Javier Hernandez Rodero
#5 Altimeter reading deviations on flight
Updated: Jun 21, 2021
We all have seen in our traces that the altitude plot features some non-expected peaks and snags when we shut motor or in the landing moment.
I have identified the following effects that will cause our altimeter, which is a device that measures just pressure, reads an altitude not in line with expected.
We want to measue the undisturbed static pressure at the location our airplane is, but certainly the only presence of the model slightly disturbs the rearing. Note that what we want is to measure the pressure, say a few meters away the model but we are measuring the pressure “somewhere inside the fuselage” of the model.
The motor and propeller flow do change as well this pressure inside the fuselage.
Main effect here is kinetic energy. When you are climbing like a rocket at the end of your climb your airplane is likely to be running at speeds around 12-15 m/s (43-54 km/hr) or even more this doubles its cruise speed which is around 6 m/s (24 km/hr). No matter the model is absolutely horizontal: a trimmed model will just float up to its cruise speed is achieved by exchanging its kinetic energy in potential energy that is increasing its altitude.
The only and last formula here is:
Delta(h)= ½ (V12-V22)/g
where Delta(h) is meters and V in meters per second and g, gravity 9,8m/s2, for the numbers above (15 m/s and 6 m/s) this Delta(h), increment of altitude due kinetic energy, is around 10 meters, which very much what you see if you are not very careful in your top of climb. and the prressure distribution will be similar
If you wanted to maintain exactly the altitude when you shut the motor you should push the elevator such the altitude is constant while the speed decreases. I made a simulation for a glider 4m span, 0,4 m/s descend speed at cruise, 1,4 kg mass and an aspect ratio of 18. It will take around 6,5 seconds to dissipate all above kinetic energy down its cruise speed. Ahh this where the famous 10 seconds of F5J altitude calculation comes…
https://leehamnews.com/2015/04/28/fundamentals-of-airliner-performance-part-7-the-wing/ )Airliners are not that different than our models…
Yellow and red areas are where the static pressure is higher than atmospheric pressure (ram effect), pink areas are those where that pressure is more or less equal to atmospheric and GREY areas are where the static pressure is LOWER than atmospheric: there is more or less where the canopy of your model is.
This is a static motor On excursion
This is an example Where you can see this effect and also the kinetik energy recovery of height
This is an explanation why you see in your gliderkeeper traces the landing moment is “a bit high” also take in account that depending on the landing manoeuvre the altimeter “may” be a little higher than the nose touching the terrain and triggering the landing time.
Another interesting effect I have seen in a couple of instances is a dynamic effect during this landing. See picture it is clearly dynamic effect and as you can see this is around -2,5m which would mysteriously match to a hammershock effect of a fuselage pod column of air of 1,5m long braking at 2g in a very hermetic fuselage cabin… or it is just inertial forces in pressure sensor (difficult to believe). That fuselage does not show much Fuselage Shape effect so it is likely to be very tight sealing canopy.
Please note all above effects are depending and may vary in your peculiar installation, Fuselage shape and as well the way you pilot your plane.For further details please contact us in firstname.lastname@example.org.
We all wish now you have a little more confidence in the readings of your favourite AMRT and this helps you to have the happiest landings.