F-16 thrust to weight ratio question?

[Shadoware]

In what conditions F-16 has a T/W ratio greater than 1?

 

T/W > 1 means it could climb 90° without losing speed?

[TLTeo]

1) The thrust of the F110 is ~29000 lb, so basically, when the jet is fairly lightly loaded.

 

 

2) No. This is a common misconception and it drives me nuts. Climbing straight vertically and not losing speed would mean having enough thrust to exceed both the jet's weight and its drag.

[Swift.]

In what conditions F-16 has a T/W ratio greater than 1?

 

T/W > 1 means it could climb 90° without losing speed?

 

Remember that drag still exists even when going upwards, so whilst yes a T/W>1 would mean you could accelerate from standstill, if you have any forward velocity that thrust will also be fighting drag.

[GGTharos]

In what conditions F-16 has a T/W ratio greater than 1?

 

T/W > 1 means it could climb 90° without losing speed?

 

No, because thrust depends on air pressure. Increasing altitude means decreasing pressure which means decreasing thrust. So even if your TWR is >1 (And for some aircraft easily exceeding weight + drag and thus accelerating going straight up!) at some altitude, it will drop well below that as you climb.

 

So the thrust thing is like this: More speed = more thrust (up to a point, after it reduces again) so the thrust of the engine at say 500kts will be higher than the bench rating (the 29000lbs you read about). The bench rating itself is 'on the bench' and you lose a bunch of thrust (10%-20%) so an aircraft taking off from a stop has that much less thrust than the bench rating.

Edited July 18, 2020 by GGTharos

[Quid]

In what conditions F-16 has a T/W ratio greater than 1?

 

T/W > 1 means it could climb 90° without losing speed?

 

You'd need the aircraft engine's measured installed thrust charts to figure that out if you were talking in real life. GGTharos explains why, but if you want an example, there are a few unclassified, unlimited distribution documents which lay it out well from an application standpoint; General Accounting Office report B-260367, "F/A-18E/F will Provide Marginal Operational Improvement at High Cost" includes several performance charts on the F/A-18C and E/F, to include installed engine thrust at different altitudes and airspeeds on report page 81. There is also a study that was done on the F-16A's F100-P-200 engine by the 26th International Congress of the Aeronautical Sciences, "A Flight Thrust Deck for the F100 Turbofan of the F-16 Aircraft," which includes a chart showing the aircraft's installed thrust curves at different altitudes from .6 to 2.0M on page 5.

 

On aircraft with a dynamic inlet, the chart will look different, more like a bow, since there will be a pronounced decrease in thrust (primarily at low altitude) as the inlet shape changes to prevent supersonic air from striking the fans, followed by a re-growth until MaxQ at the given measured altitude as the airflow/speed continues increasing, continuing the ram-air effect through the smaller area. For modern turbofans, the engine's thrust might also be programmed to decrease at higher speeds for engine life considerations.

 

As TLTeo and Swiftwin9s explain, however, you also have to accommodate for drag as well as weight, so even if you take a sample from a given chart where the jet should be at 1:1 or greater, it doesn't actually mean the plane will be able to climb without losing speed. The engine must be able to overcome the weight of the aircraft AND its drag to do so. Add to this that the air is getting thinner every second of climb, the engine's thrust is lowering in the process and even if the jet can maintain speed momentarily, it won't be able to for very long.

 

There is an easy way to figure it out for DCS: Test it! Grab your Viper, give it a number of different payloads, and figure it out. Then you'll know, and knowing is half the battle.

Edited July 18, 2020 by Quid
Spacing

[Shadoware]

1) The thrust of the F110 is ~29000 lb, so basically, when the jet is fairly lightly loaded.

 

 

2) No. This is a common misconception and it drives me nuts. Climbing straight vertically and not losing speed would mean having enough thrust to exceed both the jet's weight and its drag.

 

 

The drag is weaker when you are slower so there must be some speed and altitude that would enable you to keep speed while climbing!

 

If you are next to stall speed in about 7000 feet couldn't you point up and continue moving(with no payload and half fuel for example)?

[GGTharos]

At 7000' maybe, but not for long. Installed thrust is lower than bench thrust by some 10-20%. It will exceed bench thrust as you increase speed, but you've got to be moving. Really good info from Quid as well.

[Quid]

The drag is weaker when you are slower so there must be some speed and altitude that would enable you to keep speed while climbing!

 

If you are next to stall speed in about 7000 feet couldn't you point up and continue moving(with no payload and half fuel for example)?

 

 

The problem is as you get close to stall speed, you have to increase angle of attack to maintain level flight. You're increasing both lift and drag. Can you power out in a gentle climb from next to stall speed if you light the cans at that altitude? Absolutely! Can you stand the jet on its tail and climb straight up while accelerating? No. As you pitch the nose up, you're massively increasing the drag and by the time you get to 90, you're too slow, the engine isn't putting out nearly the thrust you need to climb.

 

 

 

Just for grins I tested it with a clean Viper at about 5000lbs of fuel. Starting at ~6800 ft and 150KTAS, I plugged in the burner and pitched the nose up towards 90. The jet didn't quite make it to 90, and the max altitude I got was 9528 feet at 38KTAS as the jet had already begun forcing the nose back down. If you start a lot faster and a little lower, you can probably get the jet to maintain speed for a brief moment (i.e., less than a second or two) before it starts decelerating. I started at 1.03M and 1500 feet, put the jet on its tail, held 630KTAS for about a second between 8000 and 8800 feet, then continued decelerating, albeit slowly, achieving a maximum altitude of 42715ft before falling out.

[Hairysteed]

On 7/18/2020 at 6:08 PM, GGTharos said:

No, because thrust depends on air pressure. Increasing altitude means decreasing pressure which means decreasing thrust.

LOL No! Just... no! 
Ever heard of Newton's 3rd law? How do you suppose rockets get their thrust in the vacuum of space?

[Spurts]

Because rockets are not jet engines.

[GGTharos]

3 hours ago, Hairysteed said:

LOL No! Just... no! 
Ever heard of Newton's 3rd law? How do you suppose rockets get their thrust in the vacuum of space?

 

By using rocket fuel which includes the oxidizer and fuel ... whereas a jet engine gets it oxidizer from ... air.   Which must be rammed into the intakes.   Congrats on your physics fail

[karasawa]

On 7/18/2020 at 4:21 AM, Shadoware said:

In what conditions F-16 has a T/W ratio greater than 1?

 

T/W > 1 means it could climb 90° without losing speed?

It seems you don't know that thrust drops over altitude?