# Drag Force Ratio: Jet vs Transport at 1200 km/h, 15 km Altitude

• Arman777
In summary, the drag on a jet flying at 1200 km/h at an attitude of 15 km is 1.79 times the drag on a plane flying at half that speed and at the same altitude with the same cross-sectional area.
Arman777
Gold Member

## Homework Statement

Calculate the ratio of the drag force on a jet flying at ##1200 km/h## at an atttude of ##15 km## to the drag force on a prop-driven transport flying at half that speed and altitude.The density of air is ##0.38(\frac {kg} {m^3})## at ##10 km## and ##0.67(\frac {kg} {m^3})## at ##5 km##.Assume that the air-planes have the same effective cross-sectonal area and drag coefficient C

## Homework Equations

##F_D=\frac 1 2CAρv^2##

## The Attempt at a Solution

Here C and A are the same so ## F_D## jet will be ##F_1## and ##F_D## plane will be ##F_2##
##F_1=ρ_1(v_1)^2## and ##F_2=ρ_2(v_2)^2##

The question ask ratio so we don't care about units ( I guess )

##ρ_1## which at 15 km is I found approximately ##0.235 (\frac {kg} {m^3})## ( from Inverse ratio )
##ρ_2## which at 7.5 km is I found ##0.525 (\frac {kg} {m^3})##

So ##\frac {F_1} {F_2}=4 \frac {0.235} {0.525}=1.79##

I don't know where I went wrong

Thank you

I don't see how you would get 3.3.

You have some rounding errors, apart from that I get the same result assuming the atmosphere has an exponential distribution in density. I guess you are supposed to make this assumption.

Arman777
mfb said:
I don't see how you would get 3.3.

You have some rounding errors, apart from that I get the same result assuming the atmosphere has an exponential distribution in density. I guess you are supposed to make this assumption.
Yeah...I don't know too.This is all writes in the question.

Thanks

You are given the densities at 10km and 5km, but asked about drag at 15km and 7.5km. Smells like a question that has been altered to different data, but not consistently.

haruspex said:
You are given the densities at 10km and 5km, but asked about drag at 15km and 7.5km. Smells like a question that has been altered to different data, but not consistently.

I know...So I think I just let it go :)

haruspex said:
You are given the densities at 10km and 5km, but asked about drag at 15km and 7.5km. Smells like a question that has been altered to different data, but not consistently.
It wouldn't fit to those values either, but of they might have altered even more values.

mfb said:
It wouldn't fit to those values either, but of they might have altered even more values.

What an interesting question...

## 1. What is drag force ratio?

The drag force ratio is a measure of the difference in drag forces between two objects moving at the same velocity and altitude. It is typically expressed as a ratio, with the higher value representing the greater drag force.

## 2. How is drag force ratio calculated?

The drag force ratio is calculated by dividing the drag force experienced by one object by the drag force experienced by the other object. In this case, it would be the drag force experienced by a jet compared to the drag force experienced by a transport aircraft at 1200 km/h and 15 km altitude.

## 3. What is the significance of studying drag force ratio between a jet and transport aircraft?

The drag force ratio between a jet and transport aircraft is important because it can impact the efficiency and performance of both types of aircraft. A higher drag force ratio means that one aircraft will experience more drag than the other, potentially leading to differences in fuel consumption and speed.

## 4. What factors affect the drag force ratio between a jet and transport aircraft?

The drag force ratio between a jet and transport aircraft can be affected by a variety of factors, including the shape and size of the aircraft, the air density at the given altitude, and the speed and direction of the wind. Other factors, such as the weight and aerodynamics of the aircraft, can also play a role.

## 5. How does the drag force ratio change at different altitudes and velocities?

The drag force ratio between a jet and transport aircraft can change at different altitudes and velocities due to the varying air density and wind conditions. Generally, as altitude and velocity increase, the drag force ratio will also increase. However, this can also depend on the specific design and characteristics of the aircraft.