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DGoncz

USA
2 Posts

Posted - 04/11/2018 :  05:04:42  Show Profile  Visit DGoncz's Homepage  Reply with Quote
From the Glider Handbook, FAA:

"
Every pilot must understand the terms “high density altitude”
and “low density altitude.”

In general, high density altitude
refers to thin air, while low density altitude refers to dense
air.

Those conditions that result in a high density altitude
(thin air) are high elevations, low atmospheric pressure,
high temperatures, high humidity, or some combination
thereof.

Lower elevations, high atmospheric pressure, low
temperatures, and low humidity are more indicative of low
density altitude (dense air).

However, high density altitudes
may be present at lower elevations on hot days, so it is
important to calculate the density altitude and determine
performance before a flight.
"

https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/glider_handbook/media/gfh_ch05.pdf

This relates to the ground HPV record attempts because the drag on a(n) HPV is (ideally) "rho" time "Cd" times "A" where rho is density in kg/m^3 for ISO units, Cd is a unitless Coefficient of drag, and Area is in ... uh, would it m^2 for ISO units? Yes, I think that is right.

So....HPV record attempt are usually run on high desert land, far as I know. Now this is hot, dry air. Please reread the above.

"However, high density altitudes
may be present at lower elevations on hot days,"

I'm not suggesting that we run HPV record attempts in Florida!

I'm suggesting that either:

1) records be kept of the humidity, pressure, and temperature for each record set; or,
2) all record nomiations be "derated" (this takes a lot of work) by the density alitude, given onsite measurements of humidity, pressure, and temperatuer (that does not take much work).

I have searched and searched and the Galaxy Note 3 is one of the most appropriate platforms for this purpose. I collect them.

It's fascinating. The Wikipedia, true to form, has a good introduction:

"
The addition of water vapor to air (making the air humid) reduces the density of the air, which may at first appear counter-intuitive.
"
https://en.wikipedia.org/wiki/Density_of_air


My own comments:

Moist air reduces engine performance by diplacing oxygen. Most air reduces glider performance by reducing lift. Most air increases ground HPV performance by reducing drag, ignoring correlations to temperature and pressure, which do exist, because weather is a reality.

Cheers,

Doug



Douglas "Dana" Goncz

DGoncz

USA
2 Posts

Posted - 04/11/2018 :  11:51:05  Show Profile  Visit DGoncz's Homepage  Reply with Quote
To follow up:
Here's a relevant calculator for "density altitude" that also gives density:
https://wahiduddin.net/calc/calc_da_rh.htm
Cheers,
Doug

Douglas "Dana" Goncz
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Don S

USA
83 Posts

Posted - 04/11/2018 :  22:38:43  Show Profile  Reply with Quote
Thanks Doug! Your post is appreciated for the referenced data and most of your observations are correct. The IHPVA does in fact recognize the effect of altitude concerning the human powered vehicle records and maintains both high altitude and low altitude records. If a low altitude speed record exceeded those set at Nevada's high altitude course it would become the new World Record.

The fact that the the current World Human Powered Speed Challenges are held at high altitude has a lot to do with the availability of an ideal course (over 5 miles of a straight, nearly flat, paved surface) on State Route 305 that the State of Nevada has allowed the IHPVA to use for the event on a repeating annual schedule and the fact that all the low altitude records were beaten on this course.

The WHPSC in Battle Mtn, NV doesn't always get ideal weather conditions during the event and some years never get the optimum
high altitude weather conditions you identified. It is not unusual for the event schedule to be interrupted due to adverse weather conditions but an ideal low altitude course (long, straight, and flat) with long range predictable ideal low altitude weather conditions hasn't been identified.

Don

"it's important to understand what makes them fast. It's more important to understand what keeps them from going faster." DS
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