Aerobatics Down Low

 

Most recreational pilots perform aerobatics up at a nice, safe altitude.In the USA, the minimum altitude is 1500 AGL.Below that you need a waiver.In Canada for some odd reason, itís 2000 AGL.Below that, you need a Special Flight Operations Certificate (SFOC).

 

Some years back, I actually made an application to CARAC to change the 2000 foot minimum in Canada to 1500, to be the same as the USA.Transport told me in so many words to bugger off.Oh well, I tried.

 

Anyways, for most pilots, itís a very good idea to do aerobatics with lots of altitude because they often fall out of maneuvers Ė Iíve seen it happen many times at aerobatic contests Ė and because they have poor control of their exit altitude.

 

If youíre going to fly aerobatics down low, itís crucial that your exit altitude of ANY maneuver that you perform is the same as your entry altitude, for obvious reasons.

 

But watch any military pilot do a roll on TV, and you will cringe Ė they all lose altitude during the roll.Douglas Bader, one of the finest Spitfire pilots in World War II, lost his legs doing (guess what) a roll down low that he lost altitude during, and crashed.Sigh.

 

One of the best aerobatic pilots that ever lived, Bob Hoover, used to practice aerobatics on top of a stratus cloud deck, so that he could practice controlling his exit altitude.

 

The bald truth is that very few aerobatic pilots have the experience and skill to safely perform aerobatics at low altitude.Please donít do it without some expert training, because a small mistake on your part will kill you, and thatís a high price to pay for a thrill today, that you will have forgotten next week.

 

A very experienced aerobatic pilot recommends that before you fly any aerobatic maneuver down low, to fly it 10 times at 2000 feet, and if you make a mistake, start over again.Once you can fly it perfectly 10 times at 2000 feet, then do it 10 times perfectly at 1000 feet.Any mistake, start over at 2000 feet.Once you have done it perfectly 10 times at 2000 feet, and 10 times at 1000 feet, then do it 10 times at 500 feet.Etc.

 

Thatís very good advice.At the very least, it should develop considerable proficiency on the part of the aerobatic pilot, which is an essential prerequisite to flying aerobatics down low.

 

However, thereís a little bit more to it than just that.Some maneuvers are inherently more dangerous than others.I am not considering out-of-control maneuvers such as tumbles (see below) but maneuvers which do not involve exceeding the stalling angle of attack of the wing.

 

 

Non-Stalling Aerobatic Maneuvers

 

Probably the safest aerobatic maneuver down low, is the aileron (aka ballistic) roll.

 

There are many different kinds of rolls, but the aileron roll, viewed from above, draws a straight line over the ground.It involves pitching up with around +2G, neutralizing the stick, applying full aileron and touch of rudder, and while inverted the G should remain slightly positive for a variety of reasons.The upright exit is generally on the same down angle as the entry, and around +2G should be pulled back to level flight at the entry altitude.

 

Note the extremely light, continually positive G.Also note that dramatic pitch attitudes were not involved, though for aircraft for very slow roll rates you will have to get the nose up substantially, and that means you will need lots of airspeed at entry because youíre going to convert it to altitude.Hopefully you already know this sort of basic stuff, otherwise you really shouldnít be considering doing aerobatics down low.

We can objectively rate the safety of any aerobatic maneuver by measuring the maximum angle that it subtends the horizon.The aileron roll only requires perhaps 30 degrees nose down, and is thus the safest aerobatic maneuver there is.However downward looping maneuvers will involve vertical downlines (eg 90 degrees below the horizon) and those are the maneuvers that are more likely to kill you.

 

Amongst aerobatic pilots, the reverse Ĺ Cuban-eight has a fearsome reputation when entered at the surface.Many highly experienced contest aerobatic pilots believe it should never be attempted because it is so dangerous.

 

However, aerobatics is merely applied physics, so if you learn a little physics and understand what it is that you are doing, you wonít kill yourself.I do reverse Ĺ Cuban eights from the surface all the time - they are easy and safe, if you do a couple of things right.

 

If you want to stay alive doing downward-looping aerobatic maneuvers down low, you need to understand and apply the TOP GATE.The top gate is a simple set of parameters, and if you donít meet them, you discontinue the maneuver and donít die.Simple.

 

For example, consider the fearsome Ĺ reverse Cuban-eight.Or even a vanilla inside loop.Both are going to take you to an inverted position, before you pull through, and if you donít make your top gate, simply drop the nose a bit and Ĺ roll upright out of it, which you will note involves a much smaller angle below the horizon.

There are two parameters for a top gate: altitude and airspeed.Altitude is the most important, and if you donít understand why, read my physics memo on vertical downline recovery.

 

In the Pitts S-2B, my top gate for a downward-looping maneuver is 1000 AGL at sea level density altitude.If I donít see 1000 on the altitmeter Ė I zero it before takeoff Ė I donít pull through.The altitude parameter of your top gate determines whether you live or die.The airspeed parameter determines how much G youíre going to pull.If your airspeed is too high Ė I like to get the airspeed down to 80 mph before I pull through Ė just keep on trucking upwards, converting airspeed to altitude.Having 1300 on the altimeter makes me very happy at the top gate, because energy is always a good thing to have.

 

So to summarize, my top gate for the Pitts S-2B is 1000 AGL (min) and 80 mph (max).Note that these numbers are a function of the density altitude Ė which affects your TRUE AIRSPEED Ė and the stall speed of the aircraft.The higher the stall speed, the higher the top gate.

 

Note that I mentioned above, setting the altimeter to zero before takeoff.I donít know any airshow pilot that doesnít do this.There was an F-16 that had a very bad day out in Colorado a few years ago.Because the altitude of the airport was over 3000, he couldnít

zero the altimeter, and instead he was going to do a little mental arithmetic for his top gate.Guess what happened.He did the arithmetic wrong, pulled through 1000 feet below his top gate, and crashed spectacularly.There is awesome video and photos all over the internet of his ejection.

 

There are many lessons to learn from this, but if nothing else, it should teach you the importance of the top gate.If you donít make the minimum altitude of your top gate before you pull (or push) through, you are going to die because you probably donít have an ejection seat like the F-16 does.

 

Once you get religion about the top gate, the next thing you need to learn about is the entry (or bottom gate).For example, in my Pitts S-2B, I know that if I have 100 mph at the surface, I donít have the energy to pull into an inside loop, so why even bother trying?

 

This is crucial for formation aerobatics.Solo aerobatics down low is really easy, in comparison Ė you can bail out of maneuvers and thrash all over the box and the crowd will be thrilled.But the lead pilot of a formation aerobatic team must have an entry gate Ė again, of minimum altitude and minimum airspeed Ė which tells him that he has enough energy for the formation to complete the maneuver.If the entry gate is not met, a wingover is performed to regain energy, and then the sequence is continued.

 

If you learn about (and apply) entry and top gates to your aerobatic maneuvers down low, you can safely perform non-stalling (eg ďin controlĒ) aerobatic maneuvers.

 

 

Stalled Aerobatic Maneuvers

 

Now letís look at the other category of aerobatic maneuvers, which I refer to as ďout of controlĒ maneuvers because they involve exceeding the stalling angle of attack of the wing.The prime example of this is the tumble.Tumbles should NOT be attempted down low until you have considerable experience and expertise in that particular type of aircraft, otherwise you are rolling the dice with your life.

 

When I perform a tumble, at least 90% of the time I exit the tumble in an inverted spin at or below 1000 feet.I actually prefer inverted spins to upright spins at low altitude, because the rudder is in clean air, and I can stop the yaw instantly.

 

Until you are completely comfortable recovering from any spin, you should NOT fly out of control maneuvers down low, because you are betting your life that you will be lucky.

 

A common error Ė which has killed many airshow pilots Ė is that they exit the spin, but start doing (often outside) snap rolls on the vertical downline.This is not good, and death generally results because they donít recover in time to avoid hitting the ground.You simply wouldnít believe the supposedly ďexpertĒ aerobatic pilots that donít understand the difference between a spin and a snap roll.

 

Around 10% of the time when I tumble the Pitts, I exit in a tailslide.The procedure for recovery from a tailslide is very different than from an inverted spin, and you must do it correctly to avoid damaging your aircraft.Again, unless you are totally comfortable instantly recognizing and recovering from tailslides at low altitude, donít do tumbles at low altitude.

 

I wish I could give you a simple set of procedures to keep you alive, performing out-of-control aerobatic maneuvers down low, but itís just not that simple.You must be an extremely experienced and proficient aerobatic pilot to fly out-of-control maneuvers down low, because you must be 100% certain that you can quickly recover from whatever develops with a minimum amount of altitude loss.

 

On that subject, I should mention that whenever you recover from a torque roll or tailslide or tumble with the nose pointing straight down Ė it doesnít get any better than that - the best recovery technique is to unstall the wing by applying full power and getting some airspeed before you haul back on the stick.A common newbie mistake in this situation is to panic because of the unusual view out the front, and haul back on the stick too soon, and stall the aircraft, which can result in mushing the aircraft into the ground.Donít do that, either.

 

Again, I cannot emphasize enough the importance of lots of experience and familiarity and skill before you start doing aerobatics down low.Please donít hurry, and kill yourself.The rest of us find it depressing when you do that.

 

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acboyd@gmail.comSept 2011