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 Post subject: Re: L-39 Operators?
PostPosted: 21 Jan 2023, 12:26 
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The big difference on the Russian VS American AI that takes getting use to is the airplane moves for the bank angle, and the background moves for pitch. It’s a bit of getting use to on the eye.

It seems there's more than one type of "Russian" AI.

There's this one in a L-39 (top photo) that has a moving "airplane" for bank, but at least the sky is on top and the ground on the bottom. This one frankly seems fairly easy to adjust to compared to the other type as shown in a Nanchang CJ-6 (bottom photo).

The Nanchang AI appears to function in the same way as a Sperry F-3 (ground on top, sky on bottom) but at least (unlike the F-3) the sky is blue and the ground is brown.

I think I read somewhere that the original Russian (Soviet) AIs were virtually a direct copy of the Sperry design except for the colors. Sperry's concept was that the moving part of the display is a sphere that is "rigid in space". The result is a display that mimics what a miniature Earth viewed from a great distance would look like on your panel. So when you pitch down you're looking at the top of the sphere (Earth).

This is how an engineer would design an attitude indicator and also what the astronauts use in a spacecraft. But that presentation seems backwards to a pilot so the modern AI flips the presentation "upside down" (from an engineer's perspective) to make it appear more natural. One downside of a "modern" AI is that it cannot properly display pitch angles greater than about 60° up or down. This limitation can be extended to almost +/- 90° on an electronic PFD. Beyond that the display has to "flip" even on an electronic display vs the Sperry gyro which simply continues to rotate in pitch throughout 360° of pitch change.
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 Post subject: Re: L-39 Operators?
PostPosted: 22 Jan 2023, 03:22 
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I think I read somewhere that the original Russian (Soviet) AIs were virtually a direct copy of the Sperry design except for the colors. Sperry's concept was that the moving part of the display is a sphere that is "rigid in space". The result is a display that mimics what a miniature Earth viewed from a great distance would look like on your panel. So when you pitch down you're looking at the top of the sphere (Earth).

This is how an engineer would design an attitude indicator and also what the astronauts use in a spacecraft. But that presentation seems backwards to a pilot so the modern AI flips the presentation "upside down" (from an engineer's perspective) to make it appear more natural. One downside of a "modern" AI is that it cannot properly display pitch angles greater than about 60° up or down. This limitation can be extended to almost +/- 90° on an electronic PFD. Beyond that the display has to "flip" even on an electronic display vs the Sperry gyro which simply continues to rotate in pitch throughout 360° of pitch change.

No doubt the Sperry F-3 is an excellent, robust design if you need an AI that will handle any attitude accurately and reliably. The problem is that it takes some getting used to. I frankly don't understand how some of them ended up in GA aircraft, like the one that took the life of Buddy Holly. They are just complete overkill for a GA aircraft (except if you really need to use an AI for aerobatics). Only military pilots need something like this (and not all military pilots).

You can see Sperry was trying to sell F-3's to GA pilots from their 1950's ad. Same stuff happens today. Pilots want the latest and greatest to be like the "pros" and manufacturers are all too happy to oblige.

I wonder if Mat can tell us if the L-39 AI (if Matt's L-39 has the same type AI as in my photo above) works throughout 360 degrees pitch. If so, I think that's a great design and I don't mind the wings moving to show bank at all. In fact I think it's more logical to have it that way, especially for a recovery from an extreme attitude.

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 Post subject: Re: L-39 Operators?
PostPosted: 22 Jan 2023, 20:22 
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Username Protected wrote:

I think I read somewhere that the original Russian (Soviet) AIs were virtually a direct copy of the Sperry design except for the colors. Sperry's concept was that the moving part of the display is a sphere that is "rigid in space". The result is a display that mimics what a miniature Earth viewed from a great distance would look like on your panel. So when you pitch down you're looking at the top of the sphere (Earth).

This is how an engineer would design an attitude indicator and also what the astronauts use in a spacecraft. But that presentation seems backwards to a pilot so the modern AI flips the presentation "upside down" (from an engineer's perspective) to make it appear more natural. One downside of a "modern" AI is that it cannot properly display pitch angles greater than about 60° up or down. This limitation can be extended to almost +/- 90° on an electronic PFD. Beyond that the display has to "flip" even on an electronic display vs the Sperry gyro which simply continues to rotate in pitch throughout 360° of pitch change.


The L-39's (and most other FSU) ADIs are a huge pain, especially when transitioning from other "normal" ADIs. It is somewhat similar as going back and forth from an HSI to a CDI. The good thing about the L-39 is that it doesn't carry that much gas, so you don't have to fly autopilot-less IMC for too long!


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 Post subject: Re: L-39 Operators?
PostPosted: 22 Jan 2023, 23:25 
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Username Protected wrote:

I think I read somewhere that the original Russian (Soviet) AIs were virtually a direct copy of the Sperry design except for the colors. Sperry's concept was that the moving part of the display is a sphere that is "rigid in space". The result is a display that mimics what a miniature Earth viewed from a great distance would look like on your panel. So when you pitch down you're looking at the top of the sphere (Earth).

This is how an engineer would design an attitude indicator and also what the astronauts use in a spacecraft. But that presentation seems backwards to a pilot so the modern AI flips the presentation "upside down" (from an engineer's perspective) to make it appear more natural. One downside of a "modern" AI is that it cannot properly display pitch angles greater than about 60° up or down. This limitation can be extended to almost +/- 90° on an electronic PFD. Beyond that the display has to "flip" even on an electronic display vs the Sperry gyro which simply continues to rotate in pitch throughout 360° of pitch change.


The L-39's (and most other FSU) ADIs are a huge pain, especially when transitioning from other "normal" ADIs. It is somewhat similar as going back and forth from an HSI to a CDI. The good thing about the L-39 is that it doesn't carry that much gas, so you don't have to fly autopilot-less IMC for too long!

I find switching from a "sky pointer" presentation to a "ground pointer" one is distracting enough, I can't imagine trying to fly or worse yet recover from an upset with a direct sensing attitude gyro like the Sperry or Russian ones..
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 Post subject: Re: L-39 Operators?
PostPosted: 23 Jan 2023, 08:54 
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Username Protected wrote:
I find switching from a "sky pointer" presentation to a "ground pointer" one is distracting enough, I can't imagine trying to fly or worse yet recover from an upset with a direct sensing attitude gyro like the Sperry or Russian ones..


Even on our jet (F-15E), there is a disparity between the HUD ILS display and the ADI ILS display. I'm not 100% sure why it's designed this way, but to fly an ILS using the HUD, you can simply put the flight path marker on the center of the ILS cross (i.e. glideslope and localizer). However, if you use the ADI ILS display, you must disregard the flight path marker and put the waterline on the ILS cross. So, if you align the FPM to the HUD ILS cross, the FPM is not aligned on the ADI.... but the waterline is... and you're good to go. It takes a little bit of thinking, especially for newer pilots, if you're HUD out. I wouldn't have designed it this way, but here we are.


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 Post subject: Re: L-39 Operators?
PostPosted: 19 Feb 2023, 09:01 
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I flew for 2 years as the safety officer on The Patriots Jet Team, a six-ship of L-39's, and we fly low-altitude, formation over-the-top maneuvers with AI-25 engines.

If you need or want to get the training to safely and comfortably fly lower aerobatics, one of the team pilots that taught me L-39 low alt flying has TONS of L-39 aerobatic and formation time and works with private L-39 owners. If you're interested, drop me a note and I'll get you two talking. He's a great instructor, wildly funny and has great stories from his years flying on the Thunderbirds, as well as other cool stuff he's done.

A 10,000' AGL floor has a lot of safety margin for an L-39. Even in the T-38, with 500 knot loop entry/exit speeds, we strive for around 8,000' AGL, with a min floor of 5,000' AGL. You may not want pull over the top at 4,000' to bottom out of your loop at 300' like The Patriots... but you can certainly get below 10,000' AGL with a little guidance and work.

When ferrying the jets to/from airshows, we stay VFR at/below 17,500 and we avoid all icing. We wear flight helmets, but a boom mic with no mask. It depends on your mission statement, but I have no desire to fly in the Flight Levels with an L-39.

If they were my jets, I'd get rid of the RAT in a heartbeat. It will create problems, not help you.

Same with the EGT Limiter (RT-12 system).

Running the AI-25 at greater than 103% will greatly reduce engine life. We avoided that range when at all possible. I've done formation training with owners that regularly run the throttle at the stop (~106%), but I don't know how their long-term engine wear is. I've had owners tell me I'm wrong... but the Patriots owner has about 20 years of owning and operating a fleet of 8 jets. He's got it pretty dialed in.

As mentioned, the canopy is a very weak setup, and must be approached with care... especially if carrying pax. I hate it.

To clarify an earlier post: 0 G flight is limited to 5" for oil limits in the Ivchenko. Negative G is 20" for fuel limits (with a 20" min reset between inverted sets).

It is really difficult to fly good, aesthetically pleasing close formation in this jet. This is due to the AI-25 fan response/lag. The former TBirds and Blues pilots on the team agreed it takes work. I've never heard of a jet where you use speed brakes to help with close formation flying... but it's pretty normal in an AI-25 equipped L-39. I've never flown the 731.

The Patriots had hot seats when I was on the team. The private owners I've given instruction to have all had cold seats. Not a big deal, except I'm not big on close formation flying with someone I don't know.

Combine the engine start sequence and taxiing the L-39... and you've got the best anti-theft system in aviation!

Enjoy the video: https://www.youtube.com/watch?v=DLvQGjlZngs


Last edited on 19 Feb 2023, 13:27, edited 1 time in total.

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 Post subject: Re: L-39 Operators?
PostPosted: 19 Feb 2023, 09:50 
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Jon-

that was a fun video- thanks for posting.

Supposed to pick mine up from Code1 this week (trying to flex around the Chicago wx). I'm pretty excited (my wife less so). just got my helmets finished- put A20s in both (and of courses cool graphics)...

love to hear more. please keep posting!

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 Post subject: Re: L-39 Operators?
PostPosted: 19 Feb 2023, 13:42 
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Supposed to pick mine up from Code1 this week..

Fantastic! Post some pics, if willing.

While at Code 1, tell Buck Wyndham and Nathan that Huggy says hi.


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 Post subject: Re: L-39 Operators?
PostPosted: 19 Feb 2023, 18:34 
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Username Protected wrote:
I flew for 2 years as the safety officer on The Patriots Jet Team, a six-ship of L-39's, and we fly low-altitude, formation over-the-top maneuvers with AI-25 engines.
...

It is really difficult to fly good, aesthetically pleasing close formation in this jet. This is due to the AI-25 fan response/lag. The former TBirds and Blues pilots on the team agreed it takes work. I've never heard of a jet where you use speed brakes to help with close formation flying... but it's pretty normal in an AI-25 equipped L-39. I've never flown the 731.


Jon,

Awesome video, I hadn't seen that one! Being able to fly formation aerobatics is a goal of mine... I'm slowly working up to that, just got my wing card a year ago.


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 Post subject: Re: L-39 Operators?
PostPosted: 19 Feb 2023, 20:58 
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Being able to fly formation aerobatics is a goal of mine...

If you can do it in the L-39, you can do it in anything.


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 Post subject: Re: L-39 Operators?
PostPosted: 19 Feb 2023, 21:41 
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Username Protected wrote:
Jon-

that was a fun video- thanks for posting.

Supposed to pick mine up from Code1 this week (trying to flex around the Chicago wx). I'm pretty excited (my wife less so). just got my helmets finished- put A20s in both (and of courses cool graphics)...

love to hear more. please keep posting!


Congrats, if you ever make it out to Wyoming/Utah let me know!


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 Post subject: Re: L-39 Operators?
PostPosted: 19 Feb 2023, 21:42 
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Username Protected wrote:
Jon-

that was a fun video- thanks for posting.

Supposed to pick mine up from Code1 this week (trying to flex around the Chicago wx). I'm pretty excited (my wife less so). just got my helmets finished- put A20s in both (and of courses cool graphics)...

love to hear more. please keep posting!


Congrats, if you ever make it out to Wyoming/Utah let me know!


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 Post subject: Re: L-39 Operators?
PostPosted: 20 Feb 2023, 11:23 
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what are you guys using for best glide speed? i'm in the books, and the Russian pubs seem to indicate ~157 kts. this seems high-- I thought I read somewhere that 140 was best glide.

Pubs indicate a glide range of about 3mi/1,000' (seems high also). Link to the pub is below. Appendix A, A6-2.

https://ninelima.org/poh/L39/Aero-L-39- ... Manual.pdf

thoughts?

will post pics as soon as i get the bird. today we had a scheds issue, Wed wx looks marginal. fingers crossed fore thurs/fri.

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 Post subject: Re: L-39 Operators?
PostPosted: 20 Feb 2023, 11:52 
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Username Protected wrote:
what are you guys using for best glide speed? i'm in the books, and the Russian pubs seem to indicate ~157 kts. this seems high-- I thought I read somewhere that 140 was best glide.

Pubs indicate a glide range of about 3mi/1,000' (seems high also). Link to the pub is below. Appendix A, A6-2.

https://ninelima.org/poh/L39/Aero-L-39- ... Manual.pdf

thoughts?

will post pics as soon as i get the bird. today we had a scheds issue, Wed wx looks marginal. fingers crossed fore thurs/fri.


Looks like 1.5 nm per 1,000 feet glide; 9.3:1 glide ratio; 6.1 degree glide path.


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 Post subject: Re: L-39 Operators?
PostPosted: 20 Feb 2023, 12:27 
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Username Protected wrote:
what are you guys using for best glide speed? i'm in the books, and the Russian pubs seem to indicate ~157 kts. this seems high-- I thought I read somewhere that 140 was best glide.


About 5 years ago, I asked myself the same question. It's been 3 years since I've flown the L-39, but I think I remember most of what I thought the answer was:

The Aero Flight Manual states that in an engine failure scenario, obtain 260 kmh IAS... which is 140 knots (page 3-5).

But if you look at the chase-through charts (Figure A6-2 on page A6-5), you will see it is 290 khm IAS... which is 157 knots.

So why the disparity? I asked around and even had a Ukrainian AF pilot that I would occasionally email with questions about oddball things that I had thought of. I never did discover the reason for the disparity.

My guess... and it's only a guess... is that the Emergency Procedure uses 260/140 because it assumes the RAT will deploy. And I'm guessing the chase-through chart is based on a clean jet. Would that RAT drag account for 17 knots? Yes, I believe it would.

Bottom line: I had no intention of using the RAT, so I used 290/157.

As I mentioned earlier, if I owned one, I'd remove the RAT and sell it on eBay to someone that wants a souvenir. Another great way to drop a few pounds too. With a Concorde 38 amp battery, I had no need for more electrical power in an engine out glide from 17,500... which should take no more than 20 minutes.

(p.s. the Patriots brief and fly everything in khm, not knots).


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