[dwilson]

Went out yesterday to test the top speed of my 9A with my new Catto two bladed prop.

Here is the setup:
160 HP o-320
Sam James cowel and pants
Altimiter 8000
Pressure 30.41
Temp. 33 F
RPM 2800 - still had throttle left
TAS 169 Kts as displayed by GRT Horizon 1. Previously checked by 3 way GPS track at 150 Kts.

My question is two fold.

1) Should I believe these numbers?
2) How much will changing pressure and temp effect the speed? Is there an equation that applies that I don't need Calc IV to interpret?

Thanks,

 

[Ron Lee]

I always fly at a given density altitude which means that you have to determine the correct Pressure Altitude (PA) that equates to your target Density altitude at that temperature.

I have a sheet that shows PA as a function of temperature for my selected density altitude.

Then you can start having reasonable comparisons.

 

[DGlaeser]

Speed numbers

Performance is a function of air density, which is a function of pressure and temperature. Performance numbers are only comparable at the same density altitude (pressure altitude - altimeter set to 29.92 - corrected for non-std temp - see a density altitude chart). The numbers you gave put you pretty close to 8500' DA.
What was your IAS? TAS is a computed value, and seems high compared to the GPS GS (not sure what you mean by 'previously checked'). Does your OAT seem correct?

You've pretty much got to believe your GPS for GS If you use one of the speed run methods correctly it should agree with your TAS, which depends on an accurate OAT.

What static RPM do you get? If you're at 2800 rpm with throttle left, at 8K, you probably could use a tad more pitch to get more speed.

 

[jetblackaircraft]

Top Speed Calc

http://en.wikipedia.org/wiki/Flight_envelope

Read the section on Top Speed and it might help to clear up what is going on. As pressure is decreased and temperature is increased the air density goes down which reduces drag. However, this reduction in density will begin to limit your engine power. So, there will be a point with a given pressure and temperature (corresponding to an altitude in the standard atmosphere) where your airplane will make it's top speed.

If the TAS calculation on your GRT Horizon 1 is accurate, then all you need to do is fly at different altitudes until you find your top speed. However, you really need to know some other things about the airplane, like gross weight at the time of your run. Ideally you would test at a constant weight, but that is not possible.

Personally what I would do is fill up the tanks and fly somewhere at a constant altitude and power setting. Over several hours you should have to slightly adjust trim to stay at your altitude. When you make those trim adjustments you will note a change in TAS. See if your TAS begins to increase as you burn fuel and then begins to decline when you get extremely light.

If you find a weight where you reach a minimum drag/max TAS, then try to do your testing around that weight and then try different altitudes to locate the optimal density altitude like Ron mentions.

Keep in mind the changes in TAS for weight and density altitude maybe be very small. Somewhere around 2 to 3 knots maybe.

To get an idea of the range you're talking about, fly at pattern altitude and full throttle, then climb to 10k and see what the difference in TAS is. I would guess your loss in power would be significant enough at 10k to reduce your TAS significantly.

Sorry for writing a novel, I'll keep the posts shorter next time.

Mike

 

[dwilson]

Performance is a function of air density, which is a function of pressure and temperature. Performance numbers are only comparable at the same density altitude (pressure altitude - altimeter set to 29.92 - corrected for non-std temp - see a density altitude chart). The numbers you gave put you pretty close to 8500' DA.

I was thinking it meant 7500', but I am not sure

What was your IAS?

I didn't think to record it, but I think it was about 155 Kts.


TAS is a computed value, and seems high compared to the GPS GS (not sure what you mean by 'previously checked').

Previously checked means I did a 3 leg run at 150 Kts TAS and compared it to the speed calculated from the 3 leg GPS speed and it matched within 1 Kt.


Does your OAT seem correct?

Have not calibrated it, but has always seemed about right.

You've pretty much got to believe your GPS for GS If you use one of the speed run methods correctly it should agree with your TAS, which depends on an accurate OAT.

What static RPM do you get?

Static RPM is around 2230

If you're at 2800 rpm with throttle left, at 8K, you probably could use a tad more pitch to get more speed.

More speed? Yeah, I would like more speed!

 

[DGlaeser]

Speed numbers

>>I was thinking it meant 7500', but I am not sure
You may be right - the delta from 30.41 to 29.92 is 490 feet - I may have adjusted the wrong way (it's Friday).

>>What was your IAS?
>>I didn't think to record it, but I think it was about 155 Kts.
A rough estimate of TAS is to add 2% per 1K of altitude. We'll use 7500' so 155 x 1.15 = 178 Kts - a bit high if your 169 TAS is correct. A TAS of 169 would have an IAS around 147.

>>Previously checked means I did a 3 leg run at 150 Kts TAS and compared it to the speed calculated from the 3 leg GPS speed and it matched within 1 Kt.
So the 169 TAS was not at the same time as the 150 Kt GS check? That is what confused me. What was your GPS GS when you had 169 TAS?

>>More speed? Yeah, I would like more speed!
Join the crowd!

 

[dwilson]

So the 169 TAS was not at the same time as the 150 Kt GS check? That is what confused me. What was your GPS GS when you had 169 TAS?

Did not look at ground speed. I know there was a pretty good wind up there. Some where in the 30Kt range.

Maybe Sunday I can go up and do a 3 leg GPS speed course and see how close the TAS matches the speed by GPS.

Still interested in a formula to calculate change in airspeed with changing pressure/temp.

Duane

 

[Ron Lee]

"Still interested in a formula to calculate change in airspeed with changing pressure/temp."

I get up to cruise altitude and the GPS tells me how long to get there. Got to tell you I don't worry about a knot or two difference...if there is one...as density altitude changes.

Terrain clearance and winds aloft are far more important.

 

[elippse]

I'll try to give it a shot at the formulae. Your power is proportional to the density of the charge entering the cylinder. So charge density ratio, which gives the ratio of altitude power to sea-level power, Prat = ALT MAP / SL MAP X (273.16+15) /(273.16+OAT(C)) X AltRHO / SLRHO. To get this ratio you need density altitude. Use PAlt to get standard altitude temperature and from this find the temperature variation at altitude from standard. TV = OAT - (15 - .001981 X Palt). Multiply this by 113.4' and algebraically add to Palt; this gives Dalt. Dratio = (1 - Dalt X 6.88E-6)^4.256. Oh, and if you are wondering how your MAP compares to static pressure, Palt, it equals PSL X (1 - Palt X 6.88E-6)^5.256. Your PSL should be the MAP the engine maker shows with the engine at 100% power at sea-level; with a carb this will be 1" to 1.5" below standard pressure of 29.92 so you should use 28.42" - 28.92". Now your speed at any given density altitude will be a function of the engine power change with altitude and the increased induced loss with altitude. Power change with altitude is given by some as (Drat - 0.15) / 0.85 and others as Drat^1.135. What this basically means is that power falls off slightly faster with altitude than does the density which causes the drag, so the plane will slow down as you go up. You have a -9A with an aspect ratio of 6.32:1, so your speed will fall off faster than one with a higher AR, but not as fast as a -6 with 4.8:1 since the induced loss (or drag) is related to AR. You can also do some extra calc's to further close in on the engine's power by taking into account the stagnation temperature rise in the induction inlet, the temperature drop after the venturi in the carb, and the induction air heating in the pan's oil. Your speed at a given Dalt is related to HP^1/3; 3% more power will give 1% more speed. So, to sum up, you wanted to know how temperature and pressure affects you speed, and as you can see for any given WOT, it's a function of density altitude. BTW; I'm not trying to bust your bubble, but that last inch of throttle movement causes very little change in power since whether the throttle valve is at an 80 deg. or 90 deg. angle has little effect on MAP; try it yourself! See how much movement it takes to decrease MAP 5% or 10%.

 

[dwilson]

Wow!

Thanks Elippse,
Glad I asked. It will take me a while to work through all this! In the meantime maybe I will go fly a while to let it all sink in.

I am grateful for all the folks on this newsgroup that are willing to share their brains and experience with us.

Duane

I'll try to give it a shot at the formulae. Your power is proportional to the density of the charge entering the cylinder. So charge density ratio, which gives the ratio of altitude power to sea-level power, Prat = ALT MAP / SL MAP X (273.16+15) /(273.16+OAT(C)) X AltRHO / SLRHO. To get this ratio you need density altitude. Use PAlt to get standard altitude temperature and from this find the temperature variation at altitude from standard. TV = OAT - (15 - .001981 X Palt). Multiply this by 113.4' and algebraically add to Palt; this gives Dalt. Dratio = (1 - Dalt X 6.88E-6)^4.256. Oh, and if you are wondering how your MAP compares to static pressure, Palt, it equals PSL X (1 - Palt X 6.88E-6)^5.256. Your PSL should be the MAP the engine maker shows with the engine at 100% power at sea-level; with a carb this will be 1" to 1.5" below standard pressure of 29.92 so you should use 28.42" - 28.92". Now your speed at any given density altitude will be a function of the engine power change with altitude and the increased induced loss with altitude. Power change with altitude is given by some as (Drat - 0.15) / 0.85 and others as Drat^1.135. What this basically means is that power falls off slightly faster with altitude than does the density which causes the drag, so the plane will slow down as you go up. You have a -9A with an aspect ratio of 6.32:1, so your speed will fall off faster than one with a higher AR, but not as fast as a -6 with 4.8:1 since the induced loss (or drag) is related to AR. You can also do some extra calc's to further close in on the engine's power by taking into account the stagnation temperature rise in the induction inlet, the temperature drop after the venturi in the carb, and the induction air heating in the pan's oil. Your speed at a given Dalt is related to HP^1/3; 3% more power will give 1% more speed. So, to sum up, you wanted to know how temperature and pressure affects you speed, and as you can see for any given WOT, it's a function of density altitude. BTW; I'm not trying to bust your bubble, but that last inch of throttle movement causes very little change in power since whether the throttle valve is at an 80 deg. or 90 deg. angle has little effect on MAP; try it yourself! See how much movement it takes to decrease MAP 5% or 10%.