No, This Was Not a Prop Strike. The Benefits of These Props Are Huge!
The old story is that an FAA inspector walked up to the pilot of a Beechcraft Bonanza and asked him how he struck the smoothly curved propeller of his aircraft. When the pilot replied that his propeller was simply designed with curved tips, the FAA inspector immediately invalidated his airworthiness certificate. Unfortunately for the inspector, the pilot was telling the truth.
Fast forward to 2015, I am a newly minted CFI sitting on the frozen FBO ramp of KEWB (New Bedford Regional Airport,) waiting for a pickup ride from a friend. A twin-engine Beechcraft Bonanza with the same style of curved propeller tips strikes my eye. Not realizing that I am verbalizing my own thoughts, the words “how in the world did the pilot do that?” slip out of my mouth in front of the maintenance staff of the nearby FBO, of course. Just like out of a “Chicken Wings” comic, the maintenance staff made sure that I left that ramp feeling a little less confident in myself.
I couldn’t stop thinking about that Bonanza, I was intrigued. What was this type of propeller, and what was its deal? I mean, people don’t want to always be asked how they bent their props tips, right?
Of course, the first place I went to was the National Aeronautics and Space Administrations archives, and by golly did I find very few articles but so much information. What I found are that these propellers are referred as the “Q-Tip” type, as well as “P-Tip,” depending on the angle of the tip bend and designer. They have impressive advantages for their users, but before I can explain what those advantages are, let’s have a simple propeller aerodynamics review.
Let’s Remember That a Propeller Is an Airfoil;
A propeller has a leading edge, a trailing edge, a chord line, and camber. When it rotates, the forward movement of the aircraft and the rotational velocity of the spinning propeller gives us the opposite moving “relative wind.” Since the propeller has a chord line, and a relative wind, it has an angle of attack. Additionally, since the propeller has camber, one can make the relation that a propeller produces lift similar to an aircrafts wing. It sure does! This lift is termed “thrust.”
Now What Do We Remember About Airfoils That Produce Lift?
They also produce drag, more specifically for this article induced drag. The high pressure underneath an airfoil always tends to travel towards the low pressure of the top portion of the airfoil. How does it do this? Via the least path of resistance of course; the wing tips. This generates “wing tip vortices,” and when coupled with the forward movement of aircraft creates swirling vortices.
Now the propeller operates under the same principles as the wing, but it is worth mentioning that it is also rotating. As the propeller has air moving about the top and the bottom of it, it is creating thrust (lift.) Since the bottom part of the propeller is the high pressure station (think little camber) and the top part is the low pressure station (think larger camber) the high pressure air will spill over it’s path of least resistance; the propeller tips.
This creates a phenomena known as “propeller vortices.” Just like wingtip vortices increase the overall drag of the aircraft, propeller vortices increase the overall drag of the aircraft. More drag = less efficiency, less thrust, greater fuel burn and decreased performance.
So How Does a Q-Tip Propeller Alleviate Propeller Vortices?
The bends in the propeller tips hold the key. Think of what a winglet does for wings, increases interference drag and disbands the tendency of the high pressure area below the wing to bleed over to the low pressure area of the wing.
The angled bends in the Q-Tip propeller does the same thing, increases interference drag and disbands vortices. The greater the bend angle, to a point, the greater the reduction of this propeller drag. The reduction in induced drag far exceeds the increase in interference drag, which is parasite drag. The benefits of this are increased thrust due to less drag, greater performance due to more increased thrust and decreased drag, and reduced fuel burn as a result of the what was previously mentioned.
What Else?
Another added benefit of Q-Tip propellers was a decrease in propeller noise. While often overlooked, propeller tip vortices are the main culprit for propeller noise. By eliminating propeller tip vortices, you will decrease the overall noise of the aircraft.
Additionally, the bends of the Q-Tip propellers increase ground clearance and decrease the chances of a propeller striking foreign objects. AOPA has a few articles on this, which are interesting reads, but I find the alleviation of tip vortices more impressive.
So, Why Don’t More Owners Install These Q-Tip Props?
As you can imagine, installing a new propeller, especially a not commonly used design, is expensive. Not to mention that the overhaul and inspection costs are going to be more expensive than staying with original stock equipment that came with the aircraft. Aircraft owners did not see the added benefit outweighing the costs, so the “Q-Tip” propeller faded into obscurity.
In summary.
Q-tip propellers gave owners a huge advantage in overcoming drag, increasing aircraft performance and giving people something to turn their heads at. Even though the high cost of installing and maintaining these Q-Tip propellers lead to their demise, if produced more economically, I can see them making a comeback in the near future.
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