Safe QNH

[HENNING JOHAN]

I want to know what a safe Qnh is to fly in?The other day it was 1028. My trike did not climb lekker out!I know 1018-1024 is safe.

[Dobbs]

Hi Henning - I am not so sure you have a complete understanding of QNH. Using your example, your aircraft would in theory, all else being equal, have performed better at the higher QNH, because the air is slightly denser, so more lift, on the wing and the propellor, more oxygen per stroke of the engine so more power. This of course is often so small that it is not noticeable. I strongly suspect that a trike with a half full tank at a QNH of 1010 will perform much the same as a trike with an additional couple of litres, at a QNH of 1020. ( I have no imperical data to prove this, it is only a gut feel, but it is that order of magnitude.)

The difference in air density between 1010 and 1011 mb is 30 feet, so really it is not that significant for us trike flyers

[vernon11]

Today I flew at 1030. No problems. Admmitedly it was in a Savannah.
Vernon.
P.S. I know Dobbs is going to have something to say!!!

[Dobbs]

Vernon, I am not so sure that your aircraft flies, it is more a case of the earth repelling it OK OK I take that back

Have you put you fat tyres on yet?

[HENNING JOHAN]

Hi Henning - I am not so sure you have a complete understanding of QNH. Using your example, your aircraft would in theory, all else being equal, have performed better at the higher QNH, because the air is slightly denser, so more lift, on the wing and the propellor, more oxygen per stroke of the engine so more power. This of course is often so small that it is not noticeable. I strongly suspect that a trike with a half full tank at a QNH of 1010 will perform much the same as a trike with an additional couple of litres, at a QNH of 1020. ( I have no imperical data to prove this, it is only a gut feel, but it is that order of magnitude.)

The difference in air density between 1010 and 1011 mb is 30 feet, so really it is not that significant for us trike flyers

Thanks Dobbs!

[vernon11]

Vernon, I am not so sure that your aircraft flies, it is more a case of the earth repelling it OK OK I take that back

What did I tell you? ( you forgiven)

Have you put you fat tyres on yet?

Not yet Donovan. I ordered new tubes through Brett. I am a bit worried about the other tubes.
Regads
Vernon.

[Android]

Following on from the above responses, you do, indeed, appear to have your understanding of QNH reversed. However, aircraft performance is based on Density Altitude.

Without getting into the details... DA is a factor of the Pressure Altitude (QNE) at a given elevation, adjusted for for non-ISA temperature.

For every 1 hectopascal decrease, "relative altitude" increases by a factor of 30ft. HOWEVER, for every 1 deg. C increase, "relative altitude" increases by a factor of 120ft.

Therefore, it is more prudent to have checked what the temperature was at the time of for flight, as it would have a greater bearing on performance.

[Dobbs]

Hi Android, thanks for reminding me that temperature in fact plays a greater role than QNH

[John Boucher]

Here is an online calc link....

http://wahiduddin.net/calc/calc_da.htm