General Category > Unofficial F/A-18 Acceleration Pack board

F/A-18 Approach speed

(1/5) > >>

neutrino:
Hi, guys. I did some flight testing with the Acceleration Hornet to test which speed gives the approach AOA of 8.1 degrees at different weights. It seems that the FSX Hornet approach speed (8.1 AOA) is about 9 knots higher than the real world Hornet. For example, at 30,000 lbs gross weight, the FSX Hornet is at 8.1 AOA at 139 knots, but the real Hornet is at 130 knots. At max trap weight (34,000 lbs) the real hornet lands at 139 kts and the FSX Hornet at 148 kts. This means that we are landing at much higher speeds in the simulator. It's not such a big deal but I wonder if anyone else has noticed that? I am sure whoever modelled the aircraft had some good data to work with, and yet 9 kts is strange difference.


Edit (September 08, 2009): OK, I found the answer to the problem. It is very interesting. The Hornet appoach speed (8.1 degrees AOA) in the simulator is higher than the speed of the real Hornet (by 9 knots), simply because it does not land with the flaps fully  extended. You can visually check the flaps in the sim at approach speeds and you will see that they are extended only about 50% of their full limit (45 degrees). This is even if you have pressed F8 and the 'FULL' flaps light is on. So the plane is actually very accurately modelled. If the real Hornet were to land with half flaps its speed at 8.1 AOA will match that of the Hornet in the simulation.

Why are the flaps not fully extended? Because of the Auto-flaps system the ACES has introduced in the Acceleration. The auto-flaps system has a logic that controls the position of the flaps depending on speed and AOA. It is always working in the background and you can know it is changing the flaps position each time the amber light 'FLAPS' goes on. It is a generic flight model system applicable to all aircraft in the simulator and can be turned on and off by the property 'maneuvering_flaps' in the aircraft.cfg file. If you put maneuvering_flaps=1 in the Cessna aircraft.cfg it will have auto-flaps just like the Hornet.

Unfortunatelly, because the system is not specific to the Hornet, it does a poor job of controlling the flaps in this particular aircraft. In the real Hornet the flaps are controlled by the flight control system based on airspeed and AOA, just like in the FSX system, but if you are in 'FULL' flaps mode, the logic will fully extend (45 degrees) the flaps at approach speeds. The FSX system however extends the flaps fully at speeds below 95 knots for the Hornet. I checked how the auto-flaps work in the Cessna, and they work fine - fully extended below 65 kts (approach speed) and fully retracted above 100 kts (cruise speed).

So you have a choice here, to leave the default <maneuvering_flaps=1> for the Hornet and enjoy the auto-flaps system, which works almost like in the real Hornet at high speeds, but at appoach speeds your flaps will not be fully extended as they are in the real Hornet. This will put an additional 9 kts to your approach speed and your nose is certain to hit the deck of the carrier if you land at close to your maximum weight. Also your VSI will be higher. The F5 to F8 keys in this mode only control the maximum deflection of the flaps and the lights 'HALF' and 'FULL' should not be used as an indication of flap position.

The other choice is to set <maneuvering_flaps=0> (for both leading edge and trailing edge flaps - [flaps.0] and [flaps.1] respectively) and manually control the position of the flaps. Make sure you are not at FULL flaps above 160 kts, and at HALF flaps above 200 kts. Switching off the auto-flaps system by changing a line in a file looks like tinkering, but in the real Hornet you can also turn off the FCS control of the flaps if you put the GAIN switch on the FCS panel (behind the throttle) in "ORIDE" instead of "NORMAL". Unfortunatelly this is not impleneted in the FSX Hornet, so feel free to change it in the aircraft.cfg :)

Bruce Hamilton:
What happens if you drop the FSX Hornet down to the real world speeds?  With full flaps, you should be able to match the real Hornet speed of 139 knots, which to me seems slow for a supersonic aircraft.  A Boeing 737 comes in hotter than that.

neutrino:

--- Quote from: Bruce Hamilton on September 05, 2009, 10:03:23 pm ---What happens if you drop the FSX Hornet down to the real world speeds?  With full flaps, you should be able to match the real Hornet speed of 139 knots, which to me seems slow for a supersonic aircraft.  A Boeing 737 comes in hotter than that.

--- End quote ---
You can do that, but the AOA will have to increase to 9.1 degrees, to support the aircraft at the lower speed, which is 1 degree more than the recommended value. You will be landing at a speed closer to your stall speed which is of course more dangerous and will make the aircraft more difficult to control. Also in carrier landings, the hook touch down point will move aft by about 4-5 feet.

SpazSinbad:
neutrino, I would be interested to know where the data is for the Hornet Optimum AoA please. Often this information is in a NATOPS graph but I do not have one for the Hornet. Thanks.

As a complete generalisation, having used in FSX only this Hornet and Dino Cattaneo's freeware Goshawk T-45C and Tomcat F-14D it would seem that it is difficult to get the Optimum AoA parameters correct (comparing sim to real world). For example (if memory serves well) the Dino Goshawk is about 5 to 10 knots KIAS slower at Optimum AoA compared to real world. HOWEVER... the aircraft performs it seems (without having flown it real world) as it should and I think same applies to Tomcat by Dino. He works on this issue though.

Another future FSX naval aircraft will be the RNZAF KAHU Skyhawk which probably will have a similar issue with Optimum AoA / KIAS matchup but for the moment it seems to fly well (with other issues yet to be solved).

I would suggest that flying at the indicated Optimum AoA on the indexer is the only way to do it in the sim with the aircraft below max. landing weight and properly configured dirty (full flap, speedbrakes out, gear down).

As noted by neutrino it is vital to approach and land at Optimum AoA for not only NOT exceeding the maximum arrest speed for the gear but possibility of breaking the aircraft and also as noted the 'hook to ramp' distance (raising or lowering hook). The mirror/fresnel lens glidepath is calibrated for the pilot eyeline at Optimum Angle of Attack. Flying outside any of the parameters required for a carrier landing is fraught (real world). In the sim one may get away with changes however my suggestion would be to stick with what is there and fly at the indexer indicated Optimum Angle of Attack below the Max. Landing Weight.

Without knowing how the Hornet specs were modelled in FSX, perhaps what is there is optimised for the model. To my knowledge from the KAHU model maker for example there are many limitations in the sim that have to be fudged to get close to real world modelling. Perhaps the difference in KIAS to Optimum Angle of Attack for the Hornet in FSX is one of those examples of compromise?

neutrino:

--- Quote from: SpazSinbad on September 05, 2009, 11:35:07 pm ---neutrino, I would be interested to know where the data is for the Hornet Optimum AoA please. Often this information is in a NATOPS graph but I do not have one for the Hornet. Thanks.
--- End quote ---

Yes, it is from NATOPS.


--- Quote from: SpazSinbad on September 05, 2009, 11:35:07 pm ---I would suggest that flying at the indicated Optimum AoA on the indexer is the only way to do it in the sim with the aircraft below max. landing weight and properly configured dirty (full flap, speedbrakes out, gear down).
--- End quote ---

Why the speedbrakes? I thought at least in the groove you don't use speedbrakes...


--- Quote from: SpazSinbad on September 05, 2009, 11:35:07 pm ---Flying outside any of the parameters required for a carrier landing is fraught (real world).
--- End quote ---

I like that statement  ;D

Navigation

[0] Message Index

[#] Next page