Yak-9U Performance Questions

[Bullethead]

I admit to being fairly ignorant of the performance details of WW2 Russian planes, compared to what I know about planes from elsewhere.  But some of the books of stats I have, which I trust for my info on other planes, contain some stats on the Yak-9U, and say it was fast as Hell.  OK, I'll believe that.

Problem is, aircraft performance numbers are notoriously subjective.  What condition was the plane it, what was the weather, is the quoted max speed IAS or TAS, and at what alt and weight?  The stat books never mention all that, so it's hard to compare the numbers straight up between planes.  But what my stat books say is that the Yak-9U had a "max speed" in the 430 range, which puts it in the same league as the 51, the Dora, and other late-war speedsters.  

But because the published stats can't be relied on absolutely, nor used for absolute comparisons, this generalization is as far as my stat books will take me.  They don't give me exact details, nor do they cover performance in the regimes common to MA combat.  Which is why I'm posting this, because I really have trouble buying that the Yak-9U was really as fast as it is in AH.  Hopefully, somebody with more knowledge than that quoted from stat books can answer this one way or the other.

For instance, the Yak-9U in AH "cruises" (as in standard flightsim thing of full military power level flight) at considerably more than 300 IAS at the lower alts common to MA fights.  This is about the fastest such speed in the game, and the Yak-9U achieves it with a mere 1600hp and no WEP at all.  Sure, it was a light plane, but so were some of the others in the game that have comparable power but are slower.  So is this really accurate?

Also, what about compression?  In AH, I am easily able to run down diving P51s with the Yak-9U primarily because the Yak doesn't seem to be bothered at all by compression until it gets somewhat over 500 knots.  The ponies hit a wall, even with their then-hightech laminar wing, and the Yak doesn't, even though its airframe seems to be based on pre-war principles.  Is that really accurate?

Like I said, I know next to nothing about Yaks in general and 9Us in particular.  But this extreme speed and, especially, the immunity to compression, strike me as odd based on comparisons to roughly analogous planes.

[whgates3]

according to AH charts Yak-9u is only faster than P-51d from about 14k - 18k, but i've heard those charts are not 100% accurate...

[dtango]

Max level speeds are usually quoted as true airspeed and at critical alt (alt of max engine max output before the drop off of turbo-supercharger performance).

Just a quick scan of the Yak-9U info I'm coming up with the same max airspeeds for the Yak-9U at around 434 mph at 18k.  Interestingly enough flight testing the Yak-9U in AH I get a max speed of about 413 mph TAS at 18k which matches the speed AH charts.

Not sure why you are having trouble buying the speed of the Yak-9U in AH?  If anything I think folks would be saying that the Yak-9U is too slow in AH.  I haven't done the math but my guess is that the 434 mph at 18k doesn't account for compressibility.

Regarding catching P-51's in a dive, the -9U has better acceleration characteristics than the P-51D which could account for how it catches unwary ponies trying to escape this way.  The secret for the P-51 is to get at max level speed for that alt and fly level and let parasite drag do the rest in slowing the Yak-9U down.

I haven't done any analysis regarding compression comparisons between the P-51D and the Yak-9U so can't really comment there.

Tango, XO
412th FS Braunco Mustangs

[Vermillion]

There two basic sets (or styles you might say, different testing methods and purposes of testing) of flight testing for Soviet Aircraft.

For the planes in AH, Pyro has chosen the "worst" for each of them. So if anything the Yak-9U is underperforming.

[Wotan]

sounds like  a VVSwhiner

[Don]

BH:
I recalled reading info about this months ago, and found a web site with the information below about the Yak.


>> The first Yak-9U, where "U" stood for "Uluchshenny", or "Improved" was flown in early 1943 with a Klimov M-107 engine, but problems with the engine led to the loss of the prototype in late February 1943.
Initial production of the Yak-9U used the M-105PF engine until the bugs were worked out of the M-107. The Yak-9U featured a number of aerodynamic improvements, such as moving the oil cooler from the nose to the left wing and placing the cockpit even farther back. A new propeller was fitted.
Fuel capacity was 355 liters (94.4 US gallons). Armament was a 20 millimeter or 23 millimeter cannon firing through the prop spinner, and two 12.7 millimeter machine guns. The result was a highly effective and extremely maneuverable fighter, with a speed of 713 KPH (433 MPH) at an altitude of 5.6 kilometers (18,500 feet). The Yak-9U was regarded as equivalent in performance and handling to its American counterpart, the P-51D Mustang. <<

[MANDOBLE]

Originally posted by Don
and handling to its American counterpart, the P-51D Mustang. <<

Our Yak9U is far better than any P51 variant.

[Bullethead]

Like I said, I can buy the fact that the 9U should be among the fastest planes in the game in level flight.  It was light, it was sleek, and it had a fair amount of power.  And if it was faster than the 51 at certain alts, I can easily see that being the case.  I'm not going to even argue which alt this should happen at, because I have far less data available than those who make the FM

What I do not see, however, it where the 9U gets the ability to run down diving 51s with a head start over a chase of a full sector or more at speeds in excess of 500 TAS at low alts.

When you get into this regime, most planes in the game start having serious compression problems.  Their controls lock up, the whole plane buffets, and your speed stops increasing; you know the story there.  Most planes encounter the problem in the lower-mid 400s, some in the upper 400s, and a few (basically the jets and the 9U) in the 500s.  I'm talking IAS in the typical low-alt MA situation here, where it's pretty much the same as TAS anyway--at strato alts, TAS seems to be way more important as to the advent of compression.

Anyway, with the 9U, the lateness of the appearance of the compression effects gives it the serious advantage of being able to run down every prop plane in the game, even the perks.  The 9U retains the ability to accelerate to higher speeds before it hits the wall, and it retains control longer.  So over the course of a full sector of chase, it will catch up with the 51 (or any other prop plane) because it's able to reach a higher top speed, and it will be able to maneuver enough to get a shot when it gets into range, while the 51 is locked up.  Trust me on this--I've been on both ends of this situation many times.

I just don't think this is right.  I would put my money on the 51 having more resistance to compression because of its laminar flow wing, compared to the 9U"s apparently standard airfoil section.  Both had about the same frontal area and both were fairly sleek, so I figure you have to look at the airfoil section to come up with the real determining factor.  So I would think that the 9U would start getting shockwaves building up on its wings way before the 51 does, and thus have a lower max uncompressed dive speed.

NOTE:  I am not talking about acceleration here.  And remember there are 2 types of acceleration:  on the level and in a dive.  The former is where power-to-weight ratio is important, while in the latter, sectional density becomes very important.  So other things being equal, the lighter plane should accelerate better on the level and the heavier plane should accelerate better in a dive.  IOW, the 9U should beat the pony on the level, but the pony should beat the 9U in a dive.

So not only should the pony pull away from the 9U at the start of the dive, due to better acceleration, but the 9U should hit the compression wall at at lower speed than the 51, so should not be able to catch up at all.  But this isn't the case in AH.  The 9U out-accelerates the 51 in a dive, and has a higher max dive speed as well, so can always run it down.

[MANDOBLE]

Bullethead, do the following:
Go to Dueling Arena and use the first field at the NW corner, from there you can test 11k dives to sea level. Pick up the P51D, Yak9U, La7, P47D30, Typhoon, 190D9 and 190A8, one by one, and try vertical dives trying to recover at 3000 feet, note the speed reached and how easily you exit the pure vertical dive with each plane.

[Bullethead]

Originally posted by MANDOBLE
Bullethead, do the following:
Go to Dueling Arena and use the first field at the NW corner, from there you can test 11k dives to sea level. Pick up the P51D, Yak9U, La7, P47D30, Typhoon, 190D9 and 190A8, one by one, and try vertical dives trying to recover at 3000 feet, note the speed reached and how easily you exit the pure vertical dive with each plane.

I didn't fly the lamer7 because I can't lower myself to that even for test purposes .  But I did all the others except the FWa8, which I hadn't noticed you mention until I started this reply.

Anyway, I took off from this NW field in the DA.  Headed N to get out over the water 11k below.  All planes at 25% fuel, went on autolevel as soon as the wheels left the ground, got to level max w/out WEP, then dove vertically with throttle full open still.  Maintained vert dive until impact w/out trying to pull out, because compression is what I'm interested in.   Results:

Plane  Max Level (IAS/TAS)   Buffet Onset (IAS)   Parts Break (IAS)
51D        315/385                         495                     510
Typh       290/385                         470                     480
Dora       315/395                         490                     N/A
Yak9U     320/410                         500                     N/A
Jug30     300/375                         475                      N/A

The buffet onset has a margin of error of +/- 5 knots or so due to the difficulty reading the gauge while buffeting.  Those planes where parts actually broke off before impact maxed out at about the indicated speed where the parts started coming off and retained that speed into the water.  Those planes that didn't break continued to accelerate to about 600 IAS before impact.

Note the inconsistency between IAS and TAS for the same IAS.  I've seen this in other planes as well, so I'm convinced this isn't me eyeballing the speeds wrong but something funky with the game.

Anyway, as you can see, parts start breaking off the 51 pretty quickly after it starts buffeting, whereas nothing comes off the Yak9U.  Also, the 51 can't get any faster than about 510 IAS at low alt, with or without breaking, whereas the 9U can reach 600+ w/out breaking.  

In the normal MA situation, of course, we're not talking vertical dives, but long shallow dives from 5-10k down to the deck, say 1 sector or so in length.  Picture a jabo pony aborting short of the target and trying to run back home when meeting a co-E 9U, dumping eggs and going full WEP (which the 9U doesn't even have).  In this scenario, the pony can get to 500 IAS and no more or risk breaking up.  OTOH, the 9U buffets but doesn't stop acceleration and doesn' t risk breaking up.  The pony driver knows that he'll break if he doesn't slow down, so he does (and he can because this isn't a steep dive).  The 9U driver can thus easily catch the pony, because buffeting isn't fatal and he can easily get into a non-buffeting regime because this is a shallow dive.  

Is that how this is supposed to work?  I don't think so.  So what I'd really like is for somebody to explain to me why it's aerodynamically possible for the 9U to get that fast and ignore compression that much, when the 51 can't.

[dtango]

Bullethead -

Glad you did the tests.  For better readings film the test flights and then review them via the stand-alone film viewer.  Speeds are displayed digitally in TAS with the viewer.

Regarding the laminar-flow wings of the P-51 I believe the current aeronautic consensus is that laminar flow didn't occur (but for small section near the leading edge) with the P-51's laminar airfoil.  Here's a couple of insightful articles on this:

NASA Laminar Flow Research
Lee Atwood's Commentary (Pres of North Amer. at time of dev of p-51) + Meredith Effect
Leonard "Kit" Carson's Commentary

Regarding compression and stuctural limits at high speeds, I need to go read up on compression before I can make an intelligent comment regarding it all.  Not sure of how one models compression and structural integrity in a sim.

BTW - again my tactic for getting away in the cases you've stated in a P-51 is to try and equalize the rate of closure by shallow diving and then go into level flight (as much as you can with a Yak on your 6) and then let parasite drag do the rest in slowing the Yak down.  Of course this only works in cases where I'm at an altitude where I know I have a higher top level speed vs. the Yak.

Tango, XO
412th FS Braunco Mustangs

[whgates3]

i've read in other threads that the cockpit instruments are slightly inaccurate so as to be realistic, but the film viewer is dead-on. In other places i've read that the critical MACH number of the P-51 was about .75 - very close to the test results.  Hubert Zemke's P-51 broke up when he was knocked about in a storm, presumably at high speeds & the mustang never had the reputation for toughness that other planes did, so the FM seems accurate in that respect.  the purpose of the laminar flow wing was to reduce drag/increase fuel efficiency and i havent read in many places that it had any value in reducing compression.  the straight wings of the P-51 grossly violate the supersonic area rule, as does having the canopy & airscoop in-line w/ the widest part of the wings, while the eliptical wings of the spitfire are less out of whack with that principle (Spit IX had a reported crtical MACH number of .91, much higher than the Meteor). i dont have a 3-view of the Yak-9u, but the Yak-9t wing has a slightly swept leading edge, with the canopy & airscoop between the widest part of the wing & trailing edge (and the airscoop looks smaller)

[Dowding]

'Lamer7'? lol And the P51D isn't a dweeb ride?

[dtango]

Here is a possible explanation why the Yak-9U compresses at a higher speed vs. the P-51D...

Airfoil Thickness Ratios:
1st an airfoil's critical mach number can be seen as a function of it's thickness ratio.  Lower thickness ratios, the higher the critical mach will be.

Airfoil thickness for the P-51D (NACA 45-100 airfoil)-
Root:16%  Tip:11%

Airfoil thickness for the Yak-9 (CLARK YH airfoil)
Root:14%  Tip:10%

So we can see that the Yak-9's airfoil is roughly equivalent and actually slightly thinner the P-51's.

CL and Critical Mach
2nd as an airfoil's coefficient of lift increases (higher angle of attack) the critical mach number also decreases.

Without running the calcs but basing it on the performance characteristics of the a/c the Yak-9U has a lower wing-loading compared to the P-51D.  What this means is the P-51D is going to have a higher angle of attack needed to maintain the appropriate lift vs. the Yak-9U.

--------
So in summary a possible explanation for the higher critical mach numbers for the Yak-9U vs. the P-51 is because (a)the Yak-9U has a slightly thinner airfoil and (b)has a lower angle of attack needed for flight the Yak-9's critical mach number is higher than the P-51.

Tango, XO
412th FS Braunco Mustangs

[Bullethead]

Dtango:

Thanks.  That's the sort of reasonable explanation I've been looking for.  OK, so I'll now buy that the 9U compresses a bit later than the 51D.  But why then does the 9U continue to accelerate while compressed and nothing breaks off, while the 51 kinda sticks a bit above 500 and quickly starts losing pieces?