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Homebuilders and Stall/Spin Safety
By Tony Bingelis (originally published in EAA Sport Aviation, April 1981)
I JUST LOST a friend. He was making that long awaited first test flight in his brand-spanking-new homebuilt. As is the case with most aircraft accidents, we don't know the officially determined cause . . . may never hear what that is. Only this much was relayed to me. "He took off and was climbin', when suddenly it nosed down and went straight in."Physical incapacitation? Possible.
Mechanical failure? Not likely. My friend was too much a stickler for detail and did good work.
More likely, immediately after take-off the airplane may have been flown into an excessively high angle of attack and stalled. That, to me, is the more believable scenario for a number of reasons.
The airplane was a small one . . . maybe even a little underpowered as many of our homebuilt creations tend to be. And, being an avid builder, my friend certainly hadn't been racking up numerous flying hours in the upkeep of that all important flight proficiency. Then, too, it was a first test flight . . . for the airplane and for the pilot. I believe my friend's airplane simply fell out of the sky, out of control.
Any airplane will fall out of the sky if its airspeed is caused to decrease enough. Remember, an aircraft is defined as "a heavier-than-air craft that is driven by a screw propeller (or by a jet) and is SUPPORTED BY THE DYNAMIC REACTION OF THE AIR AGAINST ITS WINGS." It follows, then, that when the velocity of air (dynamic action) over the wings slows, the angle of attack will increase to a point where the wing quits flying. How the aircraft reacts at that instant is an indication of its stall behavior and susceptibility to spins.
Stall/Spin Behavior Is Not Always Predictable
As a pilot of a new homebuilt you have no way, no reliable way, of determining the stall/spin characteristics of your aircraft PRIOR to making your flight tests. But you are not alone in this - neither has the aircraft designer, or NASA . . . not with any degree of accuracy.
At NASA's 1980 Stall/Spin Workshop, I learned that even they have not always anticipated the development of hazardous maneuvers. It is no wonder then that NASA pilots have performed hundreds of spins, ranging from slow and steep to fast and flat, in an attempt to correlate and validate the theoretical data accumulated from old NASA studies and their own wind tunnel and radio controlled model tests.
As part of their stall/spin program, the NASA test aircraft are subjected to all possible combinations of stalled flight conditions . . . and for good reason. In a couple of instances, what had been taken for granted to be an aircraft with docile recovery characteristics went into uncontrollable maneuvers that resulted in overloading the structure. All this happened so quickly that the pilot was unable to effect the necessary control input to counter the aircraft's antics. We may be in the "Computer Age", but it still takes flight tests to prove the theoretical.
If this be the case I think that we, who would insist on testing our own aircraft, must recognize that the potential always exists for the development of a flight situation which could suddenly terminate in an unforeseen stall/spin.
As your new airplane first breaks the "surly bonds of earth" you don't know anything about its controllability or flight behavior. You don't know what its airspeed is or should be. You don't know what the correct climb attitude should be nor do you even know how effective the thrust is that you are getting out of the propeller/engine combination installed. Under such circumstances it is easy to allow the nose attitude to get too high for the power and airspeed generated (an airplane can stall even under full power). What with the tension attending the first flight, a stall might not be recognized in time to initiate corrective control action and effect a recovery in the altitude remaining.
Just about any airplane you may fly can be flown into a stall/spin encounter.
An aircraft with an ATC (Approved Type Certificate) will have already demonstrated, prior to certification, its controllability and ability to recover from spins. A homebuilt, on the other hand, is fielded with no such guarantee.
Anyone who spins a conventional design, be it a modified drooped-leading edge wing type or otherwise, should not be lulled into assuming that the aircraft has good or acceptable spin behavior simply because it is supposed to be docile. Experience has shown that we can expect any conventional aircraft to have an acceptable one-turn spin characteristic, but this doesn't necessarily imply that its multi-turn spins will be good or recoverable. There is always a good likelihood that recovery will be impossible through the use of normal control recovery procedures.
NASA has consistently proved that regardless of control input, an airplane will settle into a spin, characteristic of its design configuration and weight distribution.
The Stall/Spin Scenario
The sudden loss of much of the wing's lift at that critical angle of attack (just beyond the maximum usable lift) marks the stall. Here is where so-called conventional aircraft react in widely differing actions which can be exciting. In some, an uncontrollable pitching, yawing or rolling departure is encountered. The nose drops as the stall takes command of your airplane. Should one wing drop, the induced rolling may well be the first indication of an inadvertent spiral or an incipient spin. Figure 1
What Do We Have Here?
It is extremely difficult for a pilot in an airplane to determine whether he is in a spin or a spiral. In both instances, the airplane descents in a helical path. However, in a spin the wing is stalled, or to be more accurate, its helical descent path is at an average angle greater than stall. In a spiral, on the other hand, the angle of attack remains at less than stall.
The NASA folks learned that their test pilots were unable to sense angle of attack and as a result had difficulty in distinguishing between the spin and spiral modes. To establish reliable monitoring evaluation of the spin characteristics of the aircraft being tested, it was necessary to provide their pilots with special instrumentation.
Many a good pilot believes he has the only sure way to determine if the condition is a spin or a spiral. If it is a spin, it cannot be controlled whereas if it is a spiral the airplane can be controlled. Not so! NASA found that this was not always true.
How NASA Provokes Deliberate Spins
Entries into spins by NASA test pilots (and used-to-be military types) are usually made by slowly decelerating the aircraft at idle power (imposing no more than 1 G) to a wings-level-attitude stall (flaps retracted) and abruptly applying prospin control input. If the aircraft has a roll-off tendency that tendency will probably be the strongest at the initial stall break and well before the elevator can be fully deflected (up).
The prospin control input consists of:
- Full aft stick (or control wheel - in some social circles).
- Full rudder deflection - in the direction of intended spin.
- Full aileron deflection against the intended spin direction.
As described, these control inputs are applied about 2 mph above the stall speed with the engine power remaining at idle throughout the spin.
Whether the ailerons are deflected against the spin or held neutral, the rate of rotation seemed to be the same. However, deflecting ailerons against the resulting spin generally established a slightly flatter spin attitude. Some airplanes will not spin unless the ailerons are deflected against the direction of spin during entry.
A high rate of turn in a spin is not usually a measure of the aircraft's recoverability. A good example is the high wing Cessna Skyhawk used in some NASA tests. Its spin recovery is quick and easy and yet its speed or rate of turn in a spin is quite high.
NASA Spin Recovery Procedures
The most effective spin recovery procedure was found to consist of:
- Full anti-spin rudder smartly deflected
- Full forward stick
- Neutral ailerons
Acceptable one turn spin characteristics previously exhibited by any airplane may not be a guarantee that it will have equally good or acceptable behavior in a fully developed multi-turn spin. It is for this reason that most NASA spin tests are of the multiple turn variety. That is, the pilot will hold the prospin control input used to induce the spin and allow it to stabilize or become "locked-in". This stabilized spin usually is established after 3 to 6 turns. Figure 2
The recovery procedure is effected by applying their standard spin recovery control input at a 6-turn point and holding that input:
- until the airplane recovers or . . .
- until the pilot is convinced it obviously will not recover. At this point all he can do is to vacate the airplane or deploy the anti-spin recovery chute . . . if installed, of course.
Control Effects On Spin Recovery
The techniques giving fastest and most positive recoveries from fully developed flat spins may be rated as follows:
- Most Effective: Full opposite rudder (anti-spin) immediately followed by down elevator.
- Effective: Simultaneous opposite rudder and down elevator.
- Sometimes OK: Neutral rudder and elevator.
Spin Factors To Think About
In general, single turn spins are recoverable although aircraft, when subjected to multiple (3 to 6) turns, often turn out to be unrecoverable . . . a very important thing to remember.
Steep spins at a relatively low rate of rotation are highly recoverable and spin recovery is very rapid. On the other hand, moderately flat spins tend to be unrecoverable. NASA found that in their test aircraft the engine would usually quit after 4 turns in flat unrecoverable spins.
In some instances cable tension could become a factor in effecting a positive spin recovery. When that control stick is popped forward, slack in the elevator control cables may jeopardize your recovery efforts. You need immediate and positive movements of the control surfaces to obtain effective spin recovery input.
You may be surprised to learn that no matter what flight conditions NASA imposed their high wing aircraft seemed to have a better stall/spin recovery record than the low wing test aircraft.
Anyone contemplating the performance of deliberate spins should have plenty of altitude. NASA conducts its spin tests at fairly high altitudes - 6700' to 9500'. They also establish decision altitudes for spin-chute deployment and, if necessary, pilot egress. A very good idea.
Here's a shocker for you. Do you believe that an aft C.G. always results in an adverse spin propensity? Well, were the NASA researchers ever surprised when they found that their modified Yankee test airplane actually experienced an improved spin behavior with an aft C.G. location.
The surprising discovery was made that a C.G. movement to a more aft position, from 26% to 35%, seemed to have very little effect on moderately flat spins. Actually, it caused the rotation rate to decrease somewhat with a slightly faster recovery rate resulting. The NASA folks, however, caution us not to draw any conclusions from this find other than that it happened with that particular airplane under conditions that may not necessarily exist in other situations. It simply goes to show you that you cannot take anything for granted in flying.
In Summary . . .
Without a stall there cannot be a spin. But close to the ground one is as bad as the other because altitude is required to recover from either condition.
As dangerous as that first test flight take-off might be, those low altitude turns in landing still account for many of the fatal stall/spin accidents in general aviation.
The classical stall/spin sequence often automatically follows a poorly timed and poorly coordinated turn on final. This ultimately leads to an unexpected loss of longitudinal or lateral directional control followed by a roll-off into a spin entry and ground impact . . . even before the spin may become recognizable as such.
The low altitude stall/spin danger is real. The scenario always seems the same . . . a pilot starts his final turn to align with the runway a bit too late, he steepens and tightens the turn. Now, under a little more tension, he unknowingly adds more rudder to speed the turn and opposite ailerons to keep the bank from getting too steep. Suddenly, the airplane quits obeying his control (input) and snaps over inverted. There is no time nor altitude for recovery.