Lawn dart tendency without CAS

[Svend_Dellepude]

I have been thinking about this for some time and it defyies my sense of logic that the F-15 doesn't have a natural tendency to want to pitch up at any airspeed and/or altitude.

On the contrary when airspeed increases you have to increase backstick pressure to keep it level.

The F-15 has a very large lifting area and a low wingload and yet it's not able to attain level flight without stick input.

At high altitude (>30k) it's unable the to keep level with or without CAS engaged, clean or loaded config, between M 0.9 and ~2.0. (depends on altitude). The PTC seems the reach its travel limit and manual trim is required.

The result of this must be that the elevators create a rather large amount of drag, because of the large deflection, which could be a contributing factor to the instability we experience at high altitude. This is of course speculation from my side.

Inverted flight with CAS off seems a bit weird to me. the F-15's tendency to pitch down keeps it level while inverted.

 

A few tracks to show what im trying to explain.

CAS Pitch off #2.trk

Inverted level flight CAS pitch off.trk

TO high alt flight.trk

[OnlyforDCS]

Correct me if Im wrong but isnt the F15 designed with relaxed stability on purpose?

[Svend_Dellepude]

AFAIK it's stable. F-16 has relaxed stability and FBW. F-15 has a control augmentation system. But even if it was it wouldn't explain this negative pitch tendency.

But since it increases with airflow increasing it seems like a weird kind of drag on the underside of the airframe.

 

Did you watch the tracks?

There is a track where is fly inverted, no input, no trim, CAS off and the F-15 wants to climb.

 

EDIT: After spending a few days reading whatever i have been able to find around the internet I'm starting to suspect the FM to have an insufficient/less airflow going over the fuselage and/or wings, compared to whats going under.

Which could, if I understand this correct, create more drag on the underside thus the increased tendency to nose-down with increasing airspeed.

Could this be correct? Anybody know about this stuff?

Edited November 22, 2014 by Svend_Dellepude