Aerobatic Figures
by Dr. Guenther Eichhorn
Barrel Rolls
The Barrel roll is a combination between a loop and a roll. You complete one loop while completing one roll at the same time. The flight path during a barrel roll has the shape of a horizontal cork screw. Imagine a big barrel, with the airplanes wheels rolling along the inside of the barrel in a cork screw path. During a barrel roll, the pilot always experiences positive Gs. The maximum during the maneuver is about 2.5 to 3 G. The minimum about 0.5 G.
Can I do acrobatics in a non-Aerobat C-152?
Sure, you *can*. HOWEVER: If the maneuvers are not listed as approved in the Pilot's Operating Handbook, you have done something illegal (and quite stupid to boot, since it would be quite dangerous).
The changes made to the C-150 & C-152 are not very visible, but are extensive. Cessna was worried about people doing aerobatics in the non-aerobatic version, so the original paint jobs given the aerobatic versions were quite distinctive.
What might confuse things a bit for the uninformed is the appearance of a "normal" airplane doing aerobatics at a local airshow. An experienced acro pilot should be able to put on a decent aerobatic show in many of the "standard" category aircraft. There are a couple of problems with this, though. THERE IS NO ROOM FOR ERROR! Aerobatic category ships have a strength reserve for the maneuvers for which they are approved. For example, even a low power/weight craft should be able to do a nice loop and not exceed +3 G's. A normal category plane is certificated to +3.8, aerobatic to +6. A Bob Hoover can do it safely, you can't. The other problem is, if you do maneuvers outside what's listed in the POH, you now have an experimental airplane. Hoover's Shrike and other "normal" category airplanes on the airshow circuit, have been re-certificated under Experimental-Airshow/Exhibition.
How an Aircraft is Certified.
Limiting loads and ultimate loads
To receive “type approval certification” the design of a general aviation aircraft must conform with certain standards, among which are the in-flight “manoeuvring loads” plus the “turbulence induced loads” which the structure must be able to sustain. The turbulence loads are called the gust induced loads. FAR part 23 is the recognised world standard for light aircraft certification and the following is an extract: "... limit loads are the maximum loads to be expected in service i.e. the highest load expected in normal operations and ultimate loads are limit loads multiplied by a safety factor of 1.5. The structure must be able to support limit loads without detrimental, permanent deformation. At any load up to limit loads, the deformation may not interfere with safe operation. The structure must be able to support ultimate loads without failure for at least three seconds ..."
The minimum positive limit load factor which an aircraft in the 'normal' operational category (at maximum take-off weight) must be designed to withstand is 3.8g. For a non-aerobatic aircraft the negative limit load factor is 0.4 times the positive limit which makes it 1.5g for the normal category. The ultimate loads for the normal category are +5.7g and -2.25g.
For aircraft with aerobatic capability the negative limit load factor must be 0.5 times the positive value. The 'utility' category (which includes training aircraft with spin certification) limit loads are +4.4g and -2.2g while the 'acrobatic' category limit loads are +6.0g and -3.0g.
The manufacturer of a particular aircraft type may opt to have the aircraft certificated within more than one category in which case there will be different maximum take-off weights and centre of gravity limitations for each operational category.
It should be borne in mind that the sustainable load factors only relate to a new factory built aircraft. The repairs, ageing and poor maintenance, to which it has been exposed since leaving the factory, may decrease the strength of individual structural members considerably.
The point I’m trying to make here is:
DON’T DO AEROBATICS IN NON AEROBATIC AIRCRAFT.