Jump to content

Elevator Locks Up: I suspect due to compression at overspeed


DieHard

Recommended Posts

It makes sense DieHard, and is yet another possible reason for the range of available positive (*) stab trim settings which aren't used most of the time in normal flight conditions :-)

 

( * ) These are actually negative, because they correspond to a decrease in incidence of the movable horizontal stab...


Edited by jcomm

Flight Simulation is the Virtual Materialization of a Dream...

Link to comment
Share on other sites

From memory, compression is not the same thing as control stiffness. Earlier P-38s suffered from compression: the elevator control became loose and ineffectual at dive speeds because there was a lack of airflow over the control surface.

 

1252px-Transonic_flow_patterns.svg.png

 

Control stiffness is the opposite problem. The pilot does not sufficient torque to overcome the force of airflow on the control surface.

P-51D | Fw 190D-9 | Bf 109K-4 | Spitfire Mk IX | P-47D | WW2 assets pack | F-86 | Mig-15 | Mig-21 | Mirage 2000C | A-10C II | F-5E | F-16 | F/A-18 | Ka-50 | Combined Arms | FC3 | Nevada | Normandy | Straight of Hormuz | Syria

Link to comment
Share on other sites

I suspect elevator locks up and freezes solid at overspeed.

 

The elevator trim still functions adequately enough to rtb.

 

I did a search within the 109 group here. I only found one guy talking about it:

 

Message #106

http://forums.eagle.ru/showthread.php?t=135100&page=11

 

Anybody else experience this?

 

Thoughts?

I haven't tried in 1.5 again, but in the old version I performed some high speed dives test just to see myself how it performed compared to the German diving tests papers out there. I noticed no elevator locks at all. Can you further explain the conditions you found that locks?

 

 

Jcomm is right about the trim limits, but the ultimate reason is a controls inversion happening at really high speeds due to compressibility where you would've to pull the stick to keep the nose down (and push to recover can be guessed) so trim is limited to eliminate that control inversion possibility. That's clearly seen in the German dive tests charts. A controls lock is mentioned in those papers but related to a wrong kind of high altitude freezing grease in the trim screws that was changed after the trials. AFAIK that's not modelled in DCS module, may be any 1.5 change?

 

S!


Edited by Ala13_ManOWar

"I went into the British Army believing that if you want peace you must prepare for war. I believe now that if you prepare for war, you get war."

-- Major-General Frederick B. Maurice

Link to comment
Share on other sites

same with flaps fully down

AWAITING ED NEW DAMAGE MODEL IMPLEMENTATION FOR WW2 BIRDS

 

Fat T is above, thin T is below. Long T is faster, Short T is slower. Open triangle is AWACS, closed triangle is your own sensors. Double dash is friendly, Single dash is enemy. Circle is friendly. Strobe is jammer. Strobe to dash is under 35 km. HDD is 7 times range key. Radar to 160 km, IRST to 10 km. Stay low, but never slow.

Link to comment
Share on other sites

From memory, compression is not the same thing as control stiffness. Earlier P-38s suffered from compression: the elevator control became loose and ineffectual at dive speeds because there was a lack of airflow over the control surface.

 

There were two different effects that the P-38 experienced in high-speed dives (above 15 or 20 thousand feet, depending on air temperature; contrary to popular impression, the P-38 did not have significant issues with high-speed dives below these altitudes). Firstly, the wing could stall (yes, in a high-speed dive) because of vortices causing loss of lift. Secondly, because of the P-38's unique wing & gondola design, "transonic shockwaves" could trap the elevator in place. This isn't wasn't quite the same as normal control stiffness due to high-speed airflow; it was a unique problem for the P-38 caused/exacerbated by the design of its (high-lift) wing & gondola. The P-38 (prior to the L and retrofitted Js with the dive flaps) had a much lower critical mach than the P-51 and P-47.

 

The Me 109 didn't have nearly as bad of a compressibility problem as the P-38, but the 109 did still have serious issues with control stiffness--to a much greater extent than the P-47, P-51, F4U, FW 190, et al.

 

Anyway, there are at least three separate effects that I'm aware of that can result in an inability to pull out of a high-speed dive: normal control stiffness, compressibility stall, and transonic shockwaves. As I understand it, extreme difficulties with the latter was more or less unique to the P-38, and while the second one wasn't, the '38 did experience it sooner (that is, at a lower Mach number) than most fighters, with and without the five flaps (but the dive flaps helped enough that, when they were functioning, dives were safe, if more energy-inefficient than usual).

 

Using trim to pull out of the dive is a good idea if you're experiencing stiffness but not the other two. It's incredibly dangerous, however, to use trim to try to pull out of the dive if you're experiencing one (or both) of the other two, however. Unlike control stiffness, which is gradual and predictable, one can unexpectedly leave the wing-stalled state, at which point the excessive pitch trim can tear the airframe apart. This is what killed Ralph Virden.

 

In the case of the 109, since it did not have an unusual problem with early-onset compressibility, but did have a problem with control stiffness, using trim to pull out of a dive is a good thing (unlike the P-38, in which trim should not be used to assist dive recovery).

 

Disclaimer: not being an engineer, I don't pretend to understand the physics of all that. I'm only reporting what I've carefully gathered from the best sources I could find, as opposed to the popular misinformation which inexplicably plagues the P-38 (now, as then). Corrections from the more knowledgeable are welcome.


Edited by Echo38
Link to comment
Share on other sites

I haven't tried in 1.5 again, but in the old version I performed some high speed dives test just to see myself how it performed compared to the German diving tests papers out there. I noticed no elevator locks at all. Can you further explain the conditions you found that locks?

 

 

Jcomm is right about the trim limits, but the ultimate reason is a controls inversion happening at really high speeds due to compressibility where you would've to pull the stick to keep the nose down (and push to recover can be guessed) so trim is limited to eliminate that control inversion possibility. That's clearly seen in the German dive tests charts. A controls lock is mentioned in those papers but related to a wrong kind of high altitude freezing grease in the trim screws that was changed after the trials. AFAIK that's not modelled in DCS module, may be any 1.5 change?

 

S!

 

Try it again using the DoW server in MP DCSW 1.5 Open Beta. I don't know what mission he was running at the time. The server being setup for random failures has occured to me as an explanation.

 

High speed dives on the ACG's WWII MP server, this same effect has not reoccurred. Have sustained dives at 700kph and pulled out satisfactory. At around 600kph controls start getting sluggish. I also establish a turn engine off and restart in the air for in-game play (probably never happened in real life).

 

I only fly in full sim realism in DCS.


Edited by DieHard

[sIGPIC][/sIGPIC]

Link to comment
Share on other sites

using trim to pull out of a dive

 

You could use trim to recover from compressibility in the Bf-109 and the FW-190 because of the adjustable stabilizer trim system.

 

Look at gavagai chart for example, even at mach .82 when the elevator is ineffective because of flow seperation behind the normal shock, the portion of the airfoil at and ahead of the normal shock is still effective.

 

In an airplane with an adjustable stabilizer trim system, that control will still work under compressiblity.

 

You are right though, that control surface is very effective and a pilot risks overloading the airframe on recovery if he is not careful in getting out of compressibility. He is already well past Vne so might as well finish his test pilot run and see how it turns out.

 

IIRC, Mtt instructs the pilot NOT to trim for the dive. This way the design stability and control characteristics require a push force in order to maintain the dive speed.

 

If you trim for the dive speed against those instructions then the stick forces will be beyond what the designer intended and will be excessive. Congratulations, you are now a test pilot.

 

but the 109 did still have serious issues with control stiffness

 

Not really nor would it under modern stability and control standards unless of course you do not pay attention to what manufacturers tells you.

 

If you do your own thing, then congratulations and welcome to world of test pilots. Hope you make out alive! :music_whistling:

Answers to most important questions ATC can ask that every pilot should memorize:

 

1. No, I do not have a pen. 2. Indicating 250

Link to comment
Share on other sites

I don't pretend to understand the physics of all that. I'm only reporting what I've carefully gathered from the best sources I could find, as opposed to the popular misinformation which inexplicably plagues the P-38 (now, as then). Corrections from the more knowledgeable are welcome.

 

IMHO, the P-38 did more to advance the NACA's practical knowledge of compressible aerodynamics than any other design.

 

I hope we get one for DCS!!

 

The general aeronautics community was suddenly awakened to the realities of the unknown flight regime in November 1941, when Lockheed test pilot Ralph Virden could not pull

the new, high performance P-38 out of a highspeed dive, and crashed. Virden was the first

human fatality due to adverse compressibility effects, and the P-38, shown in Fig. 8, was the

first airplane to suffer from these effects.

Development of High Speed flight.pdf

Answers to most important questions ATC can ask that every pilot should memorize:

 

1. No, I do not have a pen. 2. Indicating 250

Link to comment
Share on other sites

P-38 was my 1st WWII bird I learned in sim-combat flying, I think it was the Warbirds sim. It was a dream to fly about in non-combat. Aces High II sim was where I encountered elevator lock up around 400 mph usually in a dive. Later models of P-38 had speed-brakes (glorified independent x-tra flap).

 

I had read somewhere, other WWII prop planes encountered lock-up due to the sound barrier shock waves interfering with the controls hinge joint and in some cases opposite stick control was how the pilots recovered in a least ditch effort not to crash. Yo-Yo seems to be the only real aeronautical engineer here as to the last word about the real flight characteristics of such encounters.

 

Yes, I too would like to see the P-38 in DCSW someday.


Edited by DieHard

[sIGPIC][/sIGPIC]

Link to comment
Share on other sites

I had read somewhere, other WWII prop planes encountered lock-up due to the sound barrier shock waves interfering with the controls hinge joint

 

Yep, many World War II designs could reach the transonic realm and experience compressibility.

 

The German's first encountered it in the FW-190 prototype. In dives, one could see visible proof of the Prandtl-Glauert transformation when the pressure gradient across the normal shock forms condensation in humid air.

 

The Focke Wulf test pilot's had never seen such a thing and were shocked when the wings turned white in a dive. The airfoil selection on the Focke Wulf "critical mach" is fairly low. Critical mach is the velocity in which a normal shock will form on the airfoil.

 

Good description of compressibilty and "mach tuck":

 

http://www.wwiiaircraftperformance.org/fw190/fw190-0022-dive.html

 

 

The transonic realm was such a new frontier that it was not until very late in the war, the mystery of what exactly happened to an aircraft's control surfaces under compressibility was solved by a very brave USAAF pilot for the NACA.

 

Until somebody actually dove into compressibility, recorded the data, and survived it was hotly debated among engineers if the controls surfaces actually moved and several theories existed as the exact mechanism that cause them to be ineffective in compressibility.

 

One my old college textbooks devotes several pages to this investigation.

 

When I get home from work in a few days, I will dig up some good articles on the early foray's into transonic flight if folks are interested I think it is pretty cool.

Answers to most important questions ATC can ask that every pilot should memorize:

 

1. No, I do not have a pen. 2. Indicating 250

Link to comment
Share on other sites

Well, "serious" might be an overstatement, but, compared to the P-47 & P-51 ...

 

The Bf-109 did have "excessive" elevator forces given the right conditions. The main condition being the pilot not following the engineers instructions on how it needs to be flown.

 

Generally speaking, I think it was more of problem with the World War II designs going the opposite direction. It was more common for designers to set the forces too light than it was to make them excessive.

 

Some common misconceptions drove that tendency. "Maneuverability is linked to instability" is one of them. Yes, but in terms of piloted aircraft only as far as it can be controlled and maintain his physiological requirements.

 

You have to remember, in the context of the times they were outright horribly unstable aircraft in common use in General Aviation. The speeds were slow enough and weight low enough that control just was not the same issue it is with a 2000hp/10000lbs airplane brushing against the transonic realm of flight.

 

Instability is just instability without control. If it makes the pilot unconscious, damages the airplane, or does not allow the pilot to put the aircraft precisely where he wants it to be...

 

It is just instability.

 

In the absence of modern aircraft control systems the designers options were limited to gearing, rigging, and the control forces. Even then, the effects were not always well understood. We have learned a lot both during and after World War II in terms of aircraft stability and control.


Edited by Crumpp

Answers to most important questions ATC can ask that every pilot should memorize:

 

1. No, I do not have a pen. 2. Indicating 250

Link to comment
Share on other sites

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...