Variable Geometry Wing Characteristics

[BlueCanary]

I was wondering about the Tomcat's wing, was it something pilots had to pay a lot of attention to? Or could they just set it to auto and forget about it most of the time? Also, were there trim changes from the center of pressure changing as it moved? I've read that it was shown in testing that if a damaged it could with one wing swept back and the other forward, do you think this will be modelled? I'm probably being stupid, but could there feasibly be a situation where the flight controls were damaged to the point where the pilot could fly only with engine thrust and wing sweep for control?

Edited December 13, 2015 by BlueCanary

[mvsgas]

Some direct PDF links:

[ame]http://www.nasa.gov/centers/dryden/pdf/88647main_H-2368.pdf[/ame]

 

[ame]http://www.nasa.gov/centers/dryden/pdf/88125main_H-1461.pdf[/ame]

 

[ame]http://www.nasa.gov/centers/dryden/pdf/88192main_H-1598.pdf[/ame]

[BlackLion213]

From my reading, Fleet Aircrews spent about 90% of their time flying with the wing sweep mode in "Auto" - meaning the air-data computer would automatically select the best sweep position. Especially for ACM, Auto tended to be the best mode (with a few sneaky exceptions :)).

 

There were 3 modes for wing sweep that were directed through the air-data computer with a separate manual override. These were selected with a 4 position hat switch on the throttle. The modes were: auto, bomb, and manual select.

 

Auto mode works the way you might expect, it automatically moves the wing into the ideal position given airspeed, altitude, and other factors that I don't recall. This air-data computer also operated the maneuvering flaps (which incorporated AOA data into the algorithm) and the glove vanes in the F-14A (when operable). One of the unique wing sweep features of the F-14 was the rate of sweep: it can sweep from 20 deg (full forward) to 68 deg (full back) in about 3 seconds. The F-111 take about 10 seconds to do the same. I think the MiG-23 takes about 8-10 seconds per position change (so about 16-20 seconds total). This fast sweep rate for the Tomcat makes the automatic wing sweep position changes much more helpful and allows it to largely keep up with energy changes during maneuvers.

 

Bomb mode was originally designed for (you guessed it!) A-G weapons delivery. It limits wing sweep to 55 deg or greater and automatically extends the glove vanes above Mach 0.35. In practice, it was used operationally for a few specific things. Pilots would select bomb mode just after an arrested landing so the wings would start sweeping while they retracted the hook, flaps, and taxied out of the landing area. Bomb mode was reportedly the easiest position for aerial refueling, but pictures would suggest that pilots didn't use it much for this once out of the RAG. Finally, many pilots would use Bomb mode for pictures because it looked cool.

 

Here is bomb mode in an early 80s F-14A, before the integration of TCS:

 

 

Manual mode again is what it sounds like: the pilot can use the wing sweep position switch on the throttle to move the wings forward or back. They cannot use this switch to sweep into a position of less sweep than the computer recommends, but he can sweep back as far as desired. This is how Tomcats perform those sweep demos for airshows. If the aircraft accelerates to a speed where the air-data computer recommends a sweep position greater or equal to the manually selected sweep, manual mode will revert to Auto. Pilots would commonly use this for parade formation so that all the Tomcats would have the same sweep position (generally 68 deg aft).

 

There was also a manual override that would bypass the air-data computer. It had a yellow handle and was covered with plexiglass. The pilot could use it to select any sweep position and would also need to use it to select oversweep (75 deg) for parking/storage.

 

"Hoser" Satrapa also used this manual override to develop a spin recovery technique for the Tomcat. You mentioned the changing center of pressure with wing sweep. When the wings were swept aft to 68 deg, there was a big change in center of pressure and center of gravity. This exerted a significant downward force on the nose that would help to pull an F-14 out of a spin. It worked very reliably if you were above 10,000'.

 

There were a few rare instances (like <5 for the Tomcat's whole career?) of slight wing asymmetry developing in the fleet after a combined actuator/wing interconnect failure. This asymmetries (IIRC) were only a few degrees. These pictures:

 

 

Are from NASA testing where one wing was locked forward and the other slowly swept back for testing. There was no mechanism or control that would allow it to be deliberately created by the pilot, so I don't think anything like this would be modeled by Leatherneck.

 

Anyway, I hope this more-or-less answers your questions.

 

-Nick

Edited December 14, 2015 by BlackLion213

[mvsgas]

[BlackLion213]

Some direct PDF links:

http://www.nasa.gov/centers/dryden/pdf/88647main_H-2368.pdf

 

http://www.nasa.gov/centers/dryden/pdf/88125main_H-1461.pdf

 

http://www.nasa.gov/centers/dryden/pdf/88192main_H-1598.pdf

 

Nice find! Thank you for posting.

 

-Nick

[BlueCanary]

There were a few rare instances (like <5 for the Tomcat's whole career?) of slight wing asymmetry developing in the fleet after a combined actuator/wing interconnect failure. This asymmetries (IIRC) were only a few degrees. These pictures:

 

Are from NASA testing where one wing was locked forward and the other slowly swept back for testing. There was no mechanism or control that would allow it to be deliberately created by the pilot, so I don't think anything like this would be modeled by Leatherneck.

 

Anyway, I hope this more-or-less answers your questions.

 

-Nick

 

Thanks for the answer, lots of interesting stuff there.

 

About assymetrical wing sweep though, I was thinking more that it would be a thing that happened due to combat damage than something you'd do deliberately, like if you took hits to one wing that for some reason left it stuck swept back (IIRC that was what the assymetrical sweep tests were actually about) it would be quite interesting having to return to the carrier with one wing messed up, do you think that would be more likely to be modelled or would it still be unlikely?

[BlackLion213]

About assymetrical wing sweep though, I was thinking more that it would be a thing that happened due to combat damage than something you'd do deliberately, like if you took hits to one wing that for some reason left it stuck swept back (IIRC that was what the assymetrical sweep tests were actually about) it would be quite interesting having to return to the carrier with one wing messed up, do you think that would be more likely to be modelled or would it still be unlikely?

 

It could happen, but it would take a few unfortunate failures. I think there were official protocols for coming aboard the boat with asymmetric wing sweep positions from the NASA testing. However, if the wing sweep precluded operating the outboard wing flaps, then speeds would be to high for an arrested landing (too much energy for the arresting gear). I don't remember what the max sweep would be.

 

You're right, that would be a challenging situation. The F-14 was hard enough to bring aboard the boat when most things were working. ;)

 

-Nick

 

PS - I don't know if LNS will model that. There current F-14 module plans are pretty extensive from what they've said, but this would be a really rare event from what I've gathered.

[BlackLion213]

Thanks for the answer, lots of interesting stuff there.

 

About asymmetrical wing sweep though, I was thinking more that it would be a thing that happened due to combat damage than something you'd do deliberately, like if you took hits to one wing that for some reason left it stuck swept back (IIRC that was what the assymetrical sweep tests were actually about) it would be quite interesting having to return to the carrier with one wing messed up, do you think that would be more likely to be modeled or would it still be unlikely?

 

This seems like a fitting follow-up picture:

 

 

Courtesy of the Leatherneck Simulations facebook page. I LOVE this stuff! :D

 

This F-14B model looks insanely good, stunning work!

 

-Nick

[Hummingbird]

Another advantage of the automatic variable sweep mode, as far as I've been told, was that it made sure that the aircraft required very little trim in pitch with changes in speed - and that exactly due to gradually shifting the center of lift backwards as speed increased.

 

Btw, going into full sweep mode and the F-14 was capable of incredible high alpha turns. Infact I made a small video showcasing it:

 

xaufSWTsXDc

Edited December 28, 2015 by Hummingbird

[Angel101]

Sorry, but that video doesn´t show nothing about going with high alpha.

 

something like this does:

 

[Hummingbird]

The Tomcat does infact pull off a turn at pretty darn high alpha in the vid.

[captain_dalan]

The Tomcat does infact pull off a turn at pretty darn high alpha in the vid.

 

Indeed it does. Perhaps there was a confusion between the sustained VS instantaneous alpha, and loaded VS unloaded 1g alpha.

[Hummingbird]

Indeed it does. Perhaps there was a confusion between the sustained VS instantaneous alpha, and loaded VS unloaded 1g alpha.

 

Must have cause that Tomcat is high fiving that air :punk:

[BlackLion213]

The Tomcat does in fact pull off a turn at pretty darn high alpha in the vid.

 

It does! But it also manages to change it's flight path much more rapidly than the Hornet video. The Tomcat has tremendous pitch authority with it's wings swept, but it also bleeds airspeed rapidly in that configuration (like the Mirage and MiG-21, of course). With the wings swept forward, it generates less alpha in turns, but also changes it's true flight path just as well with less loss of energy.

 

Most Tomcat drivers would execute the first break of a Case III recovery with the wings swept at around 400 kts (for aesthetics...and to bleed airspeed :D). I've read that they had to be careful during that break because they could rapidly build up the alpha during the turn and occasionally experience excessive yaw or departure.

 

I've also heard (from Hornet pilots) that the Tomcat can consistently out-turn the Hornet below 325 knots. It generates less alpha and the ITR is less, but the actual mass of the aircraft changes direction more in the F-14 and the STR is better.

 

It will be interesting to compare once both modules are out.

 

Also, just as a follow-up - this image is an in-game shot for testing the hydraulics/control surfaces:

 

 

Things are certainly moving along! :thumbup:

 

-Nick

[Hummingbird]

Yep, things are looking good for an open beta release sometime in the summer 2016.

[Angel101]

The Tomcat does infact pull off a turn at pretty darn high alpha in the vid.

 

You haven´t understood me. The alpha is the angle between the relative wind and the horizontal plane of the aircraft, and it´s very difficult to judge from outside how many degrees of AOA it´s pulling in that high G´s manevours.

 

It´s more easy to judge from the video of blue angels with smoke trailing on.

 

You know what I mean?

 

I´m not saying that tomcat is unable to pull high AOA, just that video means nothing in this aspect.

[Hummingbird]

You haven´t understood me. The alpha is the angle between the relative wind and the horizontal plane of the aircraft, and it´s very difficult to judge from outside how many degrees of AOA it´s pulling in that high G´s manevours.

 

It´s more easy to judge from the video of blue angels with smoke trailing on.

 

You know what I mean?

 

I´m not saying that tomcat is unable to pull high AOA, just that video means nothing in this aspect.

 

In that case you need to wactch it again mate :)

 

Like Cpt. Dalan said there's a difference between sustained vs instantaneous alpha, and loaded vs unloaded 1g alpha.

[Angel101]

In that case you need to wactch it again mate :)

 

Like Cpt. Dalan said there's a difference between sustained vs instantaneous alpha, and loaded vs unloaded 1g alpha.

 

Ok, can you tell me just with that video how many AOAº is pulling? 20º? 25º? 30º? 80º?

[Hummingbird]

Ok, can you tell me just with that video how many AOAº is pulling? 20º? 25º? 30º? 80º?

 

Impossible to say exactly, but a lot for the speed he was going and how long he held it, and that's the point really:

 

At 1 G and slow speeds you can pull a lot more alpha and get away with it, whilst at high speed and high loads you are more limited, both structurally and aerodynamically. When you see F-18's do their real high alpha maneuvers (upwards of 60 deg AoA) they are usually either going very slow or relatively so whilst pulling an instantanous high alpha maneuver. By comparison the F-14 in the vid was coming past screaming fast. Point being what is considered high alpha at high speed and high G loads is not necessarily considered high alpha at slow speeds and low G loads.

Edited December 29, 2015 by Hummingbird

[captain_dalan]

You haven´t understood me. The alpha is the angle between the relative wind and the horizontal plane of the aircraft, and it´s very difficult to judge from outside how many degrees of AOA it´s pulling in that high G´s manevours.

 

It´s more easy to judge from the video of blue angels with smoke trailing on.

 

You know what I mean?

 

I´m not saying that tomcat is unable to pull high AOA, just that video means nothing in this aspect.

 

The instances shown in the video are very different. After the initial pitch up, the F-18 demonstrates the unloaded high alpha capability. This behavior will not be observed during a loaded turn. The F-14 video is the exact opposite. For a relatively similar (to the F-18 ) video look up youtube. There is an early high alpha demonstration of the F-14A loaded with dummy ordnance. The video shows initial pitch up and then what looks like and unloaded nose-up behavior.

 

[ame]

[/ame]

 

Ignore the misleading title. It has nothing to with Cobra. The exact numbers are hard to tell, and LERX should help the F-18 sustain this state for longer time periods and less loss of control.

Edited December 30, 2015 by captain_dalan

[Hummingbird]

The instances shown in the video are very different. After the initial pitch up, the F-18 demonstrates the unloaded high alpha capability. This behavior will not be observed during a loaded turn. The F-14 video is the exact opposite. For a relatively similar (to the F-18 ) video look up youtube. There is an early high alpha demonstration of the F-14A loaded with dummy ordnance. The video shows initial pitch up and then what looks like and unloaded nose-up behavior.

 

 

Ignore the misleading title. It has nothing to with Cobra. The exact numbers are hard to tell, and LERX should help the F-18 sustain this state for longer time periods and less loss of control.

 

The F-14A was also flying with wings unswept in that test, which will increase lift by a lot but also reduce the high alpha capability significantly, esp. sustained because of the insane amount of lift generated.

 

Infact that it was able to pull that much alpha with unswept wings is pretty impressive acually, but also likely only possible because of the lifting body design and the wing glove acting exactly as LERXs. The wings were probably past their critical AoA and momentarily stalled with the body carrying the aircraft at that point until the AoA dropped back down.

 

The F-14's high alpha capability with fully swept wings however is everybit as impressive as that of the F-18 according to what I've heard.

[captain_dalan]

Worthy of note, the end of the video even shows a sustained 1g high alpha attitude, probably at high altitude. The exact angle is impossible to state, but looks to be easily between 40-50 degrees (true).

[Peter5on]

This might be interesting NASA 1/36 scale model wind tunnel test:

 

(It takes 16 turns to recover from flat spin)

 

[ame]https://www.youtube.com/watch?v=2mb2vFrfy_I[/ame]

 

 

[ame]

[/ame]

[Hummingbird]

Lots of high AoA testing here from 6:16 min onwards:

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