Vy vs Best Energy Airspeed

[muerto]

Anybody here read Shaw's "Fighter Combat"?

In this book he talks about an airpseed that is the speed providing the best gain in total energy.  I get the impression this is not Vy.

Does anyone know for sure that it is not Vy?

And if it isn't Vy does anyone know how to determine this airspeed for AH aircraft?

[stantond]

H-M plots for supersonic fighters exhibit fundamentally different operating envelopes than subsonic fighters.  The maximum climb performance for both is along the peaks of the Ps curves up to a point.  Supersonic fighters have the ability to use a speed and altitude dependent energy-rate profile.  Subsonic prop driven fighters do not have this luxury because thrust decreases above Vy airspeed.

The maximum climb profile occurs along the peaks of the Ps curves for a propeller driven fighter, which is Vy.  Also, propeller thrust does not increase with airspeed above that for Vy.  As such, increasing speeds over Vy airspeed as no advantage.  The  trick is to determine the Vy airspeed as it changes with altitude.


Well.... you asked.



Regards,

Malta

p.s. glad to see someone actually reads Shaw's "Fighter Combat"

[flakbait]

Just so you know, V-speeds are an invention for aircraft with stinky whistle motors. Prop aircraft usually don't have V-speeds, and those that do were built in the 60s or later. Also, applying Shaw's book takes some doing because none of the AC built before or during WW2 had V-speeds. Some military transports (C46 for one) did have them, but not from official testing. The pilots and crew chiefs got together and figured them out through experimentation. You can find out Vy, Vx, Vr etc... by experimenting on your own in a given aircraft.


BTW, there is no such thing as a "speed providing the best gain in energy." Speed, and thus energy, is gained by adding power or diving. The best gain in energy is accomplished by applying maximum power or diving, followed by bleeding the opponent's speed down through maneuvering. If you tell me which page this is on I can get a better idea of what he's talking about.


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Flakbait [Delta6]

[hitech]

Flakbait:

Speed is not the same as energy.

Energy = Speed ^ 2 + Alt

HiTech

[Pyro]

Originally posted by flakbait
Just so you know, V-speeds are an invention for aircraft with stinky whistle motors. Prop aircraft usually don't have V-speeds, and those that do were built in the 60s or later. Also, applying Shaw's book takes some doing because none of the AC built before or during WW2 had V-speeds. Some military transports (C46 for one) did have them, but not from official testing. The pilots and crew chiefs got together and figured them out through experimentation. You can find out Vy, Vx, Vr etc... by experimenting on your own in a given aircraft.

Sure they did, they just didn't use that nomenclature to describe them.  But you can look in the manual of these planes and find that stuff.  

As to the original question, the answer as it relates to AH is Vy.  Bob Shaw covered this very topic during his lecture at this year's con.

[muerto]

Thanks for the replies.
I didn't understand how there could be a more efficient speed than Vy since that happens at L/D Max; I just knew he implied it, I guess it applies to high speed jets as stantond stated.

Bob Shaw covered this very topic during his lecture at this year's con.

I already was disappointed about missing the con, now I'm really depressed.

BTW, there is no such thing as a "speed providing the best gain in energy." Speed, and thus energy, is gained by adding power or diving. The best gain in energy is accomplished by applying maximum power or diving, followed by bleeding the opponent's speed down through maneuvering. If you tell me which page this is on I can get a better idea of what he's talking about.

Speed is kinetic energy, total energy is the combination of kinetic and potential as stated by HiTech's formula.  Climbing at a certain speed will cause you to gain the highest level of total energy in the shortest amount of time.  There are other factors to consider though, especially if Vy is well below the speed that gives a particular aircraft the ability to maneuver effectively.

[Virage]

If my memory serves...


A climbing profile at a speed slightly higher than Vy would allow for a higher zoom at the end of the climb, possibly creating a higher Ps than a steady Vy climb.

[Badboy]

Originally posted by muerto
Anybody here read Shaw's "Fighter Combat"?

In this book he talks about an airpseed that is the speed providing the best gain in total energy.  I get the impression this is not Vy.

Does anyone know for sure that it is not Vy?

And if it isn't Vy does anyone know how to determine this airspeed for AH aircraft?

Here is the way to optimize your energy before a fight…

Picture yourself in the cockpit of a high performance prop fighter, making contact with a distant target. At that point your priority should be to enter the fight with the highest possible energy state in the shortest possible time. You would achieve that by selecting full military power, unloading to zero G, and accelerating in a dive to the best energy transfer speed, possibly losing several thousand feet in the process. You would then begin a climb, not at the best climb speed, but at the speed for best energy transfer. What surprises most folk about this is the initial dive to a higher speed. That is the correct procedure in all fighters, regardless of type, be they subsonic or supersonic jets, or prop fighters, and regardless of altitude, providing you are not at sea level.

To understand why this works, take a look at the diagram below. This diagram shows lines of constant specific Energy (Es) in green and lines of constant specific excess power (Ps) shown in red. If you want to achieve the highest energy state at the greatest rate of energy increase, you need to maximize your Es and Ps at the same time. That can be seen to occur where the P's curves are tangent to the E's curves. The red dotted lines are drawn through all such points and represents the best energy transfer line. So for the fighter I mentioned before starting at position A the pilot should dive until he reaches the speed for best energy transfer at B and then climb at that speed to C.



The surprising thing to most people is that in order to get to the best possible energy advantage for the fight, you start by diving! You increase speed to a point well above that for minimum drag, or best climb rate. The objective, after all, was not to minimize drag or get to the highest altitude in the shortest time, it was to gain the most total energy in the least time. In WWII vintage fighters, propeller and turbocharged engine combinations were generally designed to give maximum efficiency close to top speed at the critical altitude. So both altitude and airspeed have an influence on the best energy transfer, that’s why the critical altitude also has an influence, depending on where the fight begins. If you start the fight below the critical altitude you could choose to stay below it and follow the schedule from B to D, you will have more engine power and a slightly higher air speed, resulting in closure and better energy transfer. If time permits, and closure isn’t an issue, it is better to climb to point C. If you start the engagement above the critical altitude for your supercharged engine, there is some advantage in beginning a dive towards the critical altitude to increase the available power, resulting in higher specific excess power, and zooming back into the fight using a hook type maneuver which not only optimizes your energy (providing you don’t exceed top speed in the initial stage, after which point further loss of altitude gets you nothing in return) but yields a positional advantage, if the bandit doesn’t use a similar tactic.
 
The big question is of course, are there any genuine Ps/Es curves for AH2 that will show the best energy transfer speed for some of the aircraft. Well, I was getting around to that, but I’ve just had to re-start producing EM diagrams for the AH2 aircraft (only done five so far). The good news is, that in order to produce the EM diagrams, all the work required to overlay Ps and Es curves is mostly already done, the bad news is that until I’ve produced EM diagrams for more of the AH2 aircraft, I’ve put that sort of thing on the back burner.

Hope that helps…

Badboy

[TequilaChaser]

hope you do not mind Badboy, I cut and pasted this into a file with the pic for help/training purposes to distribute as needed......I kept your name on it as the author as well :-)


this is similar to the Co-E Chase is it not indirectly anyhow?

[muerto]

Badboy, that is exactly what I was looking for.  Well, okay, the actual speeds for each aircraft is "exactly" what I was looking for, but that explanation helps a lot.

Thanks.

[hitech]

Badboy: Somthing is not making since in you chart, are you missing the time in the energy state change? Because Vy is defenitly the greatest increase in energy over time .


HiTech

[Badboy]

Originally posted by muerto
Badboy, that is exactly what I was looking for.  Well, okay, the actual speeds for each aircraft is "exactly" what I was looking for, but that explanation helps a lot.

Thanks.

 
Yep, and this partly explains why you will often see good pilots diving into the merge prior to an engagement, the natural counter, is to do exactly the same thing, but better. The hook is shown below:



That of course is how the Hook works, if you do it against someone who still attempts to pull up into the vertical. It's a shame but many virtual pilots will defeat it knowingly, and the others, just by driving for a head-on shot

However, what I often do when I’m some distance away from a fight, and purely due to laziness, is to simply go to auto climb, and then level off for speed prior to a diving merge. I do that partly because I’m a very lazy flyer, and rely on auto everything as much as possible

Hope that helps...

Badboy

[muerto]

I'm confused.  I was confused before, and I'm still confused.  I alwasy thought that Vy was the answer.  Shaw eludes to some other speed, but doesn't really define it in a way that I could understand.  Pyro says he heard Shaw talk about it and Vy is it.  HiTech says it's Vy.  And though I don't know you guys personally, based on what you have created I respect your explanations.  But then Badboy brings back an explanation that is similar to what Shaw talks about in his book.
Agggggghhhhhhhhh! head feels like it will explode.

Maybe we have two differnent things going on here.  It seems to me that Vy is the way to build E most quickly/efficiently during a climb.  Is it possible that this other explanation defines the proper way of going from Vy to the speed you need to be at to properly enter a fight?  Clearly Vy is much slower than you want to be in most aircraft if you're entering a co-alt fight.

Certainly acceleration is quickest in a zero g wingload condition. (without giving away alt unneccessarily.)

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Below is a post I submitted, but edited by mistake when I replied to Badboy.

Virage, if I'm remembering correctly, Vy happens at an airspeed ever so slightly higher than L/D Max.  So maybe the correct statement is that the greatest Ps is gained at a speed slightly higher than L/D Max, or Vy.

I fairly certain of this much, a zoom is a conversion of energy; potential to kinetic or the reverse.  So by trading airspeed for altitude you haven't gained energy you've merely converted it (actually you've probably lost some if you had to pull any Gs to do the conversion.)

A climb at any steady airspeed is not a zoom, its the conversion of the energy being released by the burning of fuel into an increased energy state of the aircraft (it's an increase assuming the aircraft is climbing and/or accelerating.)

A climb at a steady airspeed known as Vy is the most efficient way to use the energy released by burning fuel if you look at the climb and/or acceleration as the storage (however temporary) of energy.  If I understand correctly.

[Badboy]

Originally posted by muerto
A climb at a steady airspeed known as Vy is the most efficient way to use the energy released by burning fuel if you look at the climb and/or acceleration as the storage (however temporary) of energy.  If I understand correctly.

Yep, that’s what classical climb analysis predicts, and that is what HiTech is referring to, but it has long been understood that the classical climb analysis has an embedded inconsistency. Recall that we are led to choose a speed V to maximize the rate-of-climb, at a given altitude h. This analysis is repeated at a sequence of altitudes and the resulting family of best speeds defines a function Vopt(h). As the aircraft climbs and the altitude changes, the choice of best speed will vary according to this function. It's clear then that the resulting speed is generally changing with time V(t) = Vopt(h(t)). Since our choice of speed was based on maximizing the unaccelerated rate-of-climb there is an inconsistency. This was well appreciated in the days before WWII and various “corrections” were suggested. When properly considered, the accelerated rate-of-climb works out to be less than the unaccelerated prediction for best energy transfer. In energy terms it is clear that the aircraft is gaining kinetic energy so that the gain in potential energy is diminished from the earlier estimate and that total increase in energy is what is being optimized on my previous diagram.

Why isn’t all this explained better in classical climb analysis? The piston-powered aircraft of WWII vintage had a limited speed range so that the amount of stored kinetic energy was generally small compared to the potential energy. For example, a speed change from 100 mph to 400 mph requires a certain increase in kinetic energy per unit mass. To achieve the same energy change in potential form requires an altitude increase of about 5000 ft. Thus, a significant change in speed is equivalent, in an energy sense, to a rather modest change in altitude. When you consider that the difference in speeds we are discussing here is very much smaller than that, the resulting change in altitude is even less. For this reason it was mostly okay to just ignore kinetic energy for such calculations. Note, however, that the kinetic energy varies as the square of the speed, doubling the speed will increase the kinetic energy by a factor of four, so as the speed capabilities of fighters increase, the kinetic energy becomes increasingly important, which is why such things are more significant for modern fighters. Although the effect is often ignored in classical climb analysis, the correction ideas noted above were well known in the early 1940's and there is a German report detailing the best energy transfer methods used on WWII vintage fighters by German flight test engineers, which was translated to English in 1944. I also have a document detailing the same theory that was published by the USAF, and despite the fact that the subject is always contentious when posted, the method is correct.  

Hope that helps…

Badboy

[hitech]

Badboy, I Belive I know what your refering to, but gut feel tells me we are talking about somthing with in the 5mph different range.

HiTech