Question 1

How much thrust does an aircraft need to fly?

Accepted Answer

In flight, there are four opposing forces acting on an aircraft—lift, drag, weight, and thrust. In straight and level flight, these forces are balanced:
 
Thrust = Drag
     Lift = Weight
 
If we know the lift-to-drag ratio (L/D) of the aircraft, we also know the thrust-to-weight ratio.
 
Divide the weight by the L/D to determine the minimum thrust required for level flight at best L/D speed:
 
(W/T) = (L/D)
      T = W / (L/D)
 
Here is an example to calculate the minimum thrust required for level flight at best L/D speed for the BD-5.
 
Here are the parameters of the BD-5,

W = 850 lbs
L/D = 15
 
where W = takeoff weight.

Thrust required for level flight at best L/D speed:
 
T = W / (L/D)
   = 850 / 15
   = 56.7 lbs
 
Don't forget to include the extra drag of engine(s) in your calculations.

Most jet engines cannot maintain maximum thrust for extended periods. Their continuous thrust rating will be quite a bit lower than maximum. For example, the continuous thrust of the TJ-100 engine is about 70% of maximum.

Question 2

What is the rate of climb of an aircraft?

Accepted Answer

To determine an approximated rate of climb (ROC), first subtract the thrust required for level flight from the engine's maximum thrust rating. The result is the allowable thrust that is used towards climb. The ratio of climb speed to forward speed is equal to the ratio of available thrust to weight. In this case, the forward speed is the best L/D speed.
 
Continuing the example for the BD-5 from above, here are additional parameters needed to calculate the rate of climb.

T_tot  =  247 lbs
T_min = 56.7 lbs
V_g    = 120 knots
 
where T_tot is the total thrust of the engine, T_min is the thrust required for level flight, and V_g is the best L/D speed.

First calculate the thrust available:
T_av = T_tot - T_min
        = 247 - 56.7
        = 190.3 lbs
 
Now, calculate the thrust to weight ratio using the available thrust:
T_av / W = 190.3 / 850
               = 0.224
 
Finally, calculate the rate of climb at best L/D speed:
ROC / V_g = T_av / W
         ROC = (T_av / W) x V_g
                  = 0.224 x 120
                  = 26.85 kts
                  = 2719 ft / min

Question 3

What is the maximum speed of an aircraft?

Accepted Answer

The aircraft's maximum speed is the speed where the aircraft drag equals the engine's maximum thrust.

Question 4

What is the most efficient loiter speed?

Accepted Answer

The most efficient loiter speed is the max L/D speed.

Question 5

What is the most efficient cruise speed of an aircraft?

Accepted Answer

The most efficient cruise speed is usually the speed where the L/D is 86% of max L/D.

Desert Aerospace LLC

Jet Equations FAQs

How much thrust does an aircraft need to fly?

What is the rate of climb of an aircraft?

What is the maximum speed of an aircraft?

What is the most efficient loiter speed?

What is the most efficient cruise speed of an aircraft?