Mysterious tach markings

VNugget

suck squeeze bang blow
Joined
Dec 4, 2002
Posts
809
Total Time
1700
I was looking at pictures of Russian acro types, and found this Su-31 cockpit on A.net. See those stick-on markings with the decreasing numbers on the tach? What in the hell are they? I've been racking my brain over it, and the best I could come up is that it's TBO... heheh. If you want more than 3 hours out of the engine, better pull it back a little. If you only run at idle, you can make it all the way to 9 hours, if you're lucky ;) TrafficInSight suggested that it might be the maximum number of turns of a spin recovery before you gotta bail out... who knows :D

So... any ideas?
 
Last edited:

ISaidRightTurns

Well-known member
Joined
Apr 24, 2005
Posts
154
Total Time
1500
I would say quick reference to power settings for manuver entry. Loop, must be a 4. Inverted pass, must be 2......
 

U-I pilot

Relaxation....
Joined
Dec 3, 2002
Posts
537
Total Time
1500
Looks like you got it.....if you look at the pattern it has those numbers on there for each maneuver.....
Pretty neat.
 

FN FAL

Freight Dawgs Rule
Joined
Dec 17, 2003
Posts
8,573
Total Time
7,000+
U-I pilot said:
Looks like you got it.....if you look at the pattern it has those numbers on there for each maneuver.....
Pretty neat.
Yes...set max manifold pressure and then use the prop lever for power changes, that's what I would do.
 

VNugget

suck squeeze bang blow
Joined
Dec 4, 2002
Posts
809
Total Time
1700
Nah, I don't think it's that. First of all, the numbers on the pattern card are just the sequential number for each maneuver. Second, why would one not just adjust power with the throttle, like the airplane was designed? Especially on a boosted engine. Also, I've never done hard acro (or any acro on a constant speed prop) but from what I've heard, they just leave the throttle wide open for most maneuvers. Third, even if they were power settings for different maneuvers, they wouldn't be sequential, and perfectly spaced out right down to idle RPM.

Gahh.... I wanna know, dammit!
 

erj-145mech

Well-known member
Joined
Aug 13, 2002
Posts
1,071
Total Time
1350
I think that thats the power setting for the given "G" loading, ie. lower setting for a higher G, to keep from over stressing the airframe.
 

ISaidRightTurns

Well-known member
Joined
Apr 24, 2005
Posts
154
Total Time
1500
erj-145mech said:
I think that thats the power setting for the given "G" loading, ie. lower setting for a higher G, to keep from over stressing the airframe.
I would like to change my answer.
 

A Squared

Well-known member
Joined
Nov 26, 2001
Posts
3,006
Total Time
11000
erj-145mech said:
I think that thats the power setting for the given "G" loading, ie. lower setting for a higher G, to keep from over stressing the airframe.
I don't think that's very likely. The Tach is graduated in percent of max RPM

100% would be 2950 (crank speed) so you're saying ath at 9g, you would have to have the engine turning at 737 RPM or less (the 9 is stuck at 25%) in order to keep from overstressing the airframe? If it was some sort of airframe limitation don't you think it would be printed permenantly *in* the tach instead of stuck outside with little stickers?
 

sbav8r

I do deny them my essence
Joined
Mar 10, 2003
Posts
185
Total Time
low
If it was a structural load limit, I would imagine that it would be a airspeed markings not tach.
 

erj-145mech

Well-known member
Joined
Aug 13, 2002
Posts
1,071
Total Time
1350
If you notice, all of the range markings are home made, and there are no slippage marks on the bezels.

If you were at 8 g's, where would you have the power set at?

I have expereince on French, English, Polish, Czech, Japanese and Russian foreign manufacturered aircraft that are experimental/exhibition category in this country, and this is typical of the markings. Instrument shops in this country can't remark the dials because they are marked with radium paint to facilitate viewing in dark conditions. The radium is now considered haz-mat, and most shops will refuse to open the cases on the instruments.
 

sbav8r

I do deny them my essence
Joined
Mar 10, 2003
Posts
185
Total Time
low
I'm not following you on the power setting for load factor thing. I could see a power setting for various maneuvers, but for G's. The amount of available load factor will be a factor of airspeed. Granted power settings will control airspeed, but having fixed power settings for G's just doesn't make sense to me. During aerobatics the exchange of potential and kinetic energy (altitude and airspeed) will be varying widely. One power setting would certainly not protect the aircraft from over stress in a straight dive compared to the same aircraft at straight and level. It just seems to me that using the markings for G's would only work in one pitch attitude. Everything above or below that pitch attitude would change the airspeed and therefore the amount of available G's.

Example: 2200 RPM at straight and level produces about 120 kts. At 120 kts I coulld briefly pull 6 G's, anything above that would increase the angle of attack beyond the critical angle of attack and the wing would stall. If I took the same aircraft in the same power setting, except this time, I put it in a dive. That same power setting would now produce much more airspeed and therefore have more available G's. It could overstress the airframe if the aircraft can't handle a structural load limit of more then 6 G's. Same power settings on both, but due to the changes in potentail energy for kinetic energy, the aircraft can now very easily exceed 6 G's.
 

erj-145mech

Well-known member
Joined
Aug 13, 2002
Posts
1,071
Total Time
1350
That was the justification behind my opinion.

SBAV8R, do you have an opinion? Or are you just criticizing?
 

sbav8r

I do deny them my essence
Joined
Mar 10, 2003
Posts
185
Total Time
low
no and no.

I wasn't criticizing, just trying to use a little deductive reasoning to answer the original question. If it came across that I was being rude in my post, that was not my intention. I'm as curious as everyone else as to what the answer is. If your answer is right, then great. I'm just trying to test the various theories to see what works.

No harmful intent.
 

A Squared

Well-known member
Joined
Nov 26, 2001
Posts
3,006
Total Time
11000
erj-145mech said:
If you were at 8 g's, where would you have the power set at?
well, just off the top of my head, I'd think I would have a fairly high power setting, because in an 8g turn, I'd be bleeding energy pretty rapidly. But then I've never flown an unlimited aerobatics plane.

I have never heard of power setting being a limiting factor for acceleration loads. Why would it be. If you're pulling 8 g in a turn, you're pulling 8g that's what the wings feel, they don't feel the power setting.

Additionally, RPM in generally used very little in power settings compared to throttle setting. Remeber this is an engine with a constand speed prop. Manifold Pressure usually plays a much more sgnificant role in settng power. So why aren't there little "g" stickers on the maniold pressure gauge?


So, let's accept for a moment that you are correct. look at the stickers, they are evenly spaced, so the nmissing "2g" sticker, would be at about 92 percent. meaning that if you pull 2 gs, you gotta pull the power back in a 2g turn, or you risk damaging the airplane, according to your theory. Now, I dunno about you, but every plane I've flown is capable of 2 gs, 60 degree bank is 2g in a level turn. It's within the normal operating envelope for every certificated airplane. And I've never heard the you have to reduce power in a 2g turn to avoid airframe damage. So, you're saying that a power setting that any normal or utilitary category aircraft will withstand with no difficulty, will endanger the structire of an SU-31, one of the toughest unlimited aerobatics planes made?
 
Last edited:

Lilah

Well-known member
Joined
Feb 4, 2005
Posts
141
Total Time
3.1415
I agree, I don't think we've hit the nail on the head yet.

My guess, and it's a BIG reach, is that it could be a limitation related to rpm vs. altitude in meters, or perhaps some other cooling issue. Could it be settings for the "gill shutters" in front of the cylinders?

I'm doing some searching, but don't have any other ideas...yet.

Lilah
 
Last edited:

ISaidRightTurns

Well-known member
Joined
Apr 24, 2005
Posts
154
Total Time
1500
Lilah said:
Could it be settings for the "gill shutters" in front of the cylinders?
I like that idea, but it seems that would be overly complex for engine cooling. Having 10 different settings of cooling gills.

SOMEONE MUST KNOW!!!
 

VNugget

suck squeeze bang blow
Joined
Dec 4, 2002
Posts
809
Total Time
1700
Well, if it should be any valuable clue, there's pictures of other cockpits without those markings, so they're not essential.
 

Lilah

Well-known member
Joined
Feb 4, 2005
Posts
141
Total Time
3.1415
I asked a friend who has a CJ-6; and he didn't know, so I've tried going to the source. We'll see if Luiz knows english (my portugese isn't that good), and replies to my email....

Lilah
 

FN FAL

Freight Dawgs Rule
Joined
Dec 17, 2003
Posts
8,573
Total Time
7,000+
Lilah said:
I asked a friend who has a CJ-6; and he didn't know, so I've tried going to the source. We'll see if Luiz knows english (my portugese isn't that good), and replies to my email....

Lilah
I spent about 45 minutes last night reviewing a lot of what I could search on www.google.com about su's and tachometers.

The Re-occuring theme seemed to be tach generators and tach's running backwards although I couldn't find a direct source saying this or that.

Maybe Luiz put a different tach generator in and now the tach indicates reverse...his home brew markings could be in percentages of 100% RPM.
 

A Squared

Well-known member
Joined
Nov 26, 2001
Posts
3,006
Total Time
11000
FN FAL said:
I spent about 45 minutes last night reviewing a lot of what I could search on www.google.com about su's and tachometers.

The Re-occuring theme seemed to be tach generators and tach's running backwards although I couldn't find a direct source saying this or that.

Maybe Luiz put a different tach generator in and now the tach indicates reverse...his home brew markings could be in percentages of 100% RPM.
THat's the first theory that makes any sense to me
 
Top