel flow numbers
 
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It?s all about power and drag. At sea level the air is denser, so you can make more power. At full throttle you?ll normally get max speed at sea level.
OTOH, if you restrict power to 75% of max, you?ll have constant power up to about 7000?, so now drag determines max speed, which will be at 7000?. Go still higher, both power and parasitic drag decrease, but induced drag slowly increases, so speed slowly falls off.
 
Good Evening Everyone,

I always thought TAS increased with an increase in altitude, up to a point of course, but I can?t explain this....


Thanks, in advance.

It's not really that TAS increases with altitude, but more that IAS decreases with altitude even though the actual airspeed is not decreasing. It is due to instrument issues (IAS is dependant upon air density).

Larry
 
Good Evening Everyone,

I have a problem in explaining why my RV6a O320 150hp FP flies at 159 TAS full throttle at sea level, while at 5000 feet amsl it only achieves 154 TAS ????

I always thought TAS increased with an increase in altitude, up to a point of course, but I can?t explain this....


Thanks, in advance.

See reply #6. More air for the engine down low, less as you go higher. During cross-country races in the RV-8 I plan on losing 1 kt TAS per 1000'.
 
Good question... so I looked up the speeds at altitude in my Tiger's (fixed pitch prop) POH.

All at 2700 rpm and TAS at "recommended lean mixture"

2,000 ft - 141 kts - 13.3 gph
4,000 ft - 140 kts - 12.5 gph
6,000 ft - 139 kts - 11.2 gph
9,000 ft - 138 kts - 10.6 gph

All following Standard Atmosphere pressure/temperatures. As you see it does seem to drop slightly with altitude.

UPDATED
After reading post #4 I added the fuel flow numbers



Now post the corresponding chart as to % power at those altitudes...
 
It's not really that TAS increases with altitude, but more that IAS decreases with altitude even though the actual airspeed is not decreasing. It is due to instrument issues (IAS is dependant upon air density).

Larry


I hope that you are not saying that TAS doesn't increase with altitude...

Rule of thumb is TAS increases about 2% per 1000'...
 
I hope that you are not saying that TAS doesn't increase with altitude...

Rule of thumb is TAS increases about 2% per 1000'...

I think what you meant to say is that the delta between IAS and TAS increases about 2%/1000’.

TAS may or may not increase with altitude, depending on many factors.
 
Yes

I think what you meant to say is that the delta between IAS and TAS increases about 2%/1000’.

TAS may or may not increase with altitude, depending on many factors.

Yes, thank you...need to proof the replies before submitting!
 
Yes, thank you...need to proof the replies before submitting!

and that was what I meant. TAS does not increase as you go up in altitude, assuming actual airspeed remained constant (TAS always matches actual airspeed), but IAS does drop relative to actual speed as you go up.

Theoretically TAS always matches actual airspeed, but IAS does not. Altitude and temp are used to compute the difference.

In many planes, TAS will climb with altitude, to a point, if fuel flow is kept constant and flying leaner than max power mixture. Max RPM and limited manifold pressure often limit this effect though. We all see fuel flow drop off with altitude and similar speeds. When operating leaner than max power, that efficiency can be traded for a small increase in speed.

Larry
 
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Larry,
Please enlighten me about "actual" airspeed. I know indicated, calibrated, equivalent and true airspeeds but not "actual" Google doesn't recognize it either.

Thanks
 
IAS is adding an unnecessary complication to this discussion - the OP is talking about TAS.

TAS at WOT decreases with altitude, assuming a normally aspirated engine. Fuel burn (at a constant mixture ratio) also decreases. If you look at maximum cruise speeds reported by Van's or in the CAFE reports they are always obtained near sea level (or as close as they can get).

TAS at 75% throttle increases with altitude, because you're making the same amount of power (and burning the same fuel), but the air is getting thinner. This only holds true so long as you can actually get 75% power however. Above a certain point (roughly 8000-9000') you can't. This is why "normal" cruise speeds are often reported at 8000'.

The OP's observation is exactly what it should be!
 
Now post the corresponding chart as to % power at those altitudes...

Good question... so I looked up the speeds at altitude in my Tiger's (fixed pitch prop) POH.

All at 2700 rpm and TAS at "recommended lean mixture"

2,000 ft - 141 kts - 13.3 gph - 89%
4,000 ft - 140 kts - 12.5 gph - 84%
6,000 ft - 139 kts - 11.2 gph - 79%
9,000 ft - 138 kts - 10.6 gph - 72%

All following Standard Atmosphere pressure/temperatures. As you see it does seem to drop slightly with altitude.

UPDATED
After reading post #4 I added the fuel flow numbers
UPDATED
Added % power numbers
 
IAS is adding an unnecessary complication to this discussion - the OP is talking about TAS.

TAS at WOT decreases with altitude, assuming a normally aspirated engine. Fuel burn (at a constant mixture ratio) also decreases. If you look at maximum cruise speeds reported by Van's or in the CAFE reports they are always obtained near sea level (or as close as they can get).

TAS at 75% throttle increases with altitude, because you're making the same amount of power (and burning the same fuel), but the air is getting thinner. This only holds true so long as you can actually get 75% power however. Above a certain point (roughly 8000-9000') you can't. This is why "normal" cruise speeds are often reported at 8000'.

The OP's observation is exactly what it should be!

+1. The OP has a small complication: fixed pitch prop. Depending on how the prop is pitched it’s likely he cannot achieve max available engine power because he (1) hits redline rpm prior to full throttle, or (2) cannot get to redline rpm with full throttle, either of which will slightly skew the ‘max available power’ results above.
 
Larry,
Please enlighten me about "actual" airspeed. I know indicated, calibrated, equivalent and true airspeeds but not "actual" Google doesn't recognize it either.

Thanks

LOL, my thoughts exactly.

IAS is pretty much meaningless, when you are talking cruise numbers. I only care about IAS for takeoff/landing, and TAS for cruise.
 
+1. The OP has a small complication: fixed pitch prop. Depending on how the prop is pitched it’s likely he cannot achieve max available engine power because he (1) hits redline rpm prior to full throttle, or (2) cannot get to redline rpm with full throttle, either of which will slightly skew the ‘max available power’ results above.

Good point!
 
Larry,
Please enlighten me about "actual" airspeed. I know indicated, calibrated, equivalent and true airspeeds but not "actual" Google doesn't recognize it either.

Thanks

The job of any sensor is to provide an indication of a condition that is as close as possible to the actual condition. In this case, the condition being measured is speed through the air. TAS should closely represent the "actual" speed through the air, assuming working parts/calibration. Most don't speak of actual airspeed, as there is no real way to know it. We only know the various indicated speeds, and hope that they closely represent actual conditions. Things like GPS box routines can tell us how close our indicated speed is to actual speed through the air.

Larry
 
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The job of any sensor is to provide an indication of a condition that is as close as possible to the actual condition. In this case, the condition being measured is speed through the air. TAS should closely represent the "actual" speed through the air, assuming working parts/calibration. Most don't speak of actual airspeed, as there is no real way to know it. We only know the various indicated speeds, and hope that they closely represent actual conditions. Things like GPS box routines can tell us how close our indicated speed is to actual speed through the air.

Larry

Please forgive me. As a once upon a time physics teacher I want to pull out a red pen and write all over the above post. I believe nearly every sentence is incorrect.
 
TAS vs Ground speed

Should be this simple

No wind: Ground speed = TAS
Headwind: Ground speed = TAS - Head wind
Tailwind: Ground speed = TAS + Tail wind

So TAS is simply ground speed with NO WIND.
 
I don't think I completely follow Larry, but I have a hunch you're referring to environmental or mechanical error?

At work our faithful SF340's have electronically "corrected" pitot static instruments on the Captain's side (tied into the FMS) and raw data instruments for the standby and FO's side. Three separate systems and without fail the three hardly ever match - climb, cruise or descent. (For the FO, flying approaches with the head turned towards to the Captain's instruments is common place in order to prevent unnecessary deviation callouts)

Is this indicative of what you're suggesting?
 
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