Barnhart wrote:

Going thru maps tonite.  Turbonetics have maps online now.

A 20bt at 300hp will fuel burn approximately 20 gallons/hour. This would

be 20 x 6 = 120 lbs

fuel/hour and 1440 lbs of air/hour. Total 1560 lbs mass air flow/hour.

26lbs/min.

At 200hp fuel burn is about 10 gallons/hr.  780 lbs/hr mass air flow or

13 lbs/min mass airflow.

Pressure ratio is 14.3 (ambient pressure) + 10 (lbs boost) divided by

14.3 =  1.69  at 1000 ft.

How is the ambient pressure determined for higher altitudes?

Barny
MGDQ

Barny,

The thumb rule is 1" of Mercury (Hg) drop per 1,000 of altitude.  Since
standard atmospheric pressure (14.7 psia at sea level; use 15 psia for a
good approximation) is 29.92 inches of Hg (use 30 inches for a really good
approximation), that's 1/2 psi drop for every 1,000 feet of altitude.  Don't
know how well those thumb rules work up in the flight levels, but they work
real well for anything below class A.

So to get the same power you have at sea level running N/A:

    10,000 ft: 14.7 / (14.7 - 5) = 14.7 / 9.7 = 1.51
    20,000 ft: 14.7 / (14.7 - 10) = 14.7 / 4.7 = 3.1

But... you and I have been talking about a 300 Hp takeoff capability.  I'm
guessing that a street ported 13b run normally aspirated is good for ~160 Hp
(80 Hp/rotor).  If so, then the 20b is good for ~240 Hp.

That's a ratio of 300 / 240 = 5 / 4 = 1.25 right out of the gate at sea
level.
MAP = 1.25 x 14.7 = 17.15 psia for the desired full power.
5,000 ft:  17.15 psia / (14.7 psia -2.5 psia) = 1.4
10,000 ft: 17.15 psia / (14.7 psia - 5 psia) = 1.77
15,000 ft:  17.15 psia / (14.7 psia - 7.5 psia) = 2.38
20,000 ft: 17.15 psia / (14.7 psia - 10 psia) = 3.65

I doubt we can span this broad a range with a single turbo.  Staging would
work, as would an oversized turbo with a honkin' wastegate.  But who needs
all that power (and weight and complexity) at 20,000 ft?  I'm thinking
10,000 ft is the maximum that I would like full rated power climb
capability, even when mountain flying.

Please let me know what you find for a compressor map.  I would like to use
a similar scheme (though I may shoot for a top-end of ~350 Hp).

Tom Kendall

 
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