Re-Calculating Cruising Speed

mmilano

mmilano

Pre-takeoff checklist
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Mike Milano
After being on a couple XCs now, I'm pretty confident the C-152 I rent does not have a cruising speed of 107 as noted in the POH.

I'm wondering what the best way to re-calculate the cruising speed so my XC planning will be more accurate.

I had an idea of using a GPS to determine GS while flying downwind, then GS while flying upwind. By subtracting 1/2 the difference of the 2 speeds from the downwind GS, I should have an accurate GS figure with 0 wind.

i.e. downwind GS: 110kts, upwind GS: 90kts == GS with 0 wind: 100kts

GS with 0 wind would be the same as my Air speed right? Is this a good way to re-calculate this?
 
Never did it myself but was always told to fly a perfect square (360, 090, 180, 270) and note the GS on each heading then average the four readings. I would think that this would be close enough for government work :goofy:
 
mmilano said:
After being on a couple XCs now, I'm pretty confident the C-152 I rent does not have a cruising speed of 107 as noted in the POH.

I'm wondering what the best way to re-calculate the cruising speed so my XC planning will be more accurate.

I had an idea of using a GPS to determine GS while flying downwind, then GS while flying upwind. By subtracting 1/2 the difference of the 2 speeds from the downwind GS, I should have an accurate GS figure with 0 wind.

i.e. downwind GS: 110kts, upwind GS: 90kts == GS with 0 wind: 100kts

GS with 0 wind would be the same as my Air speed right? Is this a good way to re-calculate this?
Click to expand...

I guess I'm a bit confused as to what you are saying. So you don't cruise at 107 knots airspeed. What airspeed do you normally cruise at? .. Use that?

The POH is never going to be exact.


What it sounds like you are trying to do is verify your airspeed with your ground speed, Are you saying that the airspeed indicator isn't accurate or?
 
mmilano said:
After being on a couple XCs now, I'm pretty confident the C-152 I rent does not have a cruising speed of 107 as noted in the POH.

I'm wondering what the best way to re-calculate the cruising speed so my XC planning will be more accurate.

I had an idea of using a GPS to determine GS while flying downwind, then GS while flying upwind. By subtracting 1/2 the difference of the 2 speeds from the downwind GS, I should have an accurate GS figure with 0 wind.

i.e. downwind GS: 110kts, upwind GS: 90kts == GS with 0 wind: 100kts

GS with 0 wind would be the same as my Air speed right? Is this a good way to re-calculate this?
Click to expand...

What speed do you normally cruise at?
Use that value in your calculations.

BTW if the airplane is clean or dirty will change you speed by a couple of knots so get used to planning on typical values but adjusting once on the air. Winds aloft are seldom what they are forecasted and you always need to watch you leg times to ensure you are not running out of fuel.
 
thanks for the info.

Jesse said:
I guess I'm a bit confused as to what you are saying. So you don't cruise at 107 knots airspeed. What airspeed do you normally cruise at? .. Use that?

The POH is never going to be exact.


What it sounds like you are trying to do is verify your airspeed with your ground speed, Are you saying that the airspeed indicator isn't accurate or?
Click to expand...

interesting point. i'm probably making it more complicated than it has to be. my airspeed with be the same with wind or no wind.

my ATA always exceeds my ETA, clean or dirty, this is why i would like to use a different figure than in the POH.

another factor now that i think about it is that i live in southern california, where the temperature (this time of year) is always about 10-20 degrees higher than the standard temps. ( standard temps int he POH used to estimate climb is what i'm referring to )

i'm still a student, and i am almost afraid to ask this, .. but is there a chart in the POH that gives cruising speeds for different temps and altitudes? i switched CFIs in the middle of my training and i fear sometimes he may assume i know things that i don't. i'll talk to him about this next time we meet. i'll also go over my POH later to look for myself.
 
mmilano said:
i'm still a student, and i am almost afraid to ask this, .. but is there a chart in the POH that gives cruising speeds for different temps and altitudes?
Click to expand...

IIRC the performance chapter of the flight manual or POH has those figures. You may have to do some extrapolating, but that isn't too bad. Get your CFI to show you how.
 
mmilano said:
i'm still a student, and i am almost afraid to ask this, .. but is there a chart in the POH that gives cruising speeds for different temps and altitudes? i switched CFIs in the middle of my training and i fear sometimes he may assume i know things that i don't. i'll talk to him about this next time we meet. i'll also go over my POH later to look for myself.
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Here's the example. The attachment is for a 1979 C152 with the 0235 N2C, not the L2C engine.

Say you are going to cruise at 5000 feet on a day which you are 10C above standard (e.g, surface is likely to be 25C, at 5000 it's 15C).

Look for 2300 rpm on the 20C above standard columns, noting 59 and 62% HP at 2300 rpm. The mid point, for 5,000 feet is 60.5% hp and the airspeed will be 95.5 kts. The fuel flow will be 5.1 gph.

Now do the same for STD temperature. The mid point between 63 and 66% bhp will be 64.5% for 96.5 knots and 5.35 gph.

Now take the mid points between +20C and STD temp. That'll be midway between 64.5% and 60.5% bph or 62.5% which is 96 knots and 5.22 gph.

That's interpolation. Amazing how many students and PVT ASELs cannot do this. POH Cruise table attached, the cited numbers are circled.
 
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mmilano said:
After being on a couple XCs now, I'm pretty confident the C-152 I rent does not have a cruising speed of 107 as noted in the POH.

I'm wondering what the best way to re-calculate the cruising speed so my XC planning will be more accurate.

I had an idea of using a GPS to determine GS while flying downwind, then GS while flying upwind. By subtracting 1/2 the difference of the 2 speeds from the downwind GS, I should have an accurate GS figure with 0 wind.

i.e. downwind GS: 110kts, upwind GS: 90kts == GS with 0 wind: 100kts

GS with 0 wind would be the same as my Air speed right? Is this a good way to re-calculate this?
Click to expand...

Many airplanes don't make book speeds. For one thing those cruise speeds in older planes usually were from one that had no antennas, flown at less than MGW, and under optimal conditions. OTOH it's rare that it will be off more than a few percent because power required goes up at the third power of the airspeed. Other factors are the prop pitch (fixed) and the engine power instruments (the tach in a 152). You might be running less power than you think. As to flight planning, unless you are seeing a huge difference (>10 KTAS) I suggest you just plan for about 5 KTAS less. There's not much point in getting more accurate than that because the winds will probably be off by that much.
 
ouch, kind of embarrasing. i found the area in the poh, thanks bruce. i guess this is learning from experience. i'm sure my current cfi must have assumed i knew this since he drilled me on the page before it to help me determine my climb figures.

no need to re-calculate the air speed now :) thanks for the info.
 
I think there could be some confusioin between Indicated Air Speed and True Air Speed. IAS is not dependent on temperature or altitude, it's only a measure of the force of the air stream pushing against the pitot tube compared to the static pressure at the static port. TAS corrects for pressure and temperature, and all the peformance charts show these corrections. Many airpseed indicators have a dial that lets you set temperature and altitude to display TAS. For planning fuel consumption and estimating ETA, ground speed is what you need, but for landing and take-off (Vx, Vy) you want IAS. Ground speed is of course easy with a GPS, and hard without one. You can time the distance made between legs and check against your planning log. Your plane will not make book values, reported as KTAS, at anything but sea level if you are looking at KIAS and not correcting.One easy rule of thumb is to add 2% to your IAS for every thousand feet of altitude (ignoring temperature). Thus at 5,000 feet add 10% to your IAS.

My trips almost always take a little longer than planned, even with a panel mounted GPS that predicts ETA based on actual groundspeed. I think one source of this discrepancy is the time we spend manuevering in the pattern and ground operations. Now I just add 15 minutes to my plans.
 
mmilano said:
After being on a couple XCs now, I'm pretty confident the C-152 I rent does not have a cruising speed of 107 as noted in the POH.
Click to expand...
Can't say that surprises me. My experience is that even 100 KTAS is optimistic at the lower altitudes at which most Student Pilots fly.

I'm wondering what the best way to re-calculate the cruising speed so my XC planning will be more accurate.
Click to expand...
In addition to the methods posted by Steve, there's a 3-leg method you can use -- see http://reacomp.com/true_airspeed/ for how to do it and a page that will let you plug in the results and get an answer.

BTW, averaging four cardinal directions does not produce an accurate result due to the effects of wind drift -- the hypontenuse is always longer than the long side unless the short side is zero (i.e., zero wind).
 
bbchien said:
Don't be embarassed. You after all, asked. That's GOOD.:yes:
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That's why I like POA.
In past boards, you would get a student crucified & his instructor (who no one knew anything about) tarred and feathered before the student was to fire him.

Thanks for posting your Q, Mike, I learned something too. I find I am constantlly forgetting stuff and need to review all the time.

Good job, guys.
 
jdwatson said:
Do y'all use TAS for your flight planning ?
Click to expand...

No. KTAS corrected for winds aloft giving a GS estimate.
 
jdwatson said:
Do y'all use TAS for your flight planning ?
Click to expand...

Yes, compensated for winds aloft with a factor for climb time. My EFP does all this with almost no effort on my part.
 
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