gear drag ?

[bbrz]

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Edited April 8, 2018 by bbrz

[QuiGon]

Just made a quick test and noticed that gear drag is rather high.

 

Flying straight and level at 5000ft and 400km/h in clean config and dropping the gear requires a 4000ft/min ROD to maintain the speed.

 

Well, it's a big gear and the flaps are beeing dropped as well when the gear extends. Without all that drag it would be difficult to slow down for landing as the actual airbrakes have very little drag.

[Goblin]

the flaps on the canards aren't slotted or have a high deflection angle

 

Well, the canard flaps have a deflection angle of 30˚ and the elevons droop 4˚ when the gear and flaps are extended.

I can’t comment on your figures because I haven’t tested myself. They seem excessive, yes, but the Viggen should have a lot of drag in a slow speed config.

 

Edit: So, deploying the gear, doesn’t just add the parasitic drag of the gears. It also adds a lot of lift induced drag.

I have read about the engine wasn’t enough to pull out of a high AoA situation, so you had to be careful in landing config.

Edited February 3, 2018 by Goblin

[Cobra847]

We can double check this. The gear drag is approximated, but the other factors are not (data available)

[Goblin]

The canards and the elevons primarily increase lift, drag increase shouldn't be excessive at low speed.

 

Actually, it would.

Total drag is the sum of parasitic drag and lift induced drag. At slow speeds the induced drag component increases, while the parasitic drag decreases. It sounds to me as if you are only considering parasitic drag, and that is true for the gear, but as has been mentioned, the gears are extended with the flaps and elevons and as speed decreases, induced drag increase...a lot!

Like I mentioned above, not even the large engine of the Viggen was enough to recover from a max AoA situation. Recovery was only possible by trading altitude.

 

But, like I said, I can’t vouch for the numbers in the sim, but the Viggen is known for excessive slow speed drag.

 

Edit: The autothrottle in the Viggen was designed precisely because of this, so the aircraft wouldn’t risk entering the speed instability region (where thrust must be increased to fly slower).

 

https://en.m.wikipedia.org/wiki/Lift-induced_drag

Edited February 3, 2018 by Goblin

[Goblin]

Ok, I understand.

But when you say that flaps and elevons primarily increase lift and that drag shouldn’t increase at low speeds, I get confused, because increase in lift also increase drag, and there is quite a big parasite component added to this.

Again, not saying the behaviour in the sim is correct. Just mentioning that deploying gear doesn’t just add parasite drag. Those flaps generate quite a vortex... The flaps are slotted, BTW.

[Goblin]

If you are flying below Vmd and you drop the elevons and the flaps, parasitic drag increases, but since you need a lower AoA to maintain the same speed, induced drag decreases, hence the net drag increase shouldn't be excessive.

 

Again, this makes me believe that you don’t fully comprehend induced drag. Or it may be me misreading your posts. This is why I’m confused.

 

Induced drag is a product of the pressure differential spanwise flow that 'spills' around the wingtip of a three dimensional wing. It creates a vortex that induces a local decreased AoA around the tip of the wing, thus inducing an added drag component. This is especially so for low aspect ratio wings, and high lift devices.

As the very low aspect ratio of the Viggen canard and its flap basically makes it one big vortex generator, this effect is both a strength and a weakness for the Viggen. It supplies a downwash vortice to the main wing with high energy air, at the expense of drag.

 

So, even if you are riding along at speeds above Vmd, deploying the flaps will induce a big vortice that will increase induced drag, even if you lower the AoA of the wings in total.

 

But you may very well be correct in your assesment that this is excessive in the sim. I’m just saying it should be a lot of drag. A lot more than just the parasite drag of the gear.

[Eldur]

Edit: The autothrottle in the Viggen was designed precisely because of this, so the aircraft wouldn’t risk entering the speed instability region (where thrust must be increased to fly slower).

 

Then it should work well enough to provide for that. It doesn't. Just take a clean Viggen with low fuel and land it with the 15.6° AoA mode. Constantly the AoA warning comes up as the plane always hits the brink of the stall. The Throttle, regardless of the AoA mode selected, sloooooowly goes from flight idle to ~50% of its max travel all the time and the AFK does utterly fail to keep the AoA matched to what it should be. It's rather doing a ping pong by a margin as big as 4-5°.