F-18 Question For Astro ??

[billt460]

Astro, In the video below it shows a pair of F-18's taking off in a very heavy crosswind. At 0:50 to 0:55 seconds into the video the F-18 on the left manipulates all of the control surfaces before applying full power and afterburner, starting his takeoff roll.

If you look at both vertical stabilizers, the rudder control surfaces are both pitched inward against each other. Instead of both operating together in a typical left or right fashion. What is the purpose of this? The F-18 on the right does much the same thing on it's takeoff roll. You can see some movement of the rudder control surfaces during and after rotation. But why are they set to "fight" each other, and nullify the others input, instead of simply remaining straight and in alignment with the vertical stabilizer when the rudder is in the neutral position, like most aircraft? Thanks in advance.

 

[JLTD]

Subbed.

 

[tom slick]

It does that when the flaps are extended to force the nose up.

 

[d00df00d]

Originally Posted by tom slick
It does that when the flaps are extended to force the nose up.

Interesting. Guessing that's because the vertical stabs aren't entirely vertical, so the rudders induce some pitch?

 

[A_Harman]

The additional drag force of the rudders being above the CG of the aircraft would cause a pitching moment helping to raise the nose.

 

[Astro14]

Originally Posted by tom slick
It does that when the flaps are extended to force the nose up.


That's basically it.

You'll see the same rudder toe in on a cat shot.

A Hornet with the flaps extended is a VERY high drag airplane. All the result of huge, simple flaps (no slots, etc.) to enable it to fly at approach speed 145 KIAS @ 34,000#). Lots of tricks (aileron droop, rudder toe in) to make everything work.

You could not keep a Hornet (lot 14 and earlier, the model I flew) in the air with the flaps at full and an engine failed. The single engine, even in full AB, didn't have enough thrust to overcome the drag from those flaps.

Takeoff was at half flaps, like the video. But with the nose on the ground, and the flaps affecting the airflow over the horizontal tails, help was needed to raise the nose.

A Hornet, despite being much lighter than a Tomcat, rotated at much higher speeds than a Tomcat, which had an efficient wing with slotted slats and fowler flaps.

In the landing pattern, configured for landing, the 34,000# Hornet burned more fuel (per minute) than the 54,000# Tomcat. And that lighter airplane was flying faster (145) than the bigger, heavier one (140 with DLC, 132 without).

So much drag...

 

[billt460]

Thanks. From 0:52 to 0:56 seconds you also see the elevators move independently of each other, rather than in unison. I'm assuming that is to help assist the ailerons during low speed banking?

 

[JetStar]

I have a twin vert RC airplane that I have the radio set-up to do that, as a speed brake for landing. It's not enough to cause any noticeable pitch trim issues.
Some of my other planes with the flaps down, I have programmed the radio for pitch trim. Test flights required to determine how much.

 

[tom slick]

I've read that the Hornet has very little ground effect and the tail's low location makes it relatively ineffective at rotation speed The giant flaps and rudder toe-in were done to help with those issues.

It's interesting to see fly by wire systems operate. The pilot indicates to the computers what he wants it to do and the computers configures the aircraft to do that. The mechanics of that configuration is not always linear to what you think it would be.

 

[Astro14]

Originally Posted by billt460
Thanks. From 0:52 to 0:56 seconds you also see the elevators move independently of each other, rather than in unison. I'm assuming that is to help assist the ailerons during low speed banking?


Exactly.

Depending on the speed and AOA, a roll (lateral stick) inout by the pilot might result in differential tail movement, rudder movement, ailerons, or even, at very high speed, differential leading edge movement to roll the airplane.

What moves when and why is governed by flight control laws stored in EPROMS on the flight control computers.

 

[billt460]

What is also noticeable is from 1:03 to 1:07 you can see smoke come from the nozzles of the F-18 that is waiting in takeoff position. I wonder if that was caused by ingestion of the exhaust gasses from the F-18 that is taking off, resulting in incomplete combustion for a few seconds?

 

[Danno]

Interesting to see the 1st plane leave "skid marks" on the runway, while the 2nd was clean.

 

[Tikka]

Hi
I can't but think we should have bought super hornet for our two new carriers instead of F35B.

 

[BMWTurboDzl]

Originally Posted by Astro14
Originally Posted by tom slick
It does that when the flaps are extended to force the nose up.


That's basically it.

You'll see the same rudder toe in on a cat shot.

A Hornet with the flaps extended is a VERY high drag airplane. All the result of huge, simple flaps (no slots, etc.) to enable it to fly at approach speed 145 KIAS @ 34,000#). Lots of tricks (aileron droop, rudder toe in) to make everything work.

You could not keep a Hornet (lot 14 and earlier, the model I flew) in the air with the flaps at full and an engine failed. The single engine, even in full AB, didn't have enough thrust to overcome the drag from those flaps.

Takeoff was at half flaps, like the video. But with the nose on the ground, and the flaps affecting the airflow over the horizontal tails, help was needed to raise the nose.

A Hornet, despite being much lighter than a Tomcat, rotated at much higher speeds than a Tomcat, which had an efficient wing with slotted slats and fowler flaps.

In the landing pattern, configured for landing, the 34,000# Hornet burned more fuel (per minute) than the 54,000# Tomcat. And that lighter airplane was flying faster (145) than the bigger, heavier one (140 with DLC, 132 without).

So much drag...


I guess that's why the F-14 was sometimes called a Turkey.