Welcome back to Helicopter Lessons in 10 Minutes or Less!

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This video's topic covers part 2 of the Lift Equation. If you missed the first part I'd recommend watching that first. Here's the link (   • The Lift Equation  Part 1  ).

The next element in the Lift Equation deals with air density using 1/2p (Greek symbol for "rho"). At this point I note that math buffs may see the "1/2" and say this doesn't necessarily have to be here in the formula and could technically be put anywhere else in the equation while still making sense. You're right. But I'll get in to it later about why this is normally found at this point in the equation. Back to the topic, Rho covers density. Density is the measure of the thickness or viscosity of a substance. For Helicopters specifically, air density affects how easily the rotor can move the air molecules around it. For helicopters, more air density = better performance. Denser air is simply easier to move. The blades grip and push the air better. Let's cover what makes air denser.
1. Atmospheric Pressure: higher pressure means there are more molecules in 1 given area. This fluctuates day by day and region by region. Higher pressure increases air density and increases performance.
2. Altitude: as altitude increases, the air is thinner because molecules are spaced farther apart. Lower altitude increases air density.
3. Temperature: warm air expands and moves farther apart which decreases air density. Low temperatures increase air density.
4. Moisture: water vapor weighs less than dry air and displaces it. As moisture increases air density decreases. Lower moisture content increases density.
Summary: High Pressure, Low Altitude, Low Temperature, and Low Moisture increase air density and therefore improve performance.

The last element in the equation is by far from most important. This part is Velocity Squared. This is the relative velocity of the air over the airfoil. As I mentioned in other videos this can affect lift more drastically than any other variable because it affects exponentially. If speed over the airfoil doubles, lift quadruples. If speed is halved, lift is quartered. This loss of air velocity could even affect the Coefficient of Lift by causing a stall condition due to the Angle of Attack in the blade now exceeding the critical angle. The loss of air velocity could cause a reduction in surface area as well if rotor RPM is allowed to slow enough for coning to occur.

The last part I wanted to address is that the part of the formula 1/2p X V squared" is actually the measure of Dynamic Pressure. This is the same formula used to calculate airspeed in your pitot tubes. Some references even cite the Lift Equation as using Coefficient of Lift times Surface Area times Dynamic Pressure.

The biggest takeaways from this formula is that each variable can increase or decrease lift and that Velocity Squared has the greatest impact. That wraps up part 2 of 2 if the Lift Equation. Thanks for watching! Don't forget to hit like and subscribe below. As always, safe flying!


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If you're just getting started and want more information, pictures, and more explanations, I'd recommend reading the Rotorcraft Flying Handbook http://amzn.to/2ifPlnZ

If you've already got a basic understanding, and want to further your professional helicopter education with advanced helicopter concepts, I'd recommend reading Cyclic and Collective, by Shawn Coyle http://amzn.to/2ifQGLx