| 12:43p |
Landing Gear Stress
I have renewed faith in humanity. I sent a short letter about a question I had regarding landing gear and got back not only an excellent response but also a note saying that my letter would be sent to someone better able to address my question! Here are both my letter and the response in full...
bob.eckles@goodrich.com
Subject: Ruducing Stress on Landing Gear Tires
Bob-
I have not studied landing gear systems and so excuse any naivete in this question. Whenever I see a plane land I always see a puff of burning rubber come up from the wheels. This must be an extreme amount of stress the wheels are coming under, not just the vertical force of the plane's weight but also the rotational force as the wheels come up to speed. I would think that puff of smoke indicates quick wear in the tires.
Why aren't the wheels brought up to speed in the air before they touch down?
I would think the reduction in wear (which has a direct translation to dollar amount) would more than make up for the cost of putting a system in place to get those wheels spinning. While it may be tricky and too costly/unsafe to appropiate energy from the main engines to get the wheels up to speed, it would not take a powerful or complex motor to do this which could be placed in the landing gear house or just below the cargo bay.
Has this idea been explored before?
Thank you for your time. I am quite interested in the response.
-Zac Diener
Student of Structural Engineering
UC San Diego
RESPONSE
Zac,
Thank you for your question. Unfortunately, since I do not work for our Landing Gear division I don't have the design engineering answer to your question. I have forwarded your inquiry on to Landing Gear for their response however. With all of the development in aerospace over the past near century, I have to imagine the question has been explored. In response to your email there are several factors to consider with regard to bringing the a/c wheels up to speed prior to touchdown. I'll list some below in no particular order.
Physical space and weight limitations for the wheel drive motor. Airlines don't like to trade usable fuel for additional weight if not needed.
Different approach speeds depending upon landing weight require different tire rotation speed prior to touchdown.
The aircraft will decelerate upon touchdown and into the braking phase of aircraft rollout.
Depending upon a/c type, landing gear trucks have from two wheels (737) up to six wheels (777) that must receive power.
How does the drive motor affect the anti-skid system on the landing gear? The wheels would all have to rotate at the same speed upon touchdown. The anti-skid system senses different tire rotation speeds and responds accordingly in an effort to keep the airplane from skidding sideways down the runway.
What are the centrifugal forces imparted on the wheel axles of up to six wheels per landing gear truck rotating at roughly 140 mph each prior to touchdown (avg touchdown speed estimated at 130kts.)? What additional maintenance or inspection requirements are required as a result of such forces (physical wear on the landing gear truck assembly)? Keep in mind this adds to the operating cost of the airplane and the airplane will be out of service longer to perform a repetitive inspection for stress and metal fatigue on the landing gear than it would be for a tire change.
If the wheels are brought up to speed prior to landing, the power must be cut off upon touchdown so that the wheels do not become "drive wheels" in the event of a crosswind landing or the aircraft is in a slight yaw upon touchdown.
What happens if the drive motor fails to the "on" position and power is not immediately removed from the aircraft wheels upon touchdown? Remember, landing an airplane is considered a "critical phase of flight". It doesn't take long for a large commercial aircraft traveling at 130kts to depart the runway off to the side if upon touchdown there is a transverse component to its velocity vector. If the wheels become "drive wheels" at 130kts with a transverse velocity component, the landing would become ugly very quickly. There must be a failsafe system to prevent this from happening. Another system, more engineering, something else to maintain.
Airlines purchase tires from the tire OEM based upon volume pricing so the price per tire is significantly lower than a single tire purchase price. This is factored into the operating cost of the tire. Often times the tire OEM will offer performance based pricing to the airline. That is, the tire OEM will guarantee 100 landings (as an example) for each tire before it needs to be replaced. If the tire needs replacing prior to the 100th landing the airline gets credit on the tire. In the end, this is likely less expensive and much less risk involved than installing a motor to bring the wheels up to landing speed just prior to landing and subsequent deceleration upon rollout.
Who really bears the burden of the cost of the tire? The one who bought the ticket.
Best regards,
Bob
Bob Eckles
Engineering Project Manager
Aviation Technical Services, Inc.
Goodrich Corporation
Ph: 425.423.3162
Cell: 425.327.3857
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