Jump to content

Engine Thrust levels in flight model (too high at moderate RPMs)


viffviff

Recommended Posts

Just tried VNSLs. The thrust developed at lowish RPMs (40 to 60 range) appears way too high. As a rough guide, 85% RPM should allow a VNSL with nozzle angles in the 40 to 55 range ('ish!)

 

[edit: or it could be that the drag caused by nozzle deflection is not high enough, or a combo of both]

 

 

Also, on the ground, the thrust seems too low at low RPM - or maybe the rolling friction is too high.

 

 

Maybe the thrust model needs adjusting? More on the ground (or less rolling resistance) and less airborne. Not checked it a very high (VSTOL) RPMs yet...


Edited by viffviff
Link to comment
Share on other sites

When you write :"seems" and "appears" it tells me you basing this on personal opinion or expectations, not Real Life (RL) data. If you have RL aircraft information that contradict the behavior of the aircraft within DCS, please PM one of the testers with the information. New forum rules does not allow the posting of technical documents.

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

Link to comment
Share on other sites

When you write :"seems" and "appears" it tells me you basing this on personal opinion or expectations, not Real Life (RL) data. If you have RL aircraft information that contradict the behavior of the aircraft within DCS, please PM one of the testers with the information. New forum rules does not allow the posting of technical documents.

 

85% RPM for VNSL AUTO, 95-100% RPM for VNSL STOL is real life data.

Link to comment
Share on other sites

85% RPM for VNSL AUTO, 95-100% RPM for VNSL STOL is real life data.

 

You're right, it is (real life data), but the (unrealistically) high nozzle angles required to control AOA (at those power settings) points to an issue with the flight model. Either the thrust is too high for the RPM, or the aerodynamic drag from the deflected nozzles is too low (or a bit of both). I suspect it might be inaccuracy with the drag caused by deflected nozzles, since that is probably the hardest (for RAZBAM) to get any good data for.


Edited by viffviff
Link to comment
Share on other sites

I also suspect the drag model for nozzles aft is a bit low as well. The AV8B is a very draggy beast (huge intakes, big bubble canopy etc) and when you select idle in level flight for example, especially at low speed, the flight model feels more like a conventional slippery jet (like an F5 or similar).

 

 

It is a shame since a lot of the model is really well done, and since VSTOL is one of the features which makes the AV8B so great, it would be good if RAZBAM were able to get better aero data and tweak the model to be more realistic. Though since it is a current jet, I think aero data is going to be very hard to come by..


Edited by viffviff
Link to comment
Share on other sites

85% RPM for VNSL AUTO, 95-100% RPM for VNSL STOL is real life data.

 

You're right, it is (real life data), but the (unrealistically) high nozzle angles required to control AOA (at those power settings) points to an issue with the flight model. Either the thrust is too high for the RPM, or the aerodynamic drag from the deflected nozzles is too low (or a bit of both). I suspect it might be inaccuracy with the drag caused by deflected nozzles, since that is probably the hardest (for RAZBAM) to get any good data for.

I don not see what you guys see.

Looking at the 2008 A1-AV8BB-NFM-000, paragraph 7.6.5.2 Variable Nozzle Slow Landing

page7-26 to 7-27 and Figure 7-6. I do not see anywhere that says that 40°-60° of nozzle and 85% RPM you can maintain 8° to 10° AOA. I see where it says the -408 engine should use 80% to 100%. I see it says if your using less than 90% to use autoflaps. I see where it says to maintain AOA with the nozzle.

 

What engine is the VMAT-203 training syllabus procedures base on? What version of the AV-8B N/A? What year.

 

Looking at the VMAT-203 FSG version 3.0 from 2012, FAM-2-62 to 2-64 I do not see that neither. Maybe I miss it but what versions of the AV-8B is this for? Being a training unit, is this for the TAV-8B or does it apply to other version?

Edited by mvsgas

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

Link to comment
Share on other sites

there's not a massive difference I don't think between the 2 ac for VNSLs. The twin sticker is a bit heavier (about 2000 lbs) so would be just like a single seater carrying a bit more fuel. For the -105 engine, the typical power used would be 75% at low fuel weights, for the -108 (which we have in the model) it is 10% higher - it isn't developing more thrust at 85% than the -105s 75%, it is just the thrust vs RPM relationship is different. 80 - 95% is the range of RPM recommended; going towards the upper end if heavy, hot and high.. For practise at typical pattern training fuel weights (circa 4k lbs and below), 85% is the norm since it is a good compromise and makes the pattern repeatable. If you set a different power every time, it would be really hard to master when learning! (I guess same goes for learning the the DCS flight model too!).

Link to comment
Share on other sites

there's not a massive difference I don't think between the 2 ac for VNSLs. The twin sticker is a bit heavier (about 2000 lbs) so would be just like a single seater carrying a bit more fuel. For the -105 engine, the typical power used would be 75% at low fuel weights, for the -108 (which we have in the model) it is 10% higher - it isn't developing more thrust at 85% than the -105s 75%, it is just the thrust vs RPM relationship is different. 80 - 95% is the range of RPM recommended; going towards the upper end if heavy, hot and high.. For practise at typical pattern training fuel weights (circa 4k lbs and below), 85% is the norm since it is a good compromise and makes the pattern repeatable. If you set a different power every time, it would be really hard to master when learning! (I guess same goes for learning the the DCS flight model too!).

 

So if we start with wrong information, how can we tell if it is right or wrong or if it even matters

Looking at the pocket manual for DCS, the mention the F402-RR-408

The A1-AV8BB--NFM--000, mentions the F402--RR--406A, F402--RR--406B, F402--RR--408,

F402--RR--408A, or F402--RR--408B.

Where are you getting this -105 and -108?

You mention the 85% Engine RPM, but the manual says 80% to 100%, nozzle as require to maintain 8° to 10°AOA. The only thing I see that matter is the AOA. The RL engine won't give you the same power at the same RPM every day on all conditions, they will always vary. So you would not be able to use the same RPM every time.

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

Link to comment
Share on other sites

I apologise, I think I used the export designations for the engine, I should have used the US desigs instead. the 108 is equiv to the 408 etc, the 105 was the smaller engine...

 

 

You're right, airfield altitude will make a difference (and so will ac weight), but the fact that 80% (the lowest of the RPM quoted) requires very high nozzle angles for the VNSL points at the model being over-powered or 'under-dragged'.

 

 

 

The same can be said for its conventional level flight characteristics - it just goes too fast for its RPM (and very reluctant to slow down - it is a very draggy jet in RL - which may be a drag modelling issue).

Link to comment
Share on other sites

I don't see the problem. Good luck convincing RAZBAM.

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

Link to comment
Share on other sites

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...