MolaKule
Staff member
What is an Empennage structure, and what does it support? An Empennage structure is also called a tail or tail assembly.
MolaKule Q&A on Aircraft Structures IV
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How about going into the technical strengths and weaknesses of the very common monocoque and semi-monocogue designs. You had started this series with some discussion on stringers, longerons and frame questions so discussing the stress dispersal and rivet designs would be interesting. Also, there is a distinct difference between longerons and stringers but it does seem that the two terms are used interchangeably and incorrectly among those who don't skin aircraft of a regular basis.
Isn't that the rear part of the fuselage that the horizontal and vertical stabilizers are attached to?
MolaKule
Staff member
We have to start with the basics before we can discuss the minute. And since the interest in aircraft is almost as popular as thick versus thin oils, we must discuss basic components before discussing details.
That falls under Stress and Fatigue Analysis and I have a thrown a bit of that into the discussions. Connecting and Bonding of various components are done with rivets, bolts, and chemical bonding.
According to Michael C Niu and other authors, stringers and longerons can be used interchangeably because they perform the function of distributing loads over spans.
Michael C Niu, Airframe Structural Design.
MolaKule
Staff member
Yes, right on.
It has vertical and horizontal stabilizer surfaces and rudder and elevator control surfaces, and may also support the APU.
Construction-wise, the empennage has many closely spaced frames and longerons (stringers) because it acts as one end of a long lever--so the loading forces are rather high.
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Cool. Remembered the term from my former neighbor across the street. He was a former US Navy Aviator and taught aeronautics at the local community college. He would take me up about once a month in the college's Cessna 172 when he needed to get some hours.
MolaKule
Staff member
A resource on empennage design:
https://www.fzt.haw-hamburg.de/pers/Scholz/HOOU/AircraftDesign_9_EmpennageGeneralDesign.pdf
https://www.fzt.haw-hamburg.de/pers/Scholz/HOOU/AircraftDesign_9_EmpennageGeneralDesign.pdf
A tragic crash from 10 years ago, where the empennage failed. I'd say the lack of stringers and frames played a major part in the failure of this design. We spoke about damage tolerance and fail safe design in the other thread. Here is a clear example of why we try not to deviate from good design practices.
In this case the empennage was a single piece of composite, devoid of any framework. As basic carbon composites tend to do, when they yield, they do so suddenly and completely. There was nothing in this design to stop crack progression.
In this case the empennage was a single piece of composite, devoid of any framework. As basic carbon composites tend to do, when they yield, they do so suddenly and completely. There was nothing in this design to stop crack progression.
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MolaKule
Staff member
Or, the empennage/fuselage "carry-through" connecting structure failed. A designer has to concern himself with carry-through loads.
At Boeing, we had designers that had courses in Stress and Fatigue, but we also had a separate group that did the final Stress and Fatigue Analysis for good reason. I worked mostly in the Stress and Fatigue Analysis group.
MolaKule
Staff member
On an aircraft with a wing mounted at the top of the fuselage, why is an elevated "T" tail horizontal stabilizer used?
They don't always or Cessna didn't know that they should.
Where a high stab is used, I would guess that it's to avoid not having adequate clean air flowing past it under certain conditions, which could result in a loss of elevator authority, which could be really bad if the crew couldn't command the nose down.
MolaKule
Staff member
"...clean air flowing past it under certain conditions, which could result in a loss of elevator authority,"
If a high-wing aircraft has unobstructed airflow to a lower-mounted empennage and horizontal stabilizer, then you will have sufficient pitch control. Also recall that the usual Cessna has an in-line mounted engine proving centerline thrust.
A horizontal stabilizer, when mounted on top of the fin, creates a “T” shape when viewed from the front. T-tails keep the stabilizers out of engine wakes and wing-tip vortices and give better pitch control. They have good glide ratios and are more efficient on low-speed aircraft.
The British Aerospace BAe 146 is one such aircraft with a T-tail.
MolaKule
Staff member
Name some advantanges and disadvantages of empennage-mounted engines:
For example: Advantage--empennage-mounted engines are closer to the centerline than wing-mounted engines.
For example: Advantage--empennage-mounted engines are closer to the centerline than wing-mounted engines.
A notable disadvantage has been that debris from an engine failure has either taken out another engine or severed control hydraulics.
Advantage: minimized chances of ground FOD entering inlets
Disadvantage: ask anyone sitting in the back row
From what I've read, the MD-90 had exactly that problem with the main gear reliably directing FOD into the engines, resulting in relatively low hours in service of an engine that only one shop in the world could overhaul.
MolaKule
Staff member
Another obvious structural disadvantage of empennage-mounted engines is that the empennage has to be made stronger with more stringers and frames, increasing weight.
Additionally, prudent routing of hydraulics and electrical systems is required in areas parallel to the fan and turbine disk areas of the engine.
However, fan case and turbine disk containment shrouds have been improved over the years by using alternating layers of Kevlar composites and titanium sheet.
https://lsdyna.ansys.com/wp-content...et-engine-fan-blade-containment-using-two.pdf
https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_33-5.pdf
MolaKule
Staff member
Some info on FOD deflector shields:
https://aviation.stackexchange.com/...-the-dc-9-80-md-80s-nose-gear-kept-from-strik
On most business jets, the aft mounted engines keep the cabin very quiet. Yes they can be heard on takeoff and climbout. a little, but in cruise at M0.90, engine intake/fan noise simply does not make it to the cabin.
