insight into low drag problem of Jeff?

[Tiger-II]

Anyone else struggling to land? It doesn't slow down easily with idle and airbrakes. Only throwing the nose around gets it to slow down. Problem is I don't know whether "problem" is with Jeff or with other jets I've tried like mirage and hornet. Hornet is a bit greasy for me too but manageable. I can control the E bracket and the fpm. Jeff not so much. Mainly because I can't get it to decelerate.

 

It feels good. Don't try and dive for the ground! :thumbup: If > 10 deg. nose down pitch, you'll need speed brakes just to hold speed.

 

If you approach at the correct angle, it all lines up (pitch/power/speed).

 

I drop gear at 200 kts, and flaps at 180. These offset the drag of brakes, so I put those away, idle, at 10 degrees nose low, then when things start to line up for the runway, I start to level out and add a tiny bit of power.

 

Just coming over the numbers I pop the speed brakes to get it to sink onto the runway.

[Tiger-II]

Anyone else struggling to land? It doesn't slow down easily with idle and airbrakes. Only throwing the nose around gets it to slow down. Problem is I don't know whether "problem" is with Jeff or with other jets I've tried like mirage and hornet. Hornet is a bit greasy for me too but manageable. I can control the E bracket and the fpm. Jeff not so much. Mainly because I can't get it to decelerate.

 

Here's a good video of takeoff clean, and landing.

 

 

It's hard to see but he modulates the speedbrakes during the turn to final.

[shaHeen-1]

Ah so that's one thing I'm doing wrong. Not using flaps. I thought I wouldn't really need them cuz plane is fly by wire.

[Tiger-II]

Ah so that's one thing I'm doing wrong. Not using flaps. I thought I wouldn't really need them cuz plane is fly by wire.

 

I had a similar problem when I first flew the F-18!

 

You do *NOT* need flaps for takeoff, only landing.

[tweet]

This thread has really become a dog's breakfast. There's so many apples to oranges comparisons used for conclusions and total lack of understanding of complex subjects and interactions that I really don't know where to start.

 

Let's begin with the GR video that was trying to test "ferry range". Just like in real life it is a meaningless term unless clearly defined and the method of determining it fully explained. Like I said earlier, Cap's methodology and vague sense of how to test it make anything he comes up with a fairy tale. In his defense he was onto one really messy way of testing but didn't build a test that could have developed the two pieces of information he needed for a valid test: optimum altitude for each specific weight and maximum range mach for each specific weight. We require a busload of data and testing to obtain them.

 

Any decent FMS (Flight Management System) or the flight manual performance charts would give us everything but we don't have either one. There are some generalities that will help us to develop a testing method to find the numbers we need. Unfortunately they will only reduce the range of altitude, mach, and weight combinations we would need to test.

 

The aircraft configuration has to be uniformly defined across aircraft in order for the test to be valid. I'd suggest maximum internal fuel, maximum external fuel, with/without additional pylons, and with/without wingtip stores. Pick one or test all of them but be consistent. The test mission must be built for no wind.

 

The standard unit for comparing fuel efficiency by range is NAM/# fuel burned. Nautical Air Miles per pound of fuel burned. NAM eliminates wind effects that are a separate calculation and change everything. We can probably get by starting in the air at 26,000' and mach .75 and 2000# below max fuel for the heavy/twin engine fighters and 1000# below max fuel for single engine jets. Unfreeze the sim and burn 1000# while maintaining mach and altitude and keeping track of distance flown. Using autopilot and autothrust in the ones that have it will help. Distance traveled/1000 will give the number needed. Plot it and move up 2000' and do it again. Put the NAM/# in a spreadsheet.

 

The following points will start at 26,000' and .75 but 2000# lighter and do the data runs until you're out of gas or see obvious changes in the wrong direction (increasing NAM/#). Then you do both sets all over again for each mach .76 up through about .90. That's a lot of data but then you'll know which altitude and mach combination gives the best range for a specific weight. You then build best range profile by flying the lowest NAM/# data points in the ferry range test.

 

Careful looks at the data will probably give you a good idea what data points can be skipped. For instance, forget high altitudes when the jet is heavy. You can also probably start at high machs when the jet is heavy and work backwards until you see a deterioration in the numbers. You can skip the low altitudes for a light weight jet and maybe the higher machs. More will become obvious as testing goes on.

 

The maximum range test itself will have a profile, altitude and mach, to follow based on weight. Climbing every 1000# burned and adjusting mach if necessary is how to fly the test. To take the guess work out of how to fly the descent profile it's probably safe if we pick a fuel remaining figure, say 1500-1000# (heavy/twin fighter - single engine fighter), to begin a descent to 1000' MSL at the final mach until reaching 350 KIAS then maintain 350 to 1000' MSL.

 

The departure and climb profile is more problematic. Military power will be best if the jet can do it but I expect some will need burner. There will be enough variation in an optimal climb schedule that one will not work for every jet. It will introduce inaccuracy but something like 350 KIAS until initial cruise mach then fly that mach to the initial altitude would be at least consistent but will help or hurt some.

 

That will settle the ferry range question but it will not:

 

• tell us if there is a FM error in Jeff or any other aircraft.
  

How did I pick these methods for developing data for use in the test? This ferry range question is one very narrow investigation of one performance factor - maximum range. In general maximum range is:

 

• not going to be supersonic or transonic eliminating every mach above about .92, maybe lower.
  
• going to be obtained at a lower altitude for an aircraft when it is heavy than when it is light after burning fuel.
  
• Max range mach will slowly decrease with fuel burn.
  
• faster than L/D max which is essentially the speed for maximum endurance, minimum clean, best glide range, and best angle of climb.
  
• associated with an aircraft specific AOA just like L/D max.
  
• going to be higher for an aircraft with external stores/pylons than a clean jet.
  

That roughly puts us in the .73 - .90 mach range. Testing might show the need to get outside that range or go to lower altitudes for some jets (especially the Hawg).

 

There's a lot of nonsense and just poor thinking in this thread. Shall we?

 

There is no valid comparison between a straight wing jet (A-10C), a swing wing jet (F-14), a pure delta jet (M-2000), or modern blended fuselage/wing jet (JF-17, F-16, F-18, SU-33, etc) that can lead one to reasonable say that one FM is flawed. Even within the category it's not valid. Not liking the results, not believing, not feeling, not understanding is not objective or valid. Period.

 

The same engine in a different airframe will have different characteristics with respect to performance. That this has to be said is mind boggling. It's such a silly claim that I'm not even going to embarrass anyone by explaining why.

 

Using performance data from a similar type has no validity. This should be obvious.

 

L/D max is the point at which it takes more power to maintain a slower speed. Simply because the fuel burn keeps going down, less power required, at a data point means that aircraft was not operating below endurance speed. No, it is not because drag is wrong, AOA something, something, or the FM is necessarily broken.

 

Guys, you're trying to imagine something by looking a several disconnected, and most of the time unassociated, bits. Max range is obtained at a very specific speed or mach and altitude for every aircraft that is associated with a weight and atmospheric conditions (temperature and wind). This range of speeds nonsense is only confusing you. So are the graphs for different aircraft and engines as well as GR's video.

Edited January 27, 2020 by tweet
left out a bullet point

[AeriaGloria]

Great thoughts tweet hit the nail on the head.

 

And Magic,

i think..the best way is to do some wind tunnel or fluid simulation of the shape jf17 model to get the true charateristic.

 

There are many online if you search for it on google. Here is one https://www.researchgate.net/publication/257930533_ANALYSIS_OF_AERODYNAMIC_CHARACTERISTICS_OF_ORIGINAL_AND_MODIFIED_WING-STRAKE_ARRANGEMENT_OF_JF-17_THUNDER_AIRCRAFT.

 

There are many others especially about the DSI that can be viewed for free https://pdfs.semanticscholar.org/6149/e7a1c63808954c08c1eb8edd2bf11e37b328.pdf?_ga=2.219273689.1595100595.1580173083-1151606871.1575066591

 

https://download.atlantis-press.com/article/25841202.pdf

 

I have been trying to view this one in full, I might pay the $25 to see it https://arc.aiaa.org/doi/10.2514/6.2011-920

 

 

Do also note the engine is not identical as RD-33,

RD-33 50.0 kN (11,230 lbf) thrust dry, 81.3 kN (18,285 lbf) afterburning.

 

RD-93 49.4 kN (11,100 lbf) thrust dry, 85.3 kN (19,200 lbf) with afterburner

 

It produces less dry thrust and more wet thrust, in addition there is a service life change, the RD-33 has a 4,000 hr life and RD-93 has a 2200 hr life. The engine has been around for 40 years, whatever happened with the dry thrust(increase in efficiency or not) it seems they sacrificed service life for wet thrust. There could be even an argument that the standards for measuring service life has gotten more realistic since the 80s, but there is a lot more then just gearbox location change:)

 

Shaheen the flaps are an interesting topic, in external view or FCS MFD page you can see there is no auto flaps at any AOA/speed, it as all manually done with the switch. I find it very interesting that unlike planes like F-18/F-5/Flankers who automatically deploy TE and LE flaps for best L/D ratio the JF-17 is only using LE flaps. I always wondered if the strakes helped get around that and help it not need auto TE flaps at slow or high alpha

Edited January 28, 2020 by AeriaGloria

[tweet]

Thanks Aeria. I was concerned I'd over done it.

 

I know we're off topic from the thread but why not. At this moment it really doesn't matter. I may have personal perspective worthy of consideration on the LES/auto-flaps and FCS.

 

There is a FCS and then there is a FCS. The one in the JF-17 is the bare bones lightweight fighter version. It's even simpler, but probably more capable, than the F-16A. It's not even three axis. The aerodynamics of the JF are decades ahead of what was possible in the A model Viper. It also has a mature engine unlike the A. It isn't encumbered by the weight of integrated advanced systems and doesn't carry a huge payload. It's a day fighter with far more capability. In every way it seems to be a better F-16A than the Viper was new. It even has a radar missile and precision guided weapons that weren't available to the Viper for a long time. That's where a lot of Jeff's performance comes from.

 

I remember the very first time I got a chance to crawl all over a Viper. I was at Andrews for a static display in the Tweet. It was the final open house for the Thud, F-105, in 1980. A young major IP from McDill had come up single ship in a clean A with the grand total of 4.5 hours on the airframe. He was parked next to me. He gave us the grand tour. (I had a young 1LT along for this boondoggle. He was a new IP in my flight who had one last training event so he could get released from the Buddy IP program. That was our wild card to get this plum. John was later to be with me as a flight examiner at PIT and went on to fly the F-15 in Germany before being killed in an avalanche in Austria. This was a memorable cross country.)

 

It had flown from McDill to DC on internal fuel, ~5000#+, and still smelled new. The dzus fasteners on maintenace panel weren't even scratched. The paint was perfect and the major was loving life. It was that era's light weight fighter which reminds me a lot of Jeff. It was an IR missile slinging, iron bomb hauling sports car when it actually had an engine stuffed in it instead of in the engine shop waiting for parts. Forty years later the base model has a few more gadgets.

 

Back to the FCS and LES/auto-flaps. Jeff doesn't need them like the F/A-18 does. The JF airframe is far more conventional than it might appear. The Hornet isn't it needs auto-LES/flaps controlled by the FCS to have conventional handling qualities. Watch videos of a Hornet coming aboard the boat. The slab is flapping around like a hummingbird's wings. That isn't the stick actuator stirring the stick. It's the FCS keeping the thing blue side up and in the groove. Wait until later blocks of the JF when they start adding goodies and growing the airframe and stuff a big engine in it so it can haul around another 10-15,000#. Then it may need a fully capable three axis FCS and LES/auto-flaps to get close to the performance it has now. The Block 50 Viper today may be Jeff's future tomorrow.

[Tiger-II]

@tweet: Sir, you rock! I'm sorry about your friend.

 

Your posts sum up why I get annoyed reading these "sore loser" posts by everyone saying the Jeff is "wrong" because it's eating their favorite "Blue" jet for breakfast.

 

There are so many facets to ACM yet they want to dive into dissimilar combat as if it proves anything.

 

As for your comments on FCS, AFAIK the only axis that requires it is in pitch, which Jeff has. On the F-16 I think it only has a complete FCS because it can, and helps with control, but isn't necessarily required. I'd wager the F-16 only needs pitch stability as well.

 

Doesn't the F-16 maintain TEF UP even at high alpha? As I understood it the FCS blends in TEF for roll control with gear up, and auto flaperons with gear down (again to aid with roll control at low speed).

 

Jeff has auto LEF which is far more critical, and something F-5 doesn't have (it has auto flaps which camber the whole wing; if the flaps are up so too are the LEF).

 

This comment nailed the whole discussion though: Jeff is a decades newer design.

[shaHeen-1]

Just used the flaps for landing and they help quite a bit. However with landing gear out it still cuts through air like butter compared to other modules I'm used to. Maybe it's a weird combo of sleek design and fly by wire that makes it feel strange. God knows. In other news I whacked 6 planes in two sorties with Jeff today. Hornets and vipers really don't want to turn away lol. They are mesmerized by Jeff. Want to have a closer look.

[Tiger-II]

Other aircraft in DCS exhibit too much drag from what I can see. The F-5 is by far the worst.

 

As the video I linked shows, the pilot is modulating the speedbrakes as he comes around the corner to final.

 

She's sleek for sure.

 

An often overlooked part of the equation is residual thrust; as Jeff is light then even idle thrust will be relatively high, and with a light/clean jet could be significant.

[tweet]

Tiger I have no issue with the way the F-5 flies. I haven't flown it in several months so maybe they've tweaked something. I'll have to give it a go.

 

I flew the T-38 many, many laps around the suns ago. The F-5 feels and performs like a heavy T-38 as I remember the Talon. It was a clean sports car where the F-5 needs lots of heavy, drag producing externals to go anywhere outside the traffic pattern or to do it's mission. Friends and acquaintances flew it and that was their unanimous general description. It's a hot ride clean but underpowered hauling externals. That alone would suggest it will bleed energy rather quickly when you load it up. Auto-flaps will only increase the energy bleed when they deploy. Flaps and leading edge slats/flaps do give a wing a slightly higher alpha capability but always at the expense of increased drag. The drag increase is never proportionate to the amount of flap/slat deployment. You always get more drag than lift. I would expect an experienced F-5 jock to fight it with the flaps in manual and only use them to assure a kill or to try to dig himself out of a really bad place. I could be wrong on that but I'd personally not want to have to keep track of auto-flaps or have them deploy when I had other ideas.

 

A jet with three axis FCS uses all of the primary and secondary flight control surfaces to keep the jet under control and to do what the pilot asks it to do. The first thing I learned from flying these types of systems is that I could know in general what the system was doing with each control surface but it is impossible in every situation and mode. I also learned that I didn't need to know. The jet either gives you what you want or it doesn't. When it doesn't there's a reason and I need a plan B. A very good piece of advice to keep in mind is that not everything the systems do is intuitive or will seem to make sense from a conventional perspective. The more extreme and unstable the airframe's natural aerodynamics the more this phenomenon will be. The F-16, F-18, F-35, F-117, and F-22, among others, need a very good FCS to even be controllable.

 

 

As far as people whining about a module meeting preconceived notions is concerned: welcome to DCS. It's always been that way and always will be. Most who fly here only know what they read, hear, or imagine. I've often had folks get really pissed insisting on some nonsense that flat ain't so. I usually let them stew if they're resistant to learning. If I don't know something, I'll admit it and try to find what I need. I'll quickly admit when I'm wrong. It happens.

[Tiger-II]

I'm not saying that loaded the F-5 won't suffer, but taking just a clean aircraft and performing idle descents shows something's not right (descent distance is measurably less than it should be). This manifests also in top-end all-out speed, and affects other parts of the flight envelope as well.

 

+1 for manual flaps!

 

Yes... the more interesting aircraft as you mention are unflyable without FBW.

 

A major limitation of simulations are they are inherently stable, which causes problems when trying to model the edges (or beyond) of the handling characteristics.