Big Crash at Reno
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Sir George.
The engine is cooled with 'evaporative cooling'; as the coolant evaporates, it is vented out, and lost. So there would be a loss of mass associated with cg also? I believe the evaporative cooler is behind the cockpit, so would this serve to improve the aft cg? Assuming there is always liquid to cool the engine, the mass of the coolant (Water/Methanol) would diminish from behind cg?
The engine is cooled with 'evaporative cooling'; as the coolant evaporates, it is vented out, and lost. So there would be a loss of mass associated with cg also? I believe the evaporative cooler is behind the cockpit, so would this serve to improve the aft cg? Assuming there is always liquid to cool the engine, the mass of the coolant (Water/Methanol) would diminish from behind cg?
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I've heard about something called 'stick free neutral point'. If the CofG was aft and fuel was being used from the wings, could the rearward moment arm have contributed to this condition?
The problem though is that he required a lot of trim to get the stick force managable. The one trim tab did double duty and is a significant departure from the original design. They would have tested this configuration to Vdive at altitude, but the dynamic pressure is a lot lower up high as opposed to the 5,100 ft race altitude, and the test would have been done at 1 g.
In terms of stick-free stability, I've included a simple pdf explanation without the eye glossing equations.
http://www.flightlab.net/Flightlab.....c%232BA158.pdf
Intend to go back to the video to visualize a new failure sequence with the starboard elevator shearing and going stick free on that side. This would assume turbulence causing the left roll disturbance and top rudder and stick forward and to the right as recovery inputs. They're briefed, as if Leeward wouldn't already know, to fly those inputs when they find themselves even slightly inverted.
In the turn, the shock wave on the wing moves the N.P. aft and increases stability. That shock induced increase in stability goes away under 1 g.
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With the Pitch Up at wings vertical after the snap left, I'd assume the elevators did not shear, for if one did, Pitch authority would diminish, and the Pitch Up would stop. Then again, having one elevator inop at the start of the right reversal, would explain the wicked Torsion visible in the tail (video).
From ClippedCub
Speeds such as VD are specified as Equivalent AirSpeeds in the standards - hence altitude should not be an issue. The exception is when mach number is a limit - MD rather than VD.
Was this aircraft actually tested to a declared VD?
As a matter of interest, does anyone know to what extent these aircraft are tested against standards? As an experimental aircraft they are obviously not expected to meet all standards (eg FAR23), but is there a set of minimums that are applied?
They would have tested this configuration to Vdive at altitude, but the dynamic pressure is a lot lower up high as opposed to the 5,100 ft race altitude, and the test would have been done at 1 g.
Was this aircraft actually tested to a declared VD?
As a matter of interest, does anyone know to what extent these aircraft are tested against standards? As an experimental aircraft they are obviously not expected to meet all standards (eg FAR23), but is there a set of minimums that are applied?
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That is a terrific question.
If there are Standards, there have to be predecessors, How does Burt Rutan make a living? Some one has to be first?
Experimental?
Who would allow asymmetrical Tab stress? Who will disallow it? Frankly, how much power does RAR have?
If there are Standards, there have to be predecessors, How does Burt Rutan make a living? Some one has to be first?
Experimental?
Who would allow asymmetrical Tab stress? Who will disallow it? Frankly, how much power does RAR have?
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Speeds such as VD are specified as Equivalent AirSpeeds in the standards - hence altitude should not be an issue. The exception is when mach number is a limit - MD rather than VD.
Was this aircraft actually tested to a declared VD?
As a matter of interest, does anyone know to what extent these aircraft are tested against standards? As an experimental aircraft they are obviously not expected to meet all standards (eg FAR23), but is there a set of minimums that are applied?
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Lyman, fair question, I'll attempt to answer.
There are many examples of light aircraft with asymmetrical trim tabs. The concept in itself is not the problem. The application of the concept to an experimental aircraft such as GG would be an issue if complete engineering and testing was not done before adopting that design.
My understanding of the regulations affecting experimental aircraft is as follows:
(Feel free to shoot me down if I'm wrong." )
As a US based experimental aircraft, construction merely needs to be performed in a workmanlike manner and to survive a certain amount of flight time before being permitted to be flown out of the designated test area. The regulations recognize that experimental aircraft may experience unexpected events and kill the 'test' pilot. The idea is to minimize the potential for for injury to others if the worst happens.
The problem with experimental aircraft is that the test program may not evaluate the full potential aircraft envelope. In the case of air racing, it is entirely possible to be exploring new ground during a race.
Who would allow asymmetrical Tab stress? Who will disallow it?
My understanding of the regulations affecting experimental aircraft is as follows:
(Feel free to shoot me down if I'm wrong." )
As a US based experimental aircraft, construction merely needs to be performed in a workmanlike manner and to survive a certain amount of flight time before being permitted to be flown out of the designated test area. The regulations recognize that experimental aircraft may experience unexpected events and kill the 'test' pilot. The idea is to minimize the potential for for injury to others if the worst happens.
The problem with experimental aircraft is that the test program may not evaluate the full potential aircraft envelope. In the case of air racing, it is entirely possible to be exploring new ground during a race.
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Thanks Machinbird. It makes sense to me. I saw Red Baron with Steve Hinton and its six blades chopping up the air I think it was '80.
I did not see LearFang, but saw Rare Bear with Lyle Shelton in it when it was wide winged and all Navy Blue. I think Strega was all Red at one time, and forget the name of the lime colored P-51.
Hinton's Engine seized, and the props stopped just as he was about to land after a Mayday. He impacted over the hill on (near) 08 and the plume of black smoke was visible for five minutes before the announcer said he'd died. There was absolute quiet, it was near the end of the day, and everyone walked to their cars in disbelief.
He survived. These guys are a breed apart, and they take monster risks.
I've been to the show at Reno dozens of times. It doesn't get old.
There is something about a half dozen unlimiteds with 18000 horsepower that touches ones soul. Charging down the start, everything vibrates.
I fon't know who said it first, but it's rumored to be Lyle Shelton.
"What do you do at Reno, Sir?"
"We turn money into noise".
I did not see LearFang, but saw Rare Bear with Lyle Shelton in it when it was wide winged and all Navy Blue. I think Strega was all Red at one time, and forget the name of the lime colored P-51.
Hinton's Engine seized, and the props stopped just as he was about to land after a Mayday. He impacted over the hill on (near) 08 and the plume of black smoke was visible for five minutes before the announcer said he'd died. There was absolute quiet, it was near the end of the day, and everyone walked to their cars in disbelief.
He survived. These guys are a breed apart, and they take monster risks.
I've been to the show at Reno dozens of times. It doesn't get old.
There is something about a half dozen unlimiteds with 18000 horsepower that touches ones soul. Charging down the start, everything vibrates.
I fon't know who said it first, but it's rumored to be Lyle Shelton.
"What do you do at Reno, Sir?"
"We turn money into noise".
Trying to keep it simple. MD is 0.80. VD is 505 IAS at race altitude and up to a certain altitude that I forget.
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Extract of LurkerBelow post
Thus far people have been concentrating on the trim tab as the initiator of events, where as perhaps its failure was a consequence of events, as with the extension of the tail wheel and the loss of its gear doors.
It's understandable why a racer would want the CofG as far aft as possible - less drag and hence more speed. How far aft is aft enough though?
As airspeed varies from a trimmed condition, the column force required to maintain a new speed (without re-trimming) is a measure of static longitudinal stability. For any conventional airplane, the location of the CG has the strongest influence on static longitudinal stability. For a statically stable airplane the required column force, as speed varies from the trimmed condition, is less at an aft CG than it is at a forward CG. As the CG moves aft, it reaches a point where the stick force per knot drops to zero, then reverses. This location is called the neutral point. The difference between the actual CG location and the neutral point is called the static margin. With a CG forward of the neutral point, an airplane has a positive static margin and positive static longitudinal stability. At a CG aft of the neutral point, an airplane has a negative static margin, is statically unstable, and may require some form of augmentation (computers) to be flown with an acceptable workload eg relaxed stability fighters F-16.
The result of moderate instability might be a flyable aircraft, but the workload goes up. If you pulled back on the stick the aircraft will pitch up and slow. But if you let go of the stick the nose will continue to pitch up, since a positive pitching moment will remain. It will require a push force to maintain a climb angle, not the mandated pull. If you pitched down and let go, the nose will tend to tuck under. You’d have to apply a pull force to hold your dive angle, not the mandated push. That’s how the Spirit of St. Louis behaved.
Manoeuvring stability, like static stability, is influenced by CofG location. However, when the CofG is aft and near the neutral point, then altitude also has a significant effect. Since air density has a notable impact on the damping moment of the horizontal tail, higher pitch rates will result for the same elevator deflections as altitude increases. From the pilots perspective, as altitude increases, a pull force will result in a larger change in pitch angle, which translates into an increasing angle of attack and g. While a well-designed flight control system, either mechanical or electronic, will reduce the variation of stick force with CofG and altitude, it is very difficult to completely eliminate the variation due to design limitations.
For example, for the same control surface movement at constant airspeed, an aircraft at 5,000 ft experiences a higher pitch rate than an aircraft at sea level because there is less aerodynamic damping. The pitch rate is higher, but the resulting change in flight path is not. Therefore, the change in angle of attack is greater, creating more lift and more g. If the control system is designed to provide a fixed ratio of control column force to elevator deflection, it will take less column force to generate the same g as altitude increases. What was the DA at Reno at the time?
Note the increased work load. Matt is quoted as observing Leeward having difficulty flying the course. High work load due aft CofG? Perhaps the workload just got too much and the aircraft caught him out. Time will tell.
Even the experts can get the CofG wrong. An SR-71 was lost on a test flight when the aircraft pitched up uncontrollably and disintegrated. The initiating event was an unstart, but the loss of control was the result of having the CofG too far aft.
Matt believes the cause of the crash was due to The Galloping Ghost having a CG too close to the aft limit which resulted in pitch instability.
During qualifying Matt watched Galloping Ghost from inside the cockpit of Furias and could not believe how much trouble Leeward was having in keeping the Ghost in a stable pattern around the course.
During qualifying Matt watched Galloping Ghost from inside the cockpit of Furias and could not believe how much trouble Leeward was having in keeping the Ghost in a stable pattern around the course.
It's understandable why a racer would want the CofG as far aft as possible - less drag and hence more speed. How far aft is aft enough though?
As airspeed varies from a trimmed condition, the column force required to maintain a new speed (without re-trimming) is a measure of static longitudinal stability. For any conventional airplane, the location of the CG has the strongest influence on static longitudinal stability. For a statically stable airplane the required column force, as speed varies from the trimmed condition, is less at an aft CG than it is at a forward CG. As the CG moves aft, it reaches a point where the stick force per knot drops to zero, then reverses. This location is called the neutral point. The difference between the actual CG location and the neutral point is called the static margin. With a CG forward of the neutral point, an airplane has a positive static margin and positive static longitudinal stability. At a CG aft of the neutral point, an airplane has a negative static margin, is statically unstable, and may require some form of augmentation (computers) to be flown with an acceptable workload eg relaxed stability fighters F-16.
The result of moderate instability might be a flyable aircraft, but the workload goes up. If you pulled back on the stick the aircraft will pitch up and slow. But if you let go of the stick the nose will continue to pitch up, since a positive pitching moment will remain. It will require a push force to maintain a climb angle, not the mandated pull. If you pitched down and let go, the nose will tend to tuck under. You’d have to apply a pull force to hold your dive angle, not the mandated push. That’s how the Spirit of St. Louis behaved.
Manoeuvring stability, like static stability, is influenced by CofG location. However, when the CofG is aft and near the neutral point, then altitude also has a significant effect. Since air density has a notable impact on the damping moment of the horizontal tail, higher pitch rates will result for the same elevator deflections as altitude increases. From the pilots perspective, as altitude increases, a pull force will result in a larger change in pitch angle, which translates into an increasing angle of attack and g. While a well-designed flight control system, either mechanical or electronic, will reduce the variation of stick force with CofG and altitude, it is very difficult to completely eliminate the variation due to design limitations.
For example, for the same control surface movement at constant airspeed, an aircraft at 5,000 ft experiences a higher pitch rate than an aircraft at sea level because there is less aerodynamic damping. The pitch rate is higher, but the resulting change in flight path is not. Therefore, the change in angle of attack is greater, creating more lift and more g. If the control system is designed to provide a fixed ratio of control column force to elevator deflection, it will take less column force to generate the same g as altitude increases. What was the DA at Reno at the time?
Note the increased work load. Matt is quoted as observing Leeward having difficulty flying the course. High work load due aft CofG? Perhaps the workload just got too much and the aircraft caught him out. Time will tell.
Even the experts can get the CofG wrong. An SR-71 was lost on a test flight when the aircraft pitched up uncontrollably and disintegrated. The initiating event was an unstart, but the loss of control was the result of having the CofG too far aft.
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ClippedCub, is the VD / MD limit specified by the contest rules?
Hinton's Engine seized, and the props stopped just as he was about to land after a Mayday. He impacted over the hill on (near) 08 and the plume of black smoke was visible for five minutes before the announcer said he'd died. There was absolute quiet, it was near the end of the day, and everyone walked to their cars in disbelief.
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Even the experts can get the CofG wrong.
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ClippedCub, any idea if they are forming shocks on the tail at this speed?
And what balance limits do they use of the elevator? Original P51 figures?
And what balance limits do they use of the elevator? Original P51 figures?
The tail is at a much lower thickness-to-chord than the wing, so the tail wouldn't have a shock. But shock waves are sensitive to curvature too, and the bulbous elevator will cause a re-accleration of the flow. Doubt there was a shock here, but in the interest of thoroughness, it's on my list to check, if for nothing else, to determine the margin.
Don't know anything about the balance specs, but it's not unusual for the guys do flutter flight tests when needed.
the elevator has been reshaped for high speed effectiveness. It's more bulbous. This is to get it out of the boundary layer of the horizontal stabilizer and would have been selected by his aero guys. There is a basis for this from the old NACA reports that I was going to dig through again, but if a shock did form, it will form on the deflected elevator itself and not on the hinge line or ahead.
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Very interesting, thanks for that. Any idea why they felt the change necessary? Were they running into a (stick force) stability issue?
Wasn't going to bring it up. Think we can figure out the failure sequence collectively without referencing that detail. If we couldn't, was going to spend time looking into the elevator shape, and maybe shock bursting on the wing if the speed and g warranted.
CC, in #251 here there is a link to `hangartalk`,air racing; if on that link you go back to page9 #88 there are 3 photos which show the `oil-canning and the tab deflection from the rear quarter.(if you haven`t seen them already).
The Mustang 4 was limited to 505 MPH or .75 M,but had probably been flown faster in testing.Also the aircraft in those times had fabric covered elevators and wooden or plastic trim tabs and one on each elevator would reduce the loading,fabric covered surfaces tend to distort aerofoil shapes at high speed; later they were made all metal,except the rudder.Since GG is a `racer` the rudder tab is also faired in.
It would be interesting to know if the stbd.tab on GG was `locked` to the elevator,or to the tailplane as a balance tab,to reduce stick forces.
With regard to the `over-bank` at pylon 8, I would consider that to be gust induced,either by the preceding two aircraft and/or natural wind,which may have caused and been sensed by JL as a possible change to his `line` around 8,and preventing a possible `cut`,requiring a quick push,and a bit of roll-out.
It is difficult to see the pylon and aircraft to draw a reference as to how close he was now running,after having passed the `Bear`,and flying a wider pattern to do so.It is a lot harder to run closer to the pylons,with a lot more activity on the controls to get a tighter line.
With regard to the roll, I think that was induced by precession from the prop on the pull-up,causing roll,as the controls have become free,and as the aircraft starts downhill,it will try to go to a trim speed,controls free again.
On another subject,I have searched for the NTSB archive for the definitive results of the investigation into the tail failure of Miss Ashley` in `99`,but only find the standard report,which give no real structural results..Anyone got anything more definitive..?
PS.GG was reported at 495 mph; that equates to M.645 at 5500ft at an assumed *T of 20*C; M.75 would be 579 mph in the same conditions.
The Mustang 4 was limited to 505 MPH or .75 M,but had probably been flown faster in testing.Also the aircraft in those times had fabric covered elevators and wooden or plastic trim tabs and one on each elevator would reduce the loading,fabric covered surfaces tend to distort aerofoil shapes at high speed; later they were made all metal,except the rudder.Since GG is a `racer` the rudder tab is also faired in.
It would be interesting to know if the stbd.tab on GG was `locked` to the elevator,or to the tailplane as a balance tab,to reduce stick forces.
With regard to the `over-bank` at pylon 8, I would consider that to be gust induced,either by the preceding two aircraft and/or natural wind,which may have caused and been sensed by JL as a possible change to his `line` around 8,and preventing a possible `cut`,requiring a quick push,and a bit of roll-out.
It is difficult to see the pylon and aircraft to draw a reference as to how close he was now running,after having passed the `Bear`,and flying a wider pattern to do so.It is a lot harder to run closer to the pylons,with a lot more activity on the controls to get a tighter line.
With regard to the roll, I think that was induced by precession from the prop on the pull-up,causing roll,as the controls have become free,and as the aircraft starts downhill,it will try to go to a trim speed,controls free again.
On another subject,I have searched for the NTSB archive for the definitive results of the investigation into the tail failure of Miss Ashley` in `99`,but only find the standard report,which give no real structural results..Anyone got anything more definitive..?
PS.GG was reported at 495 mph; that equates to M.645 at 5500ft at an assumed *T of 20*C; M.75 would be 579 mph in the same conditions.
Last edited by sycamore; 9th Oct 2011 at 20:03.
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Other P-51 racers are saying the oil canning is normal, and transitory, though I'd think they'd eliminate that from a drag standpoint since they're meticulous on maintaining smoothness. The deflection of the port tab with the strbd in trail caught my eye as an engineer.
Placard is 0.75M, but they've tested to 0.85M where the Mach buffet shakes the scoop loose and causes rivets to pop. The aircraft was unairworthy on return. As such, MD will be 0.75M as opposed to the 0.80M Mach misspeak from earlier. This would mean demonstrating the Reno q's during the dive test would have been more unlikely even using an adjusted VEAS for 5,000 ft instead of SL. The structural engineers would have compensated to 5,000 ft q's, but it wouldn't have been demonstrated.
The conversion to metal from fabric was to limit the PIO on dive recovery at high Mach numbers for reason you've noted.
Currently the quick push, post left roll you noted, would have further increased the loads on the right elevator and that might have been what did it. The pitch change occurs immediately after the left roll. This would assume a natural disturbance to cause the roll.
Loads weren't understood as well in the forty's so the engineers overbiult everything. That's why DC-3's are still in service, and the reason these airplanes are lasting so long. But the tails are getting beaten up from the horsepower increase to 4,000 and they are in direct influence of the vibrations caused by the blade pass frequency, and are getting bombarded by the blade tip vortices.
Going to a single tab will put undue loads on the tab free side, and maybe it all just added up. Or the tab actuator rod just had enough since it was supporting double the load.
Placard is 0.75M, but they've tested to 0.85M where the Mach buffet shakes the scoop loose and causes rivets to pop. The aircraft was unairworthy on return. As such, MD will be 0.75M as opposed to the 0.80M Mach misspeak from earlier. This would mean demonstrating the Reno q's during the dive test would have been more unlikely even using an adjusted VEAS for 5,000 ft instead of SL. The structural engineers would have compensated to 5,000 ft q's, but it wouldn't have been demonstrated.
The conversion to metal from fabric was to limit the PIO on dive recovery at high Mach numbers for reason you've noted.
Currently the quick push, post left roll you noted, would have further increased the loads on the right elevator and that might have been what did it. The pitch change occurs immediately after the left roll. This would assume a natural disturbance to cause the roll.
Loads weren't understood as well in the forty's so the engineers overbiult everything. That's why DC-3's are still in service, and the reason these airplanes are lasting so long. But the tails are getting beaten up from the horsepower increase to 4,000 and they are in direct influence of the vibrations caused by the blade pass frequency, and are getting bombarded by the blade tip vortices.
Going to a single tab will put undue loads on the tab free side, and maybe it all just added up. Or the tab actuator rod just had enough since it was supporting double the load.