Can someone explain the N.28's anemometer?

[bsones]

Okay, I am super confused by the anemometer in the Nieuport 28. It has two sets of labels--some small white numbers toward the center of the gauge, and big red numbers around the outside. The numbers don't align--for any given needle position, the red ones show a higher value. At first I was like "Oh I see, the red is kph and the white is mph." Except it's not, because the values, while different, are too close in value. 130 mph /= 142 kph.

 

I zoomed WAY in and read the fine print (which is in French), and it looks like the two rings are both in kph. The reason why there are two sets of values is because this anemometer is showing you values for indicated air speed at two altitudes--the white numbers are air speed at 2,000 meters and the red numbers are air speed at 4,000 meters, neither of which is the altitude at which IAS is typically given (sea level). Annoying! I turned on the info bar, which displays my true air speed (I think?), and you can see that when I'm at 116 kph at 177 meters, the white ring on the anemometer shows my indicated air speed as 128 kph, and the red ring shows my indicated air speed as 142 kph.

 

 

Here's the problem: neither of those values are correct. Indicated air speed goes down as altitude increases, because an anemometer measures speed by measuring air pressure, and air pressure decreases as altitude increases. So, the 2,000-meter IAS value should be lower than 116 kph, and the 4,000-meter value should be lower still. But instead, they go up. 

 

Which begs the question: what the heck is up with this gauge, and how am I meant to read it? 

 

 

 

 

 

Edited April 11, 2023 by bsones

[bsones]

I just ran the numbers through a calculator, and a true air speed of 116 kph would give you an IAS of ~103 kph at 2,000 meters and an IAS of ~92 kph at 4,000 meters. So I guess I just really do not understand what this gauge is showing me. 

[LukeFF]

The anemometer shows TAS at the given altitudes shown on the device. The HUD display shows IAS.

[bsones]

1 hour ago, LukeFF said:

The anemometer shows TAS at the given altitudes shown on the device. The HUD display shows IAS.

 

Good to know--thanks!

 

That gauge seems... not very useful for flying? I'd much rather know my IAS at all altitudes than my TAS at two specific altitudes. I guess you can guestimate what it is at other altitudes based on the gauge, but again, that just doesn't seem all that useful.