I took our aeroplane out to see how fast it would fly. At the altitude I chose the engine rpm was at the redline at full throttle and I achieved a certain airspeed. This was on an ISA +7 deg C day. Does anybody know a method to estimate what the airspeed would be on an ISA day ?
The engine cannot produce any more power, I imagine on an ISA day with the increased air density I would have to reduce throttle to stay below the redline. The induced drag would be the same as the weight and thus lift would be the same. Maybe with the reduced angle of attack the form drag could be less. Maybe with a reduced elevator deflection the trim drag would be less. Do I just have to wait for an ISA day and repeat the test ?
Thanks for any ideas, Alan.
Airframe performance
Moderators: John Dean, Moderator
Alan
All this kind of data can be 'reduced' to ISA conditions. I use "Homebuilt Aerodynamics and Flight Testing" by Sonja Englert as my guide (you can buy a copy from the PFA shop).
In order to reduce your data we also need to know the density altitude (or the actual altitude flown and the altimeter setting) and the outside air temp at that altitude.
To get TAS you will also need to know the various calibration errors from the ASI. Englert explains in detail how to do this.
Taking data from single points is not generally regarded as very good practice as you have no idea what errors there are (if any). Much better to measure RPM and IAS at a range of speeds, reduce the data to ISA and then plot all the points on a graph. If they all fall in a nice line its a fair bet your errors are low (or at least uniform) and then you can read off values for speed against RPM.
As you point out, once you are at red-line the speed gains will be small. The simplest way to reduce drag is either to reduce weight or move the CG back. Most spam-cans will fly 2-3 knots faster on the same throttle setting if you move the seat back as far as you can in the cruise!
I'll be happy to help you with the Math if you are serious about doing the testing.
All this kind of data can be 'reduced' to ISA conditions. I use "Homebuilt Aerodynamics and Flight Testing" by Sonja Englert as my guide (you can buy a copy from the PFA shop).
In order to reduce your data we also need to know the density altitude (or the actual altitude flown and the altimeter setting) and the outside air temp at that altitude.
To get TAS you will also need to know the various calibration errors from the ASI. Englert explains in detail how to do this.
Taking data from single points is not generally regarded as very good practice as you have no idea what errors there are (if any). Much better to measure RPM and IAS at a range of speeds, reduce the data to ISA and then plot all the points on a graph. If they all fall in a nice line its a fair bet your errors are low (or at least uniform) and then you can read off values for speed against RPM.
As you point out, once you are at red-line the speed gains will be small. The simplest way to reduce drag is either to reduce weight or move the CG back. Most spam-cans will fly 2-3 knots faster on the same throttle setting if you move the seat back as far as you can in the cruise!
I'll be happy to help you with the Math if you are serious about doing the testing.
Most of the performance adjustments are based on air density, and 7C in density terms, is about equivalent to flying 500' higher.
Did you actually measure the temperature at the altitude you were flying?
OAT gauges will produce a few degrees of error due to the compressibility effect (i.e the temperature will indicate between the static and stagnation temperatures).
In practice 7 degrees won't make much difference, especially when considering variances in weight, CG position,flying technique, rudder trim, cleanliness of wings/prop etc. etc.
If you want to see the effect of density, try repeating the test at different altitudes, then do them again in the opposite order to take out the effect of fuel burn.
Did you actually measure the temperature at the altitude you were flying?
OAT gauges will produce a few degrees of error due to the compressibility effect (i.e the temperature will indicate between the static and stagnation temperatures).
In practice 7 degrees won't make much difference, especially when considering variances in weight, CG position,flying technique, rudder trim, cleanliness of wings/prop etc. etc.
If you want to see the effect of density, try repeating the test at different altitudes, then do them again in the opposite order to take out the effect of fuel burn.
-
Alan George
- Posts: 62
- Joined: Mon Jan 07, 2008 8:22 pm
- Location: Bristol
Thanks for this.
First off I will get a copy of the Englert book, see what I understand and then get back to you.
I did record the altitude and pressure setting. Also speed by ASI and GPS groundspeed. However I estimated the temperature. We have an OAT sensor for the Dynon waiting to be installed, that can be the second job.
However from what you are saying, at ISA+7 it is not going to make much difference when the data is 'reduced' to ISA. I can take my current test as a reasonable enough top speed figure.
Regards, Alan.
First off I will get a copy of the Englert book, see what I understand and then get back to you.
I did record the altitude and pressure setting. Also speed by ASI and GPS groundspeed. However I estimated the temperature. We have an OAT sensor for the Dynon waiting to be installed, that can be the second job.
However from what you are saying, at ISA+7 it is not going to make much difference when the data is 'reduced' to ISA. I can take my current test as a reasonable enough top speed figure.
Regards, Alan.
-
Julian Bone
- Posts: 9
- Joined: Thu Jul 03, 2008 7:19 pm
dalton computer/whiz wheel works well and easy to use
need to record altitude and pressure setting of altimeter, OAT, IAS and of course check ASI by your preferred method
need a still day to keep a/c straight and level for quite a long time
ideally use measured mile on ground but not practical for most of us
gps can be used but often pretty inaccurate
need a location away from other traffic, ideally during the week say over the sea which most avoid
why is engine redline at max power? wrong prop?
need to record altitude and pressure setting of altimeter, OAT, IAS and of course check ASI by your preferred method
need a still day to keep a/c straight and level for quite a long time
ideally use measured mile on ground but not practical for most of us
gps can be used but often pretty inaccurate
need a location away from other traffic, ideally during the week say over the sea which most avoid
why is engine redline at max power? wrong prop?