Temperature and Performance

dmccormack

dmccormack

Touchdown! Greaser!
Joined
May 11, 2007
Messages
10,945
Location
Lancaster County, Pennsylvania
Display Name
Display name:
Dan Mc
I flew solo Saturday (OAT 25-30 F): Maintained 85 MPH level cruise at 2450-2500 RPM.

I flew dual Sunday (OAT around 50 F): Airspeed 75-80 MPH/2450-2500 RPM

I flew Tuesday morning (OAT around 40): Airspeed 75 MPH/2450-2500 RPM

Tuesday afternoon (OAT 56 F): Airspeed 75 MPH/245-2500 RPM

Can the ambient air temperature affect speeds that much? I've seen airspeed differences in other airplanes -- but none this large a percentage of overall speed.
 
dmccormack said:
Only changed parameter was OAT
Click to expand...

Not true, 1-2 say SOLO and DUAL 3 and 4 do not specify.

You do not specifcy fuel burn and when if ever the plane was refueled. First guess is refueld before the DUAL and very light for flight 1.

There are other weather related factors like air pressure and altitude to "airspeed".

Are the values IAS you did not specify TAS.

You also gave yourself a 2% range on the RPM probably to maintain level flight at the current trim setting. This is a fixed pitch prop isn't it.

Did you close the cowl flaps when you trimmed for cruise, that gets me fairly often with about the difference you noted.
 
Last edited:
RMCN172RG said:
Not true, 1-2 say SOLO and DUAL 3 and 4 do not specify.

You do not specifcy fuel burn and when if ever the plane was refueled. First guess is refueld before the DUAL and very light for flight 1.

There are other weather related factors like air pressure and altitude to "airspeed".

Are the values IAS you did not specify TAS.

You also gave yourself a 2% range on the RPM probably to maintain level flight at the current trim setting. This is a fixed pitch prop isn't it.

Did you close the cowl flaps when you trimmed for cruise, that gets me fairly often with about the difference you noted.
Click to expand...

Right -- one flight was dual.

All flights were takeoff with 10 gallons of fuel, burn rate 3.5 - 4 GPH.

This is a very fixed pitch prop, and the range of RPM is due to the slight wobble of the 70 year of the tach (it was far worse, but has stabilized since removal of lead deposits and runup before shutdown).

Cowl flaps -- you funny. :D
 
Ron Levy said:
You also didn't mention altitude and whether this is true or indicated airspeed.
Click to expand...


Altitude: 2500-3200 MSL
All speeds indicated

Keep in mind -- only data available is what's on the minimal panel -- No GPS, just Whiskey compass, a map, and what's out the window.

I did have an E6b on board and did not use it -- I should have.
 
were all of these speeds measured after the same amount of time in flight?

Matt M's flybaby would gain 5-10 mph as fuel burned off. a combination of less weight and aft shift of CG.
 
tonycondon said:
were all of these speeds measured after the same amount of time in flight?

Matt M's flybaby would gain 5-10 mph as fuel burned off. a combination of less weight and aft shift of CG.
Click to expand...

Not exactly to the minute -- but +/- 10 minutes.

There may have been some improvement due to fuel burn -- the CG moving aft is an interesting consideration, as it would be more pronounced when flying dual.
 
i'm not sure what effect fuel burn has on the CG in the chief. I would assume that all of the weight, passengers, and fuel, are more or less mounted on the CG so they wouldn't have much effect. At least that seems common on 2 seaters. The flybaby has 16 gallons of fuel about 1.5 to 2 feet in front of the CG. At the beginning of a long flight you are pulling back to hold altitude, by the end you are pushing forward. It's got the most pronounced CG shift in flight that i've ever experienced in an airplane.
 
tonycondon said:
i'm not sure what effect fuel burn has on the CG in the chief. I would assume that all of the weight, passengers, and fuel, are more or less mounted on the CG so they wouldn't have much effect. At least that seems common on 2 seaters. The flybaby has 16 gallons of fuel about 1.5 to 2 feet in front of the CG. At the beginning of a long flight you are pulling back to hold altitude, by the end you are pushing forward. It's got the most pronounced CG shift in flight that i've ever experienced in an airplane.
Click to expand...

I have no time in a Fly Baby.

The Chief 12g Fuel tank is mounted immediately behind the firewall, immediately in front of the panel.

The distance from center of seat to center of tank is ~3'
 
oh, duh. for some reason i thought it had wing tanks. i knew better than that. i'd bet it's due to the CG shift aft.
 
tonycondon said:
oh, duh. for some reason i thought it had wing tanks. i knew better than that. i'd bet it's due to the CG shift aft.
Click to expand...


I'll need to do a few more straight line flights and more precisely measured fuel.

So 2 aboard shifts the CG aft enough to overcome the increased induced drag?

Remarkable...
 
dmccormack said:
I'll need to do a few more straight line flights and more precisely measured fuel.

So 2 aboard shifts the CG aft enough to overcome the increased induced drag?

Remarkable...
Click to expand...

presumably you have the W&B info somewhere, you can do the math and figure it out. most people don't realize the speed increases you can get with an aft CG. Usually when we go somewhere in the 172 we throw as much weight as possible in the baggage compartment and see decent cruise speeds as a result. on the way to gastons we were truing near actual book numbers with a weekends worth of luggage, some chairs and a case or two of water in back.
 
tonycondon said:
presumably you have the W&B info somewhere, you can do the math and figure it out. most people don't realize the speed increases you can get with an aft CG. Usually when we go somewhere in the 172 we throw as much weight as possible in the baggage compartment and see decent cruise speeds as a result. on the way to gastons we were truing near actual book numbers with a weekends worth of luggage, some chairs and a case or two of water in back.
Click to expand...

W&B data for this 1940 airplane is minimal, but that's a good idea -- I should work up a spreadsheet on a rainy-day.

It's amazing what a shift 3-4 gallons of fuel can cause... :eek:

I also gave myself a minor panic episode one flight -- I was flying along, everything trimmed, etc etc, when the noise and RPM reduced.

Airspeed droppped about 5 MPH....

I pulled carb heat and turned towards the closest airport -- still out of gliding range.

Then I checked and re-checked -- duh! Airplane had started a climb after enough fuel burned off to get out of trim.

No VSI and a very old Altimeter means it's hard to see gradual climbs.
 
Last edited:
dmccormack said:
This is a very fixed pitch prop, and the range of RPM is due to the slight wobble of the 70 year of the tach (it was far worse, but has stabilized since removal of lead deposits and runup before shutdown).
Click to expand...

And that's the thing I would get calibrated or repaired. Is it one of the old centrifugal tachs (has no zero; the needle starts at the 3000 or 3500 RPM point) or the backward-reading magnetic tach? The centrifugal tach gets sticky and can give erroneous, stable readings. The magnetic tach can suffer from dust and a weakening magnet and sticky needle shaft bearings. If it has a forward-reading magnetic tach there will be a direction-reversing gearbox behind it that likely isn't a part of the problem. Even a 25 or 50 RPM error can give significantly different cruise speeds in an old, slow airplane.

All gauges should be checked and calibrated if there's a bad indication. Oil temps and CHTs can be checked with their probes in boiling water. Tachs can be checked against the handheld electronic prop tachs, which you can buy at an RC airplane hobby shop for around $40. Check the tach checker against a neon indicator light or fluorsescent tube, which flashes with the 60 Hz AC and will give an RPM reading of 3600 (two-blade setting) if the checker is accurate. For the old tach that might be sticking, check it several times at full power and go to idle between each test. Watch the CHT.

One more thing: Is this airplane covered with a water-based finish? Some of those (like the disastrous Blue River process) absorbed moisture from the air and would get saggy-baggy in the cool of a damp morning. And loose fabric allows the top wing skin to balloon upward more between the ribs, interfering with the inward spanwise flow and causing more drag. See if the airplane is slower when the dewpoint is higher.

Dan
 
Dan Thomas said:
And that's the thing I would get calibrated or repaired. Is it one of the old centrifugal tachs (has no zero; the needle starts at the 3000 or 3500 RPM point) or the backward-reading magnetic tach? The centrifugal tach gets sticky and can give erroneous, stable readings. The magnetic tach can suffer from dust and a weakening magnet and sticky needle shaft bearings. If it has a forward-reading magnetic tach there will be a direction-reversing gearbox behind it that likely isn't a part of the problem. Even a 25 or 50 RPM error can give significantly different cruise speeds in an old, slow airplane.

All gauges should be checked and calibrated if there's a bad indication. Oil temps and CHTs can be checked with their probes in boiling water. Tachs can be checked against the handheld electronic prop tachs, which you can buy at an RC airplane hobby shop for around $40. Check the tach checker against a neon indicator light or fluorsescent tube, which flashes with the 60 Hz AC and will give an RPM reading of 3600 (two-blade setting) if the checker is accurate. For the old tach that might be sticking, check it several times at full power and go to idle between each test. Watch the CHT.

One more thing: Is this airplane covered with a water-based finish? Some of those (like the disastrous Blue River process) absorbed moisture from the air and would get saggy-baggy in the cool of a damp morning. And loose fabric allows the top wing skin to balloon upward more between the ribs, interfering with the inward spanwise flow and causing more drag. See if the airplane is slower when the dewpoint is higher.

Dan
Click to expand...

The tach on this airplane is backward (Right to left) and starts at 0

No CHT -- Oil temp and Pressure only.

Fabric is Stits Polyfiber
 

Attachments

  • Fall_2009_143 (Medium).jpg
    Fall_2009_143 (Medium).jpg
    67.8 KB · Views: 12
Last edited:
dmccormack said:
W&B data for this 1940 airplane is minimal, but that's a good idea -- I should work up a spreadsheet on a rainy-day.

It's amazing what a shift 3-4 gallons of fuel can cause... :eek:

I also gave myself a minor panic episode one flight -- I was flying along, everything trimmed, etc etc, when the noise and RPM reduced.

Airspeed droppped about 5 MPH....

I pulled carb heat and turned towards the closest airport -- still out of gliding range.

Then I checked and re-checked -- duh! Airplane had started a climb after enough fuel burned off to get out of trim.

No VSI and a very old Altimeter means it's hard to see gradual climbs.
Click to expand...

Same thing in the Porterfield, no VSI and a single pointer altimeter. For level flight I just try to maintain a constant airspeed that I know is "correct" for the conditions. If you really want to hold altitude, use a portable WAAS GPS.
 
tonycondon said:
Most people don't realize the speed increases you can get with an aft CG. Usually when we go somewhere in the 172 we throw as much weight as possible in the baggage compartment and see decent cruise speeds as a result. On the way to Gastons we were truing near actual book numbers with a weekends' worth of luggage, some chairs and a case or two of water in back.
Click to expand...

I was just reading about the effect of CG on airspeed in Barry Schiff's The Proficient Pilot (Volume 1). He says that most manufacturers' dirty little secret is that they derive their book cruise speed figures with CG at the absolute back edge of the CG envelope.

My intuition would have told me that the opposite was true, but Schiff explains it very well. I've never seen it explained anywhere else. Where did you learn this?

Actually, I had a CFI tell me exactly the opposite. On my long dual IFR XC, he scooted his chair as far forward as it would comfortably go, saying that this would give us a knot or two of extra speed. I guess he was mistaken!

One more reason to understand that weight-shift-CG formula!
 
i think i learned that in basic aerodynamics during PP training. aft CG means less downforce on the tail (less drag on the tail) and also less lift that the wing has to generate (less drag on the wing), so you go fast.
 
tonycondon said:
i think i learned that in basic aerodynamics during PP training. aft CG means less downforce on the tail (less drag on the tail) and also less lift that the wing has to generate (less drag on the wing), so you go fast.
Click to expand...

That's generally true but exceptions exist. Obviously a canard configuration doesn't behave that way but IIRC some or all Bonanzas the tail actually creates a small amount of p0sitive lift when the CG is at the aft limit.
 
You must log in or register to reply here.
Back
Top