Thrust Reverser vs Drag Chute

The only one who likes to argue (even when his point is completely demolished) is you.

Thrust reversers are more efficient than chutes. End. Of. Story. Im not going to argue basic physics with someone who keeps arguing his point even when he is presented with the facts that prove the opposite in this very thread. Here is a hint though:

To be fair, I didn't really see any official numbers, e.g. the improvement in runway length needed for a Tornado to land compared to a parachute solution.

Furthermore, does this definition of efficiency take into account extra maintenance, weight, fuel consumption, risks, etc.?

IMHO, the fact that only Viggen and Tornado have those doesn't really indicate that the gains over a parachute are worth it, unless you have some rather special requirements to meet.

I can understand why Viggen has it, but in the case of Tornado, it seems like an overkill without some official data/studies.

Not true

Much like drag chutes, thrust reversers are much more effective at high speeds, partially because of greater engine power but mostly because of the higher airspeed difference between the relative airflow and the engine output.

Umm, I don't see Drag chutes generating reverse movement.

Thrust Reversers use the Engine's thrust to Generate Force, not the Aircraft's Airspeed to Generate Drag.

Does this definition of efficiency take into account extra maintenance, weight, fuel consumption, risks, etc.?

 

No it doesn't. Typically when you talk about efficiency you talk about one thing specifically, in this case the ability to brake.

It is a more complicated system, but is so the Harrier's VTOL ability for example. Does that mean that the Harrier is useless?

Stop comparing apples to oranges, for pure braking power, you can't beat the Tornado or the Viggen.

Not true

Much like drag chutes, thrust reversers are much more effective at high speeds, partially because of greater engine power but mostly because of the higher airspeed difference between the relative airflow and the engine output.

Wrong. Thrust reversers generate the same amount of thrust, depending on the throttle setting.

No it doesn't. Typically when you talk about efficiency you talk about one thing specifically, in this case the ability to brake.

 

It is a more complicated system, but so the Harrier's VTOL ability for example. Does that mean that the Harrier is useless?

 

Stop comparing apples to oranges, for pure braking power, you can't beat the Tornado or the Viggen.

My mistake, I thought this was a serious discussion, not a sports car forum.

My mistake, I thought this was a serious discussion, not a sports car forum.

Very funny :thumbup:

The Draken had a braking chute. The Viggen had a thrust reverser instead for two reasons:

1. Shorter turn around and easier ground handling. If you're planning to get your four strikes in during the short time window where the Soviet Baltic Fleet is crossing the Baltic, you really don't want to get held up by some conscript repacking a chute, nor get stuck in your parking because the tow truck is broken down.

2. 800 meter strips were not feasible with a chute, especially not with strong crosswind. You also can't go around and try again after deploying the chute.

Here's the landing roll chart from the J 35F SFI (and keep in mind a/c 35 is several tons lighter than a/c 37), which should hopefully address the lack of sources complained about earlier in the thread:

tl;dr for the Swedish impaired: realistically you need at least 1300m, probably more.

Umm, I don't see Drag chutes generating reverse movement.

 

Thrust Reversers use the Engine's thrust to Generate Force, not the Aircraft's Airspeed to Generate Drag.

When did i say a chute create a reverse movement?

If u can find any sourse that states that reverse thrust is NOT much more effective at high speed, then please feel free to educate me.

Wrong. Thrust reversers generate the same amount of thrust, depending on the throttle setting.

Wrong? Like i replied to skatezilla, if u can find any source at all stating thrust reversers are not more effective at high speed then please show me.

If u dont know anything on the subject, dont tell people they are wrong just because you think so.

I know I am behind in the technical data and knowledge department, it being a car guy I know a little bit if stuff.... don't think reverse thrustsrs generate any drag, just reduce momentum by producing an opposing force yeah?

Thrust Reverser:

Re-Directs the Engine Thrust, which is normally used to Push the Aircraft Forward, in the Reverse Direction.

Thus Engine Force is reversed, and is now being use to oppose the Airframe's Landing Inertia.

Airspeed has Zilch to do with it really, as it's the Engine's Thrust Providing Reverse Energy, Not Airspeed Providing Drag to Reduce Energy.

You're Initiating Opposing Force, Not Reducing Current Force via Drag.

Reverse Thrust is used to Save Brakes, Land on Short Runways, Land on Ice/Wet/Snow'd Runways, to Stabilize Aircraft after touch down, as you wont be using the gear brakes and causing the tires to lose traction due to brakes causing additional friction between rubber and runway.

The Engine Thrust itself would be influenced by airspeed, but it's not the same as a drag chute.

yes, its the ejected mass of the air that is doing the majority of the work, like if you tossed a bowling ball in the same direction you were coasting on ice.

there is a certain measure of drag created by the breakup of airflow created by the reversed plume but it isnt the primary operating principle

Thrust Reverser:

 

Re-Directs the Engine Thrust, which is normally used to Push the Aircraft Forward, in the Reverse Direction.

 

Thus Engine Force is reversed, and is now being use to oppose the Airframe's Landing Inertia.

 

Airspeed has Zilch to do with it really, as it's the Engine's Thrust Providing Reverse Energy, Not Airspeed Providing Drag to Reduce Energy.

 

You're Initiating Opposing Force, Not Reducing Current Force via Drag.

 

Reverse Thrust is used to Save Brakes, Land on Short Runways, Land on Ice/Wet/Snow'd Runways, to Stabilize Aircraft after touch down, as you wont be using the gear brakes and causing the tires to lose traction due to brakes causing additional friction between rubber and runway.

 

The Engine Thrust itself would be influenced by airspeed, but it's not the same as a drag chute.

That is fine in a vacuum but in air the turbofan's increase thrust by pushing on the air. The effective exhaust velocity is increased when the flow of the air is opposing the thrust of the engine and we know the force generated by a rocket/jet engine is:

F = dm/dt x Ve where dm/dt is the rate of flow of mass and Ve is the exhaust velocity.

https://en.wikipedia.org/wiki/Specific_impulse#Actual_exhaust_speed_versus_effective_exhaust_speed

A paper by nasa investigating this very thing: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730008046.pdf

Here is another paper that investigates this (amongst other things such as crosswind): https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790006878.pdf

This paper here doesn't address the points but it illustrates why this is such a complex topic and not so black and white like all you guys have been making it out to be: https://saemobilus.sae.org/ds/10.4271/660736/ < Sorry unless you have access via an institute you will not be able to access this.

Drag Chute's Drag is Linear to the Aircraft's Speed, below a certain speed, they are useless.

Also btw, Air resistance is proportional to the square of airspeed NOT linear.

Im not going to argue basic physics

Lol basic physics, this quite clearly falls under fluid dynamics, if you've ever seen a fluid dynamics paper it is complicated, hardly simple physics.

 

You're Initiating Opposing Force, Not Reducing Current Force via Drag.

Um, both reverse thrust and drag are force and both work in the same direction. Btw, you don't have to capitalize every noun you use.

I understand what you are trying to say, but as JNelson pointed out, it's not that straightforward. Also consider that with thrust reverser you have to wait until the nosewheel touches the ground (though there are aircraft that can activate reverser in the air) and later wait for the engine to spool up from idle. It's not full reversre thrust all the time until the aircraft stops. Additional stopping power at low speeds is not that beneficial as the brakes can usually cope with that quite well, it's the slowing from high speeds that requires the most energy. You need three times as much energy to slow down from 200 knots to 100 knots as you need to slow from 100 knots to 0.

Anyway, thanks renhanxue for the input, finally a factual post in this thread. :thumbup:

Another interesting bit of information on the commercial aviation, according to Airbus documents, on a dry runway reverse thrust reduces landing distance by only 3%. That increases to 24% in the worst case scenario of ice paved runway. Wheel Brakes are really your main source of stopping power. http://www.airliners.net/forum/viewtopic.php?t=754495#p10874267

People might be talking past each other less if here if they knew the difference between

efficiency- the amount of output b that you get per unit a.

and

effectiveness- The degree to which x performs task y.

For flight performance:

Chutes are more efficient, they give more braking performance per weight, volume, cost, maintenance hour.

For landing roll and ground handling performance:

Reversers are more effective at bringing a plane to a stop in the shortest distance without special airfield equipment or procedures.

That's your tradeoff. Reversers are rare in fighters because flight performance is usually the top priority, and for a given amount of braking performance reversers are far too inefficient to compete with chutes, tailhooks, or getting a bulldozer and making the runway longer.

If minimum landing roll and high sortie rate from that minimum landing roll field are mission critical though, the effectiveness of reversers can make them worthwhile despite their inefficiency. Ease of runway operation is also a significant benefit. Clearly the people who had to pick up the chutes from the Draken had some design input for the Viggen.;)

People might be talking past each other less if here if they knew the difference between

 

efficiency- the amount of output b that you get per unit a.

 

and

 

effectiveness- The degree to which x performs task y.

 

 

For flight performance:

 

Chutes are more efficient, they give more braking performance per weight, volume, cost, maintenance hour.

 

For landing roll and ground handling performance:

 

Reversers are more effective at bringing a plane to a stop in the shortest distance without special airfield equipment or procedures.

 

 

That's your tradeoff. Reversers are rare in fighters because flight performance is usually the top priority, and for a given amount of braking performance reversers are far too inefficient to compete with chutes, tailhooks, or getting a bulldozer and making the runway longer.

 

If minimum landing roll and high sortie rate from that minimum landing roll field are mission critical though, the effectiveness of reversers can make them worthwhile despite their inefficiency. Ease of runway operation is also a significant benefit. Clearly the people who had to pick up the chutes from the Draken had some design input for the Viggen.;)

Well and succinctly put. Much better than my TL:DR attempt. You are correct I got effectiveness completely mixed up with efficiency which seems obvious now in hindsight. For the shortest landing roll, thrust reversers are more effective than drag chutes.

Lol basic physics, this quite clearly falls under fluid dynamics, if you've ever seen a fluid dynamics paper it is complicated, hardly simple physics.

You are correct, it is not as black and white as Im making it out to be. However for all practical purposes, at the landing speeds and considering the airflow through the engine at those speeds, compared to the engine at 0 knots, and considering the relative decreasing speed of the aircraft, the difference in it's effectivness is almost negligible.

you really don't want to get held up by some conscript repacking a chute, nor get stuck in your parking because the tow truck is broken down.

I'd expect the chutes are released at a certain point so I'd presume you'd get a new one from the stash of repacked ones, not wait for the ground crew to repack your chute.

Pretty sure nobody is doubting that Viggen needed the reversers due to the specific operational doctrine (dispersal plus use of the road network), but I'd like to know more about the studies on Soviet conventional strike capabilities which led to the rather tight Tornado program requirements regarding the short take-off and landing distances (wrong forum, I know).

Try deploying a drag chute on a road strip in the forests of Sweden. This is a design criteria for the Viggen.

The ones i have seen all had trees standing right next to the road. If you don't get it that thrust reversers are a huge asset and safety in this scenario, then you just don't get it.

I'd expect the chutes are released at a certain point so I'd presume you'd get a new one from the stash of repacked ones, not wait for the ground crew to repack your chute.

Yes, inserting a new parachute pack takes only a few minutes. The parachutes are prepared sepearately as they have to be dried before packing. Interesting bits on the F-16 topic:

http://www.f-16.net/forum/viewtopic.php?t=2524

And yes, those F-16 can also land on a 800 meter strip without problem, with a chute of course.

The F-16 does not have a delta wing. However, it does have fly by wire. If you don't know know how this relates to landing speed and distance, what point is there to argue with you ?

Please enlighten me, how fly by wire afftects landing speed and distance. Also, I suggest you read a bit why Viggen has those huge canards with flaps in front of the delta wings. You can do that on wikipedia.

F-16 actually has a higher typical landing speed than Viggen by a few knots, even though Viggen is slightly heavier. So your argument is completely invalid.

"F-16 actually has a higher typical landing speed than Viggen by a few knots"

Is the reason for that is because of wing area and how much lift is generated?