Author Topic: I really wish Pyro would chime in soon (Read 1507 times)

Montezuma · « **Reply #45 on:** June 18, 2002, 10:21:54 PM »

Quote

Originally posted by Sachs
I would rather see the 190A5 speed fixed then new planes added as well. SO i second that motion.

You get nothing.

Hristo · « **Reply #46 on:** June 19, 2002, 12:19:08 AM »

Someone get Hernán Cortés here !!

MANDOBLE · « **Reply #47 on:** June 19, 2002, 02:51:02 AM »

Hristo, Cortés read my advice in the zuma's footer, he has better things to do

RRAM · « **Reply #48 on:** June 19, 2002, 03:03:12 AM »

I don't know what he said...ignore list is SO useful!

Wilbus · « **Reply #49 on:** June 19, 2002, 04:11:24 AM »

Indeed it is Ram

Montezuma · « **Reply #50 on:** June 19, 2002, 10:39:47 PM »

I think you luftwhiners are on Pyro's ignore list

RRAM · « **Reply #51 on:** June 20, 2002, 11:30:04 AM »

Up again for Pyro, or someone at HTC to say something

Also, please don't forget to look at the rollrate chart posted in this thread

HoHun · « **Reply #52 on:** June 22, 2002, 09:05:38 AM »

Hi Dtango,

>As far as I can tell though using the engine BHP figures that RRAM provided there might be reason to make a change in the SL speed of the A-5. This assumes that RRAM's data is very close to what HTC is using for their flight models.

Have a look at the exhaust thrust: At sea level, the BMW801D provides 1740 PS in high speed flight, and another 75 kp of direct exhaust thrust. At 21000 ft, it's 1445 PS plus 110 kp of direct thrust.

Regards,

Henning (HoHun)

dtango · « **Reply #53 on:** June 22, 2002, 09:33:27 PM »

Thanks Henning. I saw the exhaust thrust on RRAM's charts but didn't think it would factor in that much. Here is the analysis one more time - factoring in ~150 lbs exhaust thrust at SL and ~250 lbs exhaust thrust at 21K.

(1) Drag = Thrust approximations
@ SL with BHP 1775, .8 Prop Eff., exhaust thrust ~ 150 lbs
If max level speed = 335 mph, D= 1740 lbs
If max level speed = 351 mph, D= 1667 lbs

@21K with BHP 1425, .8 prop eff., exhaust thrust ~ 250 lbs
if max level speed = 407 mph, D= 1300 lbs

(2) Parasite Drag Coefficient Approximations
Using the above estimates:
SL max level speed 335 mph, CD0=.028
SL max level speed 351 mph, CD0=.025
21k max level speed 407 mph, CD0=.027
[EDIT: just to be clear, Di was factored in and I applied the CD = CDi + CD0 equation to derive CD0]

(3) Expected Max Speed at 21k using CD0=.025
Assuming 1300 lbs drag, max level speed at 21k = 431mph

(4) Expected Max Speed at 21k using CD0=.028
Assuming 1300 lbs drag, max level speed at 21k = 407mph

CONCLUSION:
Exhaust thrust makes an impact. Looking at the physics CD0 ~ .027-.028 seems to be in line while the CD0~ .025 is not. The math here says that if the SL topspeed of the A-5 is 351mph, then the expected 21k topspeed should be 431mph.

Fellas looking to change the SL speed of the A-5, sorry for leading you guys on. I should have factored in the exhaust thrust. It looks like HTC has done their homework as usual. Like I said, I was more concerned that I was missing something. I should know better since I've asked others to remember to factor such things in as exhaust thrust into the equation.

This was a good exercise however. I didn't have the BMW801D charts and what I hope this serves as notice for others that you can't forget to factor in (a) alt impact on engine performance, and (b) other not so obvious variables such as exhaust thrust.

Tango, XO
412th FS Braunco Mustangs

Naudet · « **Reply #54 on:** June 24, 2002, 05:48:05 PM »

@dtango: Were did you get your propeller efficiency from? Or is it just a guess?

And a question about thrust:

thrust = engine output x prop efficiency

this should be roughly right, right?
And if this is so, can you calculate speed with thrust data without the incertainty of prop efficiency included in the calculation?

dtango · « **Reply #55 on:** June 24, 2002, 07:52:16 PM »

Naudet:

Keep in mind the calculations are all approximations. Let me just say that anything having to do with thrust calculations can be incredibly difficult.

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Were did you get your propeller efficiency from? Or is it just a guess?

Prop efficiency ~.80-.85 is about the best that any prop is going to get and is a common "standard" factor to use for approximations. If you wanted to be exact prop eff. varies based on a lot of variables. Constant speed props enables maximum prop efficiency across varying velocities. For my calcs, I'm assuming that at the highest velocity for the A-5 the prop is still at max efficiency.

Quote

thrust = engine output x prop efficiency

Actually thrust = engine output x prop eff / velocity

Thrust can be an incredibly difficult value to calculate. I'm using a simplified equation based on the above:

Thrust = ((Engine BHP * prop. eff. * 550) / velocity) + exhaust thrust

I'm assuming it is safe to use this equation for this case since:
(a) I'm approximating,
(b) we are analyzing at a specific point - max level-speed at some assumed max Engine BHP that we can see on BMW801D charts
(c) the velocity of the aircraft is a given and,
(d) we are analyzing a level-flight situation

Tango, XO
412th FS Braunco Mustangs

Hortlund · « **Reply #56 on:** June 25, 2002, 09:35:32 AM »

Dtango, just one question. (since I am in wayy over my head when it comes to calculations like this)

What would the result be if you calculated with a speed of 416mph at 21k instead of the 407 mph figure you have used.

HoHun · « **Reply #57 on:** June 25, 2002, 10:09:45 AM »

Hi Naudet,

>And if this is so, can you calculate speed with thrust data without the incertainty of prop efficiency included in the calculation?

While we're at it, have a look at

http://jagdhund.homestead.com/DoraCharts.html

and specificially

http://server3003.freeyellow.com/jagdhund/FW190D-9/D9speedWeb_01.jpg

That's a newly posted chart that shows a speed of 611 km/h for the Fw 190D-9 using 1925 HP ("Schwebeleistungen im Schnellflug").

Regards,

Henning (HoHun)

dtango · « **Reply #58 on:** June 25, 2002, 01:33:14 PM »

Hortlund:

Quote

What would the result be if you calculated with a speed of 416mph at 21k instead of the 407 mph figure you have used.

I'm at work and don't have access to my spreadsheets that I have set up to run the calculations so I can't easily run the calcs for CD0. However I can easily do some other calcs.

If top max-level speed = 416 MPH at 21K, then with a CD0=.025 (351mph SL max speed) the expected top speed at 21K would be 445mph vs. the 431mph.

For CD0=.028 (335 mph SL max speed) the expected top speed at 21K would be 420mph.

Just by rough comparison this still demonstrates that CD0~ .027-.028 which again points out that CD0~.025 does not meet the physics sanity check.

There are other questions/replies posed from the "other" thread that I would like to address. I'm at work and heading off into another meeting (this one 4 hours long) so give me some time and I'll respond to them when I get home.

Tango, XO
412th FS Braunco Mustangs

dtango · « **Reply #59 on:** June 25, 2002, 07:46:24 PM »

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What would the result be if you calculated with a speed of 416mph at 21k instead of the 407 mph figure you have used.

I've run the calcs. I goofed a little on trying to make the "easy" calcs in the post above. (I used the CD0 values and not CD). I tried to rush the calcs and missed a detail

. At any rate if the max speed at 21K was 416 mph (vs. 407 mph) then we find the following:

(1)@21K drag=thrust approximation:
D=1277 lbs (vs. 1300 lbs)

(2)Form Drag Coefficient:
@ 21k and 416mph topspeed, CD0 = .0263

(3)If SL topspeed = 351mph, @21K = 438 mph (vs. 431 mph)

(4)If SL topspeed = 335mph, @21 = 410 mph (vs. 407 mph)

So if we assumed 416 mph was the top speed at 21k, it really doesn't change the complexion much.

At any rate the 407mph is what RRAM's chart shows vs. 416mph factored in for compressibility as already previously noted by others.

Mr Ripley:

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What's striking is that now Dtango's calculations based on guesses outrun the factory speed chart presented..

I can't see the logic there..

How does Dtango's exhaust thrust impact the original chart presented? One player presents a calculation estimate of flight performance that overruns the original chart?

I'm sorry you missed the logic. An aircrafts top-end level speed is mostly determined by form drag of the aircraft which is roughly constant for a given airframe. I'm using this aerodynamic fact to do a check against the factory speed chart. The calculations show that the form drag based on the charts have significant variance. This doesn't equate aerodynamically because form drag should remain roughly constant. If the speed at SL is 351mph like it says on the chart then based on the form drag the speed at 21K should be 431mph not 407mph as it indicates on the chart.

Hortlund:

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And he is saying he is using approximations, surely that must have some effect on the calculations? I really know too little about this stuff, but I find it very hard to believe that the factory charts are so wrong.

The "approximations" I'm using are accepted common aerodynamic methods used for performance analysis. I warn that they are approximations in the since that they are not to the 99.99% precision and not what you would use for basing flight models off of. I would be surprised if they aren't in the 90-95% accuracy range though.

Are the factory numbers wrong? I can't say for sure. However I'm trying to point out that if you're going to base flight models on physics then the physics has to equate. In this case we find that the chart has a discrepancy vs. the physics. We've already corrected for alt. variations on engine output as well as exhaust thrust. I'm at a loss to explain the discrepancy. The other possible explanation could be prop. efficiency. If prop eff. ~.66 at 407mph then you could end up with what the charts indicate. This doesn't make sense however since we are talking about a constant speed prop and you would expect a much higher efficiency. I'll see if I can double check this.

Tango, XO
412th FS Braunco Mustangs