Exhaust Thrust ????

[Anatoli-Kagari9]

- Intentionally left blank -

Edited August 17, 2015 by jcomm

[Kurfürst]

... and to add to that, there is even coolant radiator thrust (cold air entering into the radiator, warms up, expands, leaves at the back creating some pressure). Its basically a miniature jet engine, only here you do not heat up the air by burning fuel but by cirvulating hot fluid in the radiator. Of course its not that great thrust, but its still thrust and at high speed, where the propeller starts to struggle to convert its motion into thrust it can be more significant and at the very least, it makes up at least some of the drag the radiator system creates. On some designs, such as the P-51 its even claimed to provide a net positive thrust. Though this is debated back and forth, it still shows that how important careful designing of the radiators were to the overall design.

 

There are a few engine charts that show this effect - this one is for the DB 601Aa series, the top line showing the equivalent horsepower of the engine output PLUS the exhaust thrust at 600 km/h. It IS a significant boost, adding the equivalent of about 100 horsepower due to exhaust thrust (Rückstossenergie).

 

Out of interest, this power curve also shows the effect rammed power (Stau, Staudruck on the right, given at different pressures, equivalent to specific speeds/atmoshpeheric conditions). When the airplane travels very fast, it slams a lot of extra air into the supercharger air intake - in effect, its own motion provides a lot of "free" supercharging. That is why aircraft higher rated altitudes at high speed compared to slow speed climbing conditions.

Edited March 10, 2015 by Kurfürst

[Yo-Yo]

And, by the way, the amount of this jet thrust can be individual because of different exhaust nozzles construction.

 

Generally, to add significant amount of jet thrust the nozzle must be converged to convert gas internal energy to kinetic. So, if the nozzles are converged the calculations are not simple but they give quite accurate result.

 

At a glance estimation for 601a shows me that its stack was not definitely jet thrust oriented, though... 40 kg for 1100 bhp is not high enough.

[Yo-Yo]

... and to add to that, there is even coolant radiator thrust (cold air entering into the radiator, warms up, expands, leaves at the back creating some pressure). Its basically a miniature jet engine, only here you do not heat up the air by burning fuel but by cirvulating hot fluid in the radiator. Of course its not that great thrust, but its still thrust and at high speed, where the propeller starts to struggle to convert its motion into thrust it can be more significant and at the very least, it makes up at least some of the drag the radiator system creates. On some designs, such as the P-51 its even claimed to provide a net positive thrust. Though this is debated back and forth, it still shows that how important careful designing of the radiators were to the overall design.

 

There are a few engine charts that show this effect - this one is for the DB 601Aa series, the top line showing the equivalent horsepower of the engine output PLUS the exhaust thrust at 600 km/h. It IS a significant boost, adding the equivalent of about 100 horsepower due to exhaust thrust (Rückstossenergie).

 

Out of interest, this power curve also shows the effect rammed power (Stau, Staudruck on the right, given at different pressures, equivalent to specific speeds/atmoshpeheric conditions). When the airplane travels very fast, it slams a lot of extra air into the supercharger air intake - in effect, its own motion provides a lot of "free" supercharging. That is why aircraft higher rated altitudes at high speed compared to slow speed climbing conditions.

 

Is there something similar for 605? :)

[Crumpp]

And, by the way, the amount of this jet thrust can be individual because of different exhaust nozzles construction.

 

Yes, that is one of the design changes of the BMW801 series. The BMW801D2 combined cylinder's 9 and 10 (IIRC) exhaust nozzles.

 

By separating them and easy redesign of the exhaust nozzles, the BMW801S series was able to add ~100Kg increase in exhaust thrust.

[Crumpp]

Yo-Yo "DA MAN!" who gives us this great models and flight / overall physics modeling that come with them :-)

 

:thumbup:

 

Yes, Thank you Yo-Yo for knowing what you are doing as well as being forthright when mistakes are found.

[NineLine]

Yes, that is one of the design changes of the BMW801 series. The BMW801D2 combined cylinder's 9 and 10 (IIRC) exhaust nozzles.

 

By separating them and easy redesign of the exhaust nozzles, the BMW801S series was able to add ~100Kg increase in exhaust thrust.

 

If they were making changes like that, there must be studies on that very subject, be nice to find those documents.

[Crumpp]

If they were making changes like that, there must be studies on that very subject, be nice to find those documents.

 

I got the report, it is in my files somewhere.

 

They also changed the piston design as part of the BMW801 series. The pistons developed for the S series ended up being incorporated in the production BMW801D2 and are part of the design changes required to raise the manifold pressure to 1.58ata/1.65 ata.

[NineLine]

Now if some of those engineers were still around to see what they are doing with exhaust thrust now :)

 

Half the reason I enjoy flight sims is seeing all the tech changes through the years, I dont claim to understand half of it, but its still cool :)

[Crumpp]

Half the reason I enjoy flight sims is seeing all the tech changes through the years, I dont claim to understand half of it, but its still cool

 

:thumbup:

 

You can read the attached article and see how many design changes had to be made to the Merlin series to raise the Manifold pressure from +12lbs for 5 minutes to +18lbs for 5 minutes.

 

The changes are not excessive or unusual. It is a typical engineering process.

 

When you consider the engine was designed to run at maximum continuous (100% design power) at +4 1/2lbs...+18lbs is four times what it was originally design to produce (400% overload condition).

 

In the BMW801 we see the same thing. The engine was designed to run at 1.15 ata (100% design power) and ended up at 1.82ata or 1.6 times. The BMW801 started with higher compression ratio so even though it seems less stress than the Merlin series, it is not.

 

I attached a curve from the BMW801D2 development. It represents power capability at 1.65ata.

 

Both engines were developed to their potential and required a lot of engineering changes to get there.

merlin-lovesey.pdf

[Pilum]

Glad you found the posts on exhaust thrust interesting Jcomm! And yes, as the figures show it affects the performance a lot so it is certainly important to have it accounted for when estimating performance :smilewink:

 

Speaking of that, I am now really looking forward to the release of the Spitfire in DCS to see how exhaust thrust will be added there. Having both studied historical data and flown a number of different flight sims over the years one has become accustomed both to certain absolute levels of performance and also certain performance relationships between different aircraft (e.g. climb) so it will in this context be very interesting to see how the Spitfire is modeled and performs in DCS I think.

[Anatoli-Kagari9]

Glad you found the posts on exhaust thrust interesting Jcomm! And yes, as the figures show it affects the performance a lot so it is certainly important to have it accounted for when estimating performance :smilewink:

 

Speaking of that, I am now really looking forward to the release of the Spitfire in DCS to see how exhaust thrust will be added there. Having both studied historical data and flown a number of different flight sims over the years one has become accustomed both to certain absolute levels of performance and also certain performance relationships between different aircraft (e.g. climb) so it will in this context be very interesting to see how the Spitfire is modeled and performs in DCS I think.

 

Yes, and indeed DCS together with PS1 and now PSX, have become my study sims.

 

I'd love to have something similar for gliders, and would certainly have good use to it at our airclub :-) Lot's of youngsters and even grownups would profit from a good / sound soaring sim... Maybe one day we can get a glider, from the "D" day, in DCS :-)

[Pilum]

I'd love to have something similar for gliders, and would certainly have good use to it at our airclub :-) Lot's of youngsters and even grownups would profit from a good / sound soaring sim... Maybe one day we can get a glider, from the "D" day, in DCS :-)

 

Yeah that would be nice! Trouble with glider sims is you miss the seat of the pants "push" to center in thermals no? How ever would you model that? :smilewink:

[Yo-Yo]

Sometimes engineering researches must be conducted to obtain missed data. I will try to unveil some backstage investigation work.

 

 

Some interesting points can be obtained from real tests of FW-190A with BMW-801D.

What we know about it? First of all, its real L/D polars from wind tunnel tests and (L/D) max - 12. Presuming some trim losses let's get it as 11.5.

We know its exact mass for the climb test - 3827 kg

And we can use the known graph for BMW 801D with jet thrust on it to obtain incoming energy.

 

Below is a simple math that can provide climb rate for a small set of input parameters.

Let IAS for climb is constant and equal to 290 kph and we will use Gamma parameter equal to sqrt(Rho_H/Rho_SL).

 

And let's try Prop_eff = 0.78-0.8 that is very typical for the most prop for the climb mode and what is the result of prop model calculation in DCS.

 

THen N = N_shaft * 0.736 * Prop_eff + P*g, N_shaft is shaft power in PS or hp (neglecting the slight difference) and P - is thrust in kg.

Climb L/D can be presumed as (L/D)max because any function near its extremum changes very slightly and best climb IAS is very close to max (L/D) IAS.

 

TAS = IAS/Gamma

N_flight = m * g / K_max * TAS

Vy = (N - N_flight)/(m*g)

 

We also neglected small flightpath angle that could be used in this way in one or two iterations:

 

N_flight = cos (Theta)* m * g / K_max * TAS

Vy = XXXXXXXXXXXXX

Theta = asin (Vy / TAS),

 

but this addition is not very necessary as it is smaller than known sources of errors in measurements of input parameters.

 

The calculations give us about 17 m/s for the first blower speed, 13.5 m/s for the second blower speed and 4 m/s at 9 km.

Bingo...

So, we did not bend any parameter to match the answer in the end of a book. Then we can use these parameters to recalculate for any necessary mass, for example.

Or try to evaluate Flugmechanic-L documents whether they can be presumed exact references.

THis is one of 190A5 calculated performance

http://www.wwiiaircraftperformance.org/fw190/190a5-climb-20-10-43.jpg

 

or http://www.wwiiaircraftperformance.org/fw190/fw190-a8-climb-13nov43.jpg

 

All we can see that the climb rate is lower than in the trial. 15 and 11.5 m/s. What could be a reason? Let's switch off the jet thrust and the calculations

for the certain mass give us almost exact results - 15 and 11 m/s.

 

Of course, the difference is quite small but always the calculated values are less than the trials results, so the suggestion about jet thrust is very plausible...

 

Now, we are confident that the basic parameters are correct and now we can examine Dora 9, where the possible diferences will be greater.

 

Let's try this diagram

 

http://forums.eagle.ru/attachment.php?attachmentid=111673&d=1421674460

 

Dora 9 having the same induced drag has less non-induced drag, so its K is greater and some estimations give it about 13.4.

 

 

Let's start from 3000 rpm - the caculations for 4070 kg with jet thrust and the new K gives 19.5 m/s at SL that matches very well with this graph

http://www.wwiiaircraftperformance.org/fw190/Fw_190_V53_climb.jpg

 

 

THis is Flugmechanic-L data

http://www.wwiiaircraftperformance.org/fw190/fw190d9-climbchart-flugmechanik-24-3-45.jpg

 

As we remove exhaust thrust and compare SL climb rate using prop efficience difference between power ratings and rpm we can obtain from our prop model:

3250 mw50 22.6 m/s 21 m/s

3000 16.3 m/s 15 m/s

2700 12.7 m/s 11.5 m/s

(all comparative results is for 4300 kg)

We see small difference and if we reduce the basic efficience for 3000 Steig-Kampfleistung for 2% we will have

3250 mw50 21.9 21

15.8 15

12.2 11.5

Then return the jet thrust

3250 mw50 24.6

3000 17.6 (bingo to the trial results for 4070 kg - 18.9 vs 19 m/s)

2700 13.5

Leaving the old efficiency will give for 4300 kg

25.3

18

13.9

 

Though this perl hunting is really within +-10%.... but anyway - if the test flights for Dora in DCS gives good match for known

3000 rpm power rating, the FM itself does the rest of a job to get about 25 m/s for 3250rpm-MW50.

[Echo38]

Have I ever told you how much I freakin' love you, Yo-Yo?

 

: )

[Kwiatek]

Hmm loking from Yo-Yo calculation jet trust give D-9 at least 4 m/s better climb rate?

 

21 m/s for D-9 4270 kg at 3250 RPM MW 50 ( German chart) vs 25.3 m/s for D-9 4300 kg ( Yo-Yo calculated)?

 

It looks like very high jet trust boost.

 

BTW something i dont buy also with FW 190 A climb rate.

 

German data claimed for A-3 ( 3850 kg) 16 m/s ( A-3a -export version) - 16.5 m/s ( German version)

 

For A-5 ( 4000 kg) - 15 m/s. I think these data are not calculated but rather come from test flights? It corensponded well casue A-5 got the same engine power but was 150 kg heavier then A-3 thats why it got worse climb rate.

 

What a reason would have Germans to put on all Fw 190 charts data ( speed or climb) with not full performacne ( or without calulated jet trust?)? Dont understand these.

Edited March 10, 2015 by Kwiatek

[Crumpp]

Hmm loking from Yo-Yo calculation jet trust give D-9 at least 4 m/s better climb rate?

 

21 m/s for D-9 4270 kg at 3250 RPM MW 50 ( German chart) vs 25.3 m/s for D-9 4300 kg ( Yo-Yo calculated)?

 

It looks like very high jet trust boost.

 

And what does your math say?

[Kwiatek]

My math says where is logic which ordered to German put on all Fw 190 variants charts data without full performacne of all these planes ( jet trust?) ?

 

Something is not right here for me.

[Solty]

And what does your math say?

Math does not give an answer for a question "What a reason would have Germans to put on all Fw 190 charts data ( speed or climb) with not full performacne ( or without calulated jet trust?)?"

 

And no, that can't be calculated. So...My question is.

 

Where is the note that states that exhaust thrust was not included in the test/document?

 

As far as I know, there is no such note. So that should not be overinterpreted.

 

 

III Reich was looking for high performance planes. "Wunderwaffe" So not including it (extra thrust) and showing the plane as weaker than it suppose to be is not only not realy justified but just wierd.

 

Fw190D9 is still a heavy aircraft. Much heavier than Spitfires and Bf109.

 

Fw190D9 empty weight is 3,490 kg while loaded Bf109G6 is 3,148 kg... so how on earth... is it possible that D9 is capable of greater ROC than Spitfire and Bf109?

 

Not to mention that Spitfire is actually more aerodynamic.

 

Standard 22m/s seems very accurate measurment of ROC to me. Taking into consideration that Spitfire MkXIV had 25m/s climb rate and Bf109K4 had 24m/s.

[Kwiatek]

Still i think that actual ( real ) performance between D-9 and P-51 is nicely done in DCS. I feel that D-9 got at low alts some climb advetange over P-51 but these is not overdone. Even if we compare known charts D-9 got 4 m/s better climb rate then P-51 at low alts ( which is huge difference). So maby P-51 got also better climb rate in game? ( casue i dont feel like D-9 got 8 m/s better climb rate then P-51 in DCS).

Edited March 11, 2015 by Kwiatek

[Solty]

Still i think that actual ( real ) performance between D-9 and P-51 is nicely done in DCS. I feel that D-9 got at low alts some climb advetange over P-51 but these is not overdone. Even if we compare known charts D-9 got 4 m/s better climb rate then P-51 at low alts. So maby P-51 got also better climb rate in game?

 

Last time I checked it it was 18m/s at SL.

[Kwiatek]

At 100% fuel?

[Crumpp]

My math says where is logic which ordered to German put on all Fw 190 variants charts data without full performacne of all these planes ( jet trust?) ?

 

Kwaitek,

 

You are a smart guy. Re-read Yo-Yo post. It is explained very well.

 

The only data that the DCS Dora does not match is the ONE calculated performance report. That calculated report does not include exhaust thrust effects.

 

All the flight tested data at a lower power setting; Steig u Kampfleistung; the DCS Dora matches almost perfectly.

 

That Focke Wulf FLIGHT TESTED data is 10% better than Focke Wulf's calculated performance.

 

There is NO flight tested data of an FW-190D9 climbing using MW-50 at 1.8ata @ 3250rpm.

 

When you run the math it all agrees...The Dora in DCS climbs as it should with the additional power of 1.8ata @ 3250rpm.

 

What do you propose ED does? Not model the aircraft with any additional power, just limit it to the lower power setting in a climb?

 

:noexpression:

[Crumpp]

Last time I checked it it was 18m/s at SL.

 

At 4175Kg.....

 

And it should be 19.5 m/s at sea level at 4070kg....

 

http://www.wwiiaircraftperformance.org/fw190/Fw_190_V53_climb.jpg

 

Which means the FM is spot on and additional climb performance is the result of additional power at 1.8ata using MW-50.....

 

Understand? It is just physics man!!!

Edited March 11, 2015 by Crumpp

[Crumpp]

Math does not give an answer for a question "What a reason would have Germans to put on all Fw 190 charts data ( speed or climb) with not full performacne ( or without calulated jet trust?)?"

 

Let's see.

 

If you ran a company that guaranteed a 10% performance variation by contract meaning the customer does not have to pay for any aircraft that fall below that 10% margin....wouldn't you do something to ensure you could always meet that requirement?

 

I would.

 

:smilewink: