33:1 glide ratio?

[TAW_Impalor]

Hello again,

 

I bought this module quite recently (at flash sale) and noticed right away there was something wrong with its flight model. I just couldn't land the thing without air brakes and constant slipping. Today I ran a simple test, it flew 7.5 km, engine idle with AIS 200, and only lost 225 meters of altitude. This gives a glide ratio of 33:1 - crazy for a jet!!! Wheel brakes also seem week when landing.

 

Belsimtek, please check your friction parameters.

 

Thanks!

[SgtPappy]

Concerning the wheel brakes, there are tonnes of other threads that can help you find out how to use them properly.

 

If you're flying with engine at idle, that does not mean off. A small amount of thrust is still being produced. Turn the engine off, wait for total spool down and then see what the glide ratio is. According to the 1F-86E-1, the glide ratio in clean condition is 14:1 with speed maintained at 185 kn (giving 2700 fpm descent at 40000' and 1500 fpm at 10000').

 

I don't have time to test it now, but see if you can replicate those results. Based on how hard it is to slow down and land, I may agree with you, but we'll have to confirm with an actual dead engine.

[Aginor]

33:1 would be pretty over the top, that's true. That's approximately the glide ratio of a ASK-21 glider.

[xxJohnxx]

33:1 would be pretty over the top, that's true. That's approximately the glide ratio of a ASK-21 glider.

 

Yes, but the ASK-21 glider is not having a turbine that despite being on idle still creates a significant amount of thrust.

On modern airliners the idle thrust of the turbines is often enough to taxi.

 

To prove that the air friction is not right, I guess you will have to perform that test again, but this time with the engine off. Then compare those values to what's given in the handbook.

[cofcorpse]

Belsimtek, please check your friction parameters.

 

You didn't provide enough information. What was your mass? What was weather conditions? Temperature, wind?

 

Also, if you think something is wrong, it is quite useful to save the track.

[effte]

Glide ratio depends only on L/D ratio. Mass is not a factor (unless you dive deep into the marginal effects), nor does it depend significantly on temperature.

 

Wind will of course affect the achieved distance over the ground, so if that's how you arrive at a glide ratio it does need to be compensated for.

 

Jets are surprisingly slippery. Not 33:1 slippery, mind you, but they often glide better than you would think (with quite a few noteworthy exceptions :) ). Here, I'm with the rest thinking residual thrust from the idling engine.

 

Cheers,

/Fred

[cofcorpse]

I ask exact conditions for this reason: it is much faster to test with known parameters or watch track than to try to find this exact situation.

[effte]

Fair call. Tracks are indeed invaluable!

[streakeagle]

While I am sure the "weight doesn't affect glide performance" is a useful rule of thumb for pilots, it can't be completely true. Mach number affects the lift and drag curves. Since the heavier aircraft must fly at a higher speed to hold the same path and AoA, the Mach number must be higher. Unless the Mach effects cancel out perfectly, the L/D ratio must change with Mach number. Depending on the shape of the curves and the ideal glide speed, it is possible that the heavier aircraft could actually have a higher L/D and therefore a better glide slope. I am guessing that the rule of thumb is taught (and generally holds true) because of the low Mach numbers involved in the ideal glide speed may have almost flat responses to changes in Mach numbers (at least near sea level? less than 10,000 feet?). At 36,000 feet where flying at the same IAS means flying at much higher Mach numbers, I am willing to bet the rule of thumb is a tad less accurate ;)

Edited November 2, 2014 by streakeagle

[TAW_Impalor]

Yep. With engine off I got exactly 14:1. Idle engine must be doubling it. It is just a sharp contrast to how landing a Mig-21bis feels...

[SgtPappy]

Yep. With engine off I got exactly 14:1. Idle engine must be doubling it. It is just a sharp contrast to how landing a Mig-21bis feels...

 

Good. As indicated by another member, idle thrust is enough to taxi the airplane at normal combat load after the static friction on the wheels has been overcome. So it's no surprise that while in the air, a significantly larger "glide" ratio is possible since you're not actually gliding.

 

While I am sure the "weight doesn't affect glide performance" is a useful rule of thumb for pilots, it can't be completely true. Mach number affects the lift and drag curves. Since the heavier aircraft must fly at a higher speed to hold the same path and AoA, the Mach number must be higher. Unless the Mach effects cancel out perfectly, the L/D ratio must change with Mach number. Depending on the shape of the curves and the ideal glide speed, it is possible that the heavier aircraft could actually have a higher L/D and therefore a better glide slope. I am guessing that the rule of thumb is taught (and generally holds true) because of the low Mach numbers involved in the ideal glide speed may have almost flat responses to changes in Mach numbers (at least near sea level? less than 10,000 feet?). At 36,000 feet where flying at the same IAS means flying at much higher Mach numbers, I am willing to bet the rule of thumb is a tad less accurate ;)

 

The difference is that the airplane isn't maintaining steady level flight. Gliding is falling out of the sky so there are formulas that can find glide ratio (at speeds where compressibility isn't an issue) very accurately without the aircraft mass.

Edited November 4, 2014 by SgtPappy
Added last paragraph

[towsim]

To underline the difference between engine idle and engine off, all fighter jets, practicing SFO (simulated flame out), use the speed brakes to simulate the drag of a cold engine. All aircraft have a 'best glide speed' measured under standard conditions (weight, external loads). All speeds higher and lower cause increasing drag. Higher speeds cause the exponential increase of body drag because of increasing speed. Lower speeds cause increasing drag because of increasing AOA.

[Rongor]

I tried to turn attention to this problem in this thread I created in early September realistic drag? Or gaining too much lift?

[SgtPappy]

I tried to turn attention to this problem in this thread I created in early September realistic drag? Or gaining too much lift?

 

What problem? The glide ratio is confirmed to be correct. Your thread is about high speed drag.

[Rongor]

Sorry sir, but as long as this is entirely your own opinion, I see no confirmation yet. But I appreciate your advice what my thread was about. In fact I created it, to publicly discuss the exact symptoms described also in this thread here.

[Rongor]

Good. As indicated by another member, idle thrust is enough to taxi the airplane at normal combat load after the static friction on the wheels has been overcome..

I think you misunderstood the other member then, he stated that

On modern airliners the idle thrust of the turbines is often enough to taxi.

This statement may provide a point where to investigate, but despite that doesn't tell explicit what the F-86F is doing in our case.

When I startup the F-86F, idle throttle isn't sufficient to move the aircraft, nor can it prevent it from coming to a halt when taxiing. So we can rule this one out and the observations remain unresolved.

[KombatMilc]

Keywords being: on modern airliners

[sobek]

So we can rule this one out and the observations remain unresolved.

 

No, you really can't. Let alone the fact that flight idle isn't the same as ground idle, being able to taxi or not to taxi isn't any indicator of what glide ratio you should get with engine at idle. The fact remains that with engine off, the glide ratio is 14:1, which is correct. So what exactly is the problem?

[Rongor]

I don't understand your question.

 

It is the first time a participant of this discussion provides us with the statement of that glide angle being correct. Thank you!

Do you have any links or further insight on that topic? This would be appreciated, as some people might be interested.

[effte]

Ehm. Post #2? :)

[sobek]

Ehm. Post #2? :)

 

And post #10. The OP himself acknowledged that the glide ratio is correct. Are we done with the FM fearmongering now? :)