High-alt cruising needs to be "on step"?

[Fuggzy]

Hi guys, another dumb observation/question probably -

I've noticed when heavily loaded, leveled-out at high alt (25k+), and going slow to wait up for my dufus AI wingmen, too slow in fact, something interesting happens.  At a certain RPM, if I let the speed drop below 300 knots while straight and level (AP) speed will continue to bleed off as the pitch increases, exacerbating the speed loss.  But when I punch the AB, get it to about 340 knots, then return the throttle to the same setting as before, now she not only holds speed but slowly increases it.  Looking external at both situations, I wonder if the Viper has a need to be "on step" kinda like a boat to be efficient.  God forbid anyone thinks I'm calling Viper a boat, I'm just saying it has a weird step function of speed efficiency that reminds me of boating.  Curious if anyone has observed it too, and if it's realistic.  I think it is, but love to hear the experts' take.  Thx 

[104th_Money]

If you are talking about the engine rpm indicator, you will notice that the rpm indicator increases with speed and fuel flow. There is only one way to set the rpm to an exact number and that is when the thrust/weight and drag are all equal giving you an exact maintained airspeed. If you increase the rpm by 5 percent from that, you will notice it moving up even more as the speed increases until all things are equal again. It sounds like you are getting behind the power curve when heavily loaded at altitude at 300kts, 340kts is getting you above the power curve.

So to reiterate, in level flight your rpm increases with acceleration and decreases with deceleration.

 

[Dragon1-1]

This is correct, with jet engines you need to go fast to go fast. By going faster, you're pushing more air through the engine, which gives it more power. The Viper doesn't like being slow, so at high altitudes in particular, you need to be above 300kts.

[metzger]

Lower speed = Higher AoA to maintain level = Higher Drag

[alexscott292]

You're also running into one of the limitations of the FLCS - always maintaining 1G.  As you slow, the jet will pitch up to avoid descending.  The jet slows, so it pitches up more. . . .

@Mover covered it early in the DCS Viper development.

There are of course comments about this in flight manuals which can't be linked here.

Mover  Video - see from about 5 mins in.

Edited August 30, 2022 by alexscott292

[Frederf]

All airplanes have a U-shaped power (thrust) required curve. It depends on your drag and weight but even relatively fat/heavy at 25kft best endurance would be 350-370 KTAS (240-255 KCAS). You would be on the normal side of the curve although not by a bunch. The throttle position sets the fractional power between minimum and maximum so the same position isn't the same power if conditions have changed.

It's possible to be "behind the power curve" (e.g. normal landing approach). Every thrust has two equilibrium speeds where forces cancel. At high thrust these speeds are far apart. As thrust decreases these speeds come together and at th minimum level flight thrust the high/low speeds are the same number.

[Sandman1330]

 

Some have mentioned engine power due to speed, this is a factor but minor.

What you are seeing is a result of the graph above. At lower airspeeds, to maintain altitude, you need higher angle of attack. This leads to more induced drag. As you get into high AOA regimes, induced drag spikes.

Conversely, as you go faster, AOA reduces (therefore less induced drag), but parasite drag (drag from air resistance on the airframe) increases exponentially. 

The dip at the bottom, where the sum of the two drags is least, is called L/D max (lift/drag maximum). This is your most efficient airspeed. If you get slower than this without increasing power, you'll keep slowing down as induced drag will continue to increase. If you get faster than this then set the same power setting, eventually your aircraft will find the same equilibrium where your power equals your drag, except it'll be on the right hand (faster) side of the curve. If you're closer to L/D max when this happens, you'll actually observe that you accelerate until the parasite drag increases enough to equal your thrust.

[Machalot]

5 hours ago, Sandman1330 said:

What you are seeing is a result of the graph above.

Ha, I wrote the same post, with a very similar graph, and exactly the same points about induced and parasite drag, earlier today but didn't post it because I hadn't had the time to edit and refine it in between work.

Edited September 1, 2022 by Machalot

[Theodore42]

Interesting note: a high alpha pass (AoA about 20) in a clean F-16 takes more power than a high speed pass (mach .86).

[Fuggzy]

Guys I really appreciate all these outstanding answers, much to read and learn about.  Thank you @Sandman1330 for the graph, it's worth >1000 words to me. My only RL flying experience is in gliders, and I know a similar graph .  So, what have I gleaned from this regarding jets?  Yes, there is a minimum speed Viper needs to fly, above L/Dmax, to not be a pig -- and that minimum seems to be about >300 knots KTAS.  (That is a two part efficiency of aerodynamics and engine thrust yes?).  For real efficiency, I believe @Frederf is saying if loaded up it's more like 350 KTAS to not be wasting fuel pushing through mud.  That's basically what it feels like to me -- all else equal and regardless of armament, if I let the speed drop below 300 in any situation, it just flies poorly and needs substantially more throttle to mush ahead. Above 350 it feels "alive" and slick. 

I've also noticed when dogfighting that if I lose focus on my airspeed and let it drop to 250 KTAS or lower, fuggedaboutit, I might as well punch-out and save everyone the time. Thrust feels like it falls off a cliff somewhere in the mid 200's, and I have to fully unload all forces I can to get the speed back in such a low-thrust state.  As it builds back closer to 300 the acceleration (thrust) improves, but by that time I'm usually dying.  Someday I'll learn to manage energy.  

[104th_Money]

@Fuggzy

You want to be at least 350kts during a dogfight and in a good old fashion 1 circle fight more like 425kts. The new FM lets you pull quite a bit longer to take a shot but dont give up everything if you're not sure. I won't hesitate to get down to 100kts if my opponent is but just know that when the Viper is that slow, you dont have much nose pointing authority and its gonna take a bit to get that energy back.

As far as EM, it shouldnt take long to get into the jet and fly offline just doing 360's until you get the muscle memory and audio cues down.