Flight model of the Viggen

[outbaxx]

One thing I have a hard time wrapping my head around is that when accelerating after takeoff (M0-0.8) the Viggen has a “nose down momentum”, I have to stick aft or trim up or it will fly into the ground.

Reading a book about the development of the Viggen there is a picture of the zero moment Mach characteristic and that show a positive zero moment at M0-0.8 (Cm 0.003-ish), then there is a steep negative moment at 0.8-0.9 (Cm -0.009 to -0.01) followed by a fast positive at M1.0 (Cm 0.005 to 0.006) and then a sloping curve where at M1.3 the Cm is negative again.

(In DCS over M0.8 the behavior is what I expect it to be)

 

The problem I have is that I don’t understand why the Viggen in DCS wants to dive below M0.8 if the zero moment is positive? Shouldn’t it be the other way around? A small nose up moment?

 

Regards

F

[Wintermute74]

Do you have a reference for that book section? Are you sure it’s for the final (serial production) version and for the full aircraft configuration?

 

In the Aerodynamics of the Viggen 37 aircraft compendium (part 1) by Krister Karling there is a quite thorough discussion about the pitching moment of the Viggen at low speeds, see especially sections 2.4-2.14. In Fig. 47 on page 57 there is a pitching moment curve of the Viggen in comparison to other aircrafts, which suggests a small negative Cm at alpha zero (zero moment coefficient). On page 69 there is also a typical (negative) magnitude mentioned for an unloaded Viggen (-0.006). It is however mentioned that the actual magnitude is dependent on a number of factors, including external loading, flaps and elevator positions, etc. I failed to find any data for higher speeds, most likely since the publication was on the low speed characteristics (maybe this is covered in other parts of the compendium?).

Edited March 2, 2020 by Wintermute74

[outbaxx]

Flight model of the Viggen

 

 

This is the image I looked at.

I have no aerodynamic schooling so I assumed positive moment meant nose up :)

But if it’s like you say, then we should have a nose down up to M0.8 then a large nose up followed by a a nose down to about M1.3 and then a raising nose again?

(Edit: someone answered the thread and said that positive meant nose down but I can’t see that post now?)

The image show the momentum at alpha 0 so it’s not an absolute truth off course.

 

Regards

F

Edited March 2, 2020 by outbaxx

[Wintermute74]

Interesting and little contradictory to the aerodynamics compendium I mentioned! Mind sharing the book title and author as well? Do you have it has a pdf to share with the DCS community (there is a separate thread covering real life documentation)?

 

Positive moment usually means "nose-up" so I think you were correct in your assumption. :thumbup:

 

There are multiple ways of interpreting the figure I guess. Does it represent the impact of the hump on the back only (in comparison to a fuselage without one) or does it represent the complete aircraft (being the sum of many parts)? :huh:

 

Since I find no reference to the hump in the compendium, it could also be that the compendium describes an earlier (pre-hump) design...(EDIT: The bulge is actually mentioned on page 68 and illustrated in Fig. 56). I guess ultimately HB will have to jump into this discussion if we want the final answer as to which flight model is implemented. :smilewink::helpsmilie:

Edited March 2, 2020 by Wintermute74
Correction

[outbaxx]

The title is : The Saab 37 Viggen. The development of a new multi purpose military aircraft for the Swedish Airforce 1952-1971

Author: Krister Karling

 

So it’s the same author as the compendium mentioned above.

 

Can’t say for sure but as I interpreted the text it’s the whole clean aircraft.

 

There are graphs for aircraft with wing ordinance and for aircraft with center placed ordinance but they are the same up to M0.8 where the curves start to differ. But I’m only interested in the M0-0.8 regime where there is a positive pitch moment at alpha 0 but I need to trim nose up to go level flight and I don’t understand why.

[TLTeo]

Could it be either a) some artifact of the stabilization system or b) something that happens with a certain fuel load?

[InTrustWeThrust]

Are you saying that the faster you go between M0-0.8 the more nose-up trim you need to apply?

[outbaxx]

Are you saying that the faster you go between M0-0.8 the more nose-up trim you need to apply?

 

No, I fly almost only fly with a clean aircraft so I start with 0 trim. I guess I have 2-3 up trim at M0.8.

But I thought I would have some down trim when I read the picture above.

[InTrustWeThrust]

But you are not flying at 0 alpha are you? Look in the “Aerodynamics...” doc link above at page 61. It just needs a little alpha to have a negative momentum (I e Viggen has a pitch-stable configuration, unlike for example F-16 - see aerodynamic stability). As you also can tell by other graphs Viggen doesn’t have positive lift at zero alpha either so it will always fly at a little angle. The faster you go, the less alpha you need, so unless something is weird you’ll need to adjust trim forward as you speed up if you have achieved a stable trim at lower speeds.

[outbaxx]

But you are not flying at 0 alpha are you? Look in the “Aerodynamics...” doc link above at page 61. It just needs a little alpha to have a negative momentum (I e Viggen has a pitch-stable configuration, unlike for example F-16 - see aerodynamic stability). As you also can tell by other graphs Viggen doesn’t have positive lift at zero alpha either so it will always fly at a little angle. The faster you go, the less alpha you need, so unless something is weird you’ll need to adjust trim forward as you speed up if you have achieved a stable trim at lower speeds.

 

 

 

Yes, you are correct, and I found these two:

 

Describing the trim angle for 1G (level flight?) at height 1km and as I understand it show that there should be about 1degree up trim at M0.8 for a clean aircraft?