The effect of density altitude on powerplant 
thrust depends much on the type of power- 
plant. An increase in altitude above standard 
sea level will bring an immediate decrease in 
power output for the unsupercharged or ground 
boosted reciprocating engine or the turbojet 
and turboprop engines. However, an increase 
in altitude above standard sea level will not 
cause a decrease in power output for the super- 
charged reciprocating engine until the altitude 
exceeds the critical altitude. For those power- 
plants which experience a decay in thrust with 
an increase in altitude, the effect on the net 
accelerating force and acceleration can be ap- 
proximated by assuming a direct variation 
with density. Actually, this assumed vari- 
ation would closely approximate the effect on 
airplanes with high thrust-to-weight ratios. 
This relationship would be as follows: 
a2 Fm P -=-=-En 
al Frill PO 
where 
ai, Fn, = acceleration and net accelerating 
force corresponding to sea level 
aa, Fn, = acceleration and net accelerating 
force corresponding to altitude 
~=altitude density ratio 
In order to evaluate the effect of these items on 
takeoff distance, the following relationships 
are used : 
if an increase in altitude does not alter ac- 
celeration, the principal effect would be 
due to the greater TAS 
;=(g,yxe) 
where 
f2 1 -=- 
$1 (T 
Si=standard sea level takeoff distance 
St= takeoff distance at altitude 
o-altitude density ratio 
if an increase in altitude reduces accelera- 
tion in addition to the increase in TAS, the 
NAVWEPS 00-805-80 
AIRPLANE PERFORMANCE 
combined effects would be approximated 
for the case of the airplane with high in- 
trinsic acceleration by the following: 
g=(gyx(~) 
g=(i)x(;) 
s2 12 -= - 
0 J-1 a 
where 
S,= standard sea level takeoff distance 
Ja= takeoff distance at altitude 
o=altitude density ratio 
As a result of these relationships, it should. 
be appreciated that density altitude will affect 
takeoff performance in a fashion depending 
much on the powerplant type. The effect of 
density altitude on takeoff distance can be 
appreciated by the following comparison: 
sealevel.... 
I.cmft..... 
Z,cmfC..... 
,,mfi..... 
4.@JJfc..... 
5.Ccnft..... 
6.-xafC..... 
-- 
1 
1 
I 
I 
1 
I 
- 
..om 
.0?.98 
..c605 
L. wls 
L. 126 
L. 1605 
1.1965 
L.cca 
L.oa5 
1.125 
1.191 
1.264 
1.347 
1.431 
- 
P 
-- 
- 
drirude 
-- 
0 0 
2.98 6.05 
6.05 12.5 
9.28 19.5 
12.6 26.4 
16.05 34.7 
19.65 0.1 
0 
9.8 
19.9 
30.1 
40.6 
52.3 
65.8 
- 
From the previous table, some approximate 
rules of thumb may be derived to illustrated 
the differences between the various airplane 
types. A 1,ooo-ft. increase in density altitude 
191