I conceded Dick's point earlier. It seems valid to assume that a lighter airplane will accelerate more rapidly to Vne than an otherwise identical airplane, even straight down, when each is producing the same amount of propeller thrust. I think both the more conventional "glide angle", and Dick's addmittedly gedanken experiment interpretations are internally consistent.


added "when each is producing the same amount of propeller thrust".

I hope my mind games have not left you ferklempt -

Had to look it up [sm=lol.gif]

Warning, good-natured ribbing follows:

Your name wouln't happen to be James Joyce, would it?

Mesae, you gave up too easily -- although in either case the lighter aircraft wins.

Lets just pose the scenario for discussion. I suggest a heavy transport aircraft -- something like a C-130, as the test case.

Two planes flying side-by side -- one fully loaded (~150,000 lb) & one empty, with min fuel (~100,000 lb), cruising @ 300 kts. Both have a Vne of 400 kts (I don't know if those #'s fit the Herc scenario, but they seem reasonable to me). This makes lots of excess power available for initial acceleration.

Green flag --- throttles to the firewall, while simultaneously pushing over.

The unladen Herc with its greater power-weight will initially out-accelerate the heavy bird, as they push over into the dive.

As the dive angle increases, additional power is added by gravity & drag becomes increasingly important. At some point the heavier aircraft has gained sufficient power from gravity that it has an advantage in power-drag. It begins to close the gap as Vne is approached by both planes -- but the heavier plane is now moving faster.

Throttles are now closed to avoid exceeding Vne & the heavier plane, moving faster, is forced to reduce the dive angle first, regardless of whether it has overtaken the lighter aircraft. Moreover, the pull-up rate will be lower for the heavier aircraft to avoid exceeding structural limits, giving the lighter aircraft an additional advantage in being able to more closely approach Vne before pulling up.

Both aircraft are now diving at Vne with throttles closed, but the lighter aircraft must dive more steeply to derive sufficient power to achieve Vne, and thus steadily increases its advantage.

No contest.

Dick's argument is valid provided that both planes are capable of sustaining a verticle dive without exeeding Vne. I don't think there are very many full scale aircraft that can do that.

If a plane's drag at Vne is more than the weight of the plane, it can do a sustained verticle dive without exeeding Vne. This would include light and dirty planes like many RC models. It would probably also include most supersonic fighters. At supersonic speeds, the drag is probably more than the weight of the plane, at low altitudes anyway.

Super clean models like RC sailplanes can reach dangerous speeds in a steep dive. When escaping from a strong thermal, it may be safer to put the plane in a deep stall instead of diving it down. The fancier ones also have spoilers and/or crow to keep the speed below Vne when diving out of a strong thermal.

I don't recall that the aircraft were necessarily to be man carrying --.
however
way back when Wendel Wilkie was still a viable candidate -
the vertical dive bomber was developed .
The objective was to get there - get into position drop the load and depart -all in minimum time
from all the old pics I have seen - the dive angle was pretty steep on some
I also have some on board camera footage of airshow stuff where th camera clearly shows an extended vertical dive and a roll rate of about 720 per second---

I agree completely. But I still think Dick's purpose-built vertical diving dragsters idea has merit, assuming they both have the drag devices to keep right at Vne in a vertical dive. The both reach Vne quickly, but the lighter one a little quicker. Now they are both at the same speed straight down, but the lighter one is a little ahead. Now the only way the heavier one can win is if the ligher one slows down (or breaks up).

Didn't most dive bombers have dive brakes? They kind of looked like flaps but had a lot of holes in them. Before "smart bombs", dive bombing was the most effective way to make a precision airstrike.

I agree -- in that particular scenario.

One could construct many different scenarios.

There was actually some similar real-world testing conducted by the British Air Ministry in 1944.

The P-47, P-51 & Spitfire XI were formally compared for the purpose of determining whether either of the US fighters were more suitable than the Spit as reconnaisance aircraft. One of the tests was maximum diving speed. The Mach meter had been developed by that time & diving speeds would be measured as Mach number.

Production Spit fighters were limited to 0.85 Mach, although in combat that was occasionally exceeded. It was considered that the recce birds, which were often (usually) unarmed, should be capable of either out-climbing, higher-flying, out-running, or out-diving any potential adversary.

The P-47 was the also-ran at only 0.78 Mach, the 'Stang hit a solid 0.80 Mach & the Spit XI reached a stunning 0.90 Mach --a pace that went unmatched until the arrival of the XP-86.

One of the problems with high-speed diving was engine failure from over-revving, despite closed throttles -- this sometimes occurred with the Spit combat fighters if they ventured past 0.85 Mach, so the Mk XI was fitted with a full-feathering Rotol prop to permit sustained dives from ~45,000 ft.

It would have undoubtedly reached the ground first.

As Robert Johnson observed in "Thunderbolt"... "They never learn.." as he dove down behind a Bf-109 (lighter) trying to escape his P-47 (heavier)..
It's a combination of gravity, thrust and drag. The plane with an excess of thrust over drag will accelerate -faster- than a plane with less of a thrust-drag delta. The faster accelerator gets there first, as gravity is the same for both.
In the quoted instance, the reason "they" never learned was that Johnson killed them, and whatever "they" had learned died with them, Johnson coming down behind them like an anvil and blowing them out of the sky.
And from the P-51 flight manual...

Thrust has to overcome inertia too. And at the beginning of an aerial drag race, where speed and total drag are low, inertia is more important. Between two airplanes with the same thrust and drag but different masses, the ligher one will accelerate more rapidly, which is exactly what Dick specified. The heavier one could reach a higher top speed at the same glide angle if allowed (because it has more thrust), but the scenario doesn't allow it. This assumes the airplanes are not already AT Vne when they start the race.

BTW, love the page from the manual.

edit: spelling, dammit.

Gee, that explains why the featherweight Spit was ~ 80 MPH faster in a dive than the Jug.

Compared to the Bf 109 G6, the fact that the Jug had a 40 MPH advantage in level speed, an advantage of 825 HP, & a lower Cd had absolutely nothing to do with it, of course.


Johnson's empirical observations are valid enough, but his conclusions are something else altogether.

The Spit's light weight wasn't its advantage, its thinner wing was!
The critical Mach says that.
I have never seen a Spit competing in an unlimited air race.
Corsairs... Sea Furys.. big round motors.. lots of drag, lots of excess thrust over drag.
A Corsair could suck a Zero (1/2 the weight) into a dive.. the Zero would reach a pont where its control would be unmovable.. about 330 mph or so, while the Corsair could get past 400, zoom back up past the Zero stuck in the dive, and shoot it down.

Couple more pages from the P-51 manual...
Despite the -suggested- Vne, it is quite possible to get the plane past that to where the ground riseth up to smite...

There were almost no Spits left to race with -- they were largely broken-up or sold to back-water nations where they rotted away. The survivors were immensely valuable as collectibles -- much too valuable to go racing with.

--- and the round-engine guys have never been beaten by a 'Stang of course.

Re. the Bf 109, I don't disagree, but the "Gustav" was a flying aero-brick & the Jug had thin wings like the Spit, plus a nice low Cd. Like the Zero, the 109 had never-resolved control-force problems at high speed, so in a dive it could neither out-run nor outmaneuver a Jug -- the dumb ones died.

At low level it was a different story -- both the 109 & 190 could give a Jug fits, as could a Zero. Some of the speed advantage was gone & the 109, 190 (& Zero) could outfight the Jug if its movements were constrained by ground proximity. Problem was -- there were few top German drivers left in the air at that point in the war. Most of the experts were gone by that time -- Spit & Yak fodder.

Even the 'Stang wasn't much of a dancer on the deck, but it was a lot better that a Jug.

Apples to oranges comparisons don't get us anywhere with this entertaining but meaningless discussion -- I'm sticking to my "solution".

Front view.. Jug vs Spit.. to scale
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In a post-war analysis of which plane was the better fighter.. the P-47 was selected over the P-51 in almost every respect.. Which probably explains
why I saw the Jug in Germany in 1950, no P-51s.. These were mothballed or placed in areas where imminent contact with the Sov AF was expected.
The Jugs were replaced by the F-86 and F-84.

There are so many questions to answer. So let me just give it away and see what you guys think now.

[xxx I wrote this before I saw Britbrats post, so I've basically restated what he said xxx]

Two airplanes (powered or not) identical in everything but their weight are flying at the same height above ground over a runway (or a field, or whatever). They decide to race to the ground to see who gets there first. So they both hit full power and nose the airplane down. The acceleration of the two planes due to gravity is identical. The drag of the airplanes is identical. The acceleration due to thrust is larger for the lighter plane due to this equation.

Thrust(Force) = Mass*Acceleration --> So, Acceleration = Thrust/Mass

If they both have the same thrust, the airplane with less mass will have a higher acceleration. So, the lighter plane already has an advantage, but let's not dwell on this too long because this isn't the main part of the argument.

So, for arguments sake, let's ignore the thrust and assume both have lost power (Or they are gliders to begin with) thus taking the advantage of the lighter plane away.

Now, they both are in a dive and accelerating (increasing speed) until they both reach Vne, which will happen at the same time. At this point, the pilots will both pull up until their respective planes are at the glide slope that results in Vne. The principle here is that at every angle of descent, the aircraft will speed up until the vertical components of lift and drag equal its weight. This means they reach equilibrium. The pilot of the lighter plane will be able to stay at a steeper angle of descent than the heavier airplane without overspeeding. As many people have pointed out, the rate of descent is equal to the speed, Vne, times the sine of the angle of descent. Therefore, the lighter airplane will be able to come down faster.

The same would be true if both airplanes had the same weight, but one was using speed brakes. The one with more drag could descend at a higher angle. This is how pilots (full-scale) view flaps. Airplane designers think of flaps as a means to increase maximum lift coefficient. But most pilots view flaps as a means to take a steeper approach without increasing speed.

The best real world demonstration of the principle behind this question is the post about landing a foamy without power. You practically have to point the nose straight down to keep your speed up.

There was also a post about wing loading affecting L/D. You are correct wing loading doesn't really affect the max possible L/D, it just increases the speed at which you achieve max L/D. However, for almost all airplanes, you will not be operating at the speed for best L/D. So, increasing wing loading will increase the cruise L/D even though the max L/D is unchanged.

Sounds familiar

Sig, I would like to correct your lift argument, primarily the underlined comment.

In order to maintain a certain speed (Vne in this case) the airplane must achieve a state of equilibrium, meaning the sum of all forces in all directions is zero. If not, then there will be an acceleration or change of speed. So, now assuming that the plane is in equilibrium, the sum of the vertical components of lift and drag must be qual to the aircraft weight. Since both aircraft have the same drag, the lift of the heavier airplane must be more than that of the other. This means the airplane will have a higher angle of attack.

The point is if two airplanes with the same geometry, but different weight are flying at the same speed in a state of equilibrium then both the lift force and lift coefficient are different for the two airplanes. The only way to achieve this is by increasing AOA.

It's a difference between rates and accelerations. If an airplane is producing less lift than weight it will be accelerating downward, meaning it is increasing speed. This violates the constraints of the problem.

That's quite interesting. The Jug wing looks remarkably like the Typhoon wing in section.

I suppose that the superiority of the Jug was the reason why the USAF & RCAF kept P-51's on strength into the 1950's, including sending them to Korea, while Jugs were relegated to USAF reserve air-defense Sqn's (they could climb fairly quickly & had lots of nasty 50 cals).

I know it's true, but I can't imagine why anything would be replaced by an F-84. Manned target drones. We used to love "waxing" RF-84F's.

Selecting which weapon is used / retained. HAS and ALWAYS BE CONTROLLED BY INSIDER POLITICS in the final selection.

Including the F-22 over it's competitor. Pilots inputs about which was better was totally disregarded in the decision.

Nothing changes.

Except who has more skill and luck.

Thank goodness we train the daylights out of them and are still allowing a cannon in the plane for doing the job after a VISUAL VERIFICATION is MANDATED. [sm=thumbup.gif][sm=thumbup.gif][sm=thumbup.gif]

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You say that like it's a good thing. The Jug wing is the same as on the P-35 and P-43. Far from laminar.
The wing on the Tempest/Fury series was laminar,and a lot thinner, which permitted the Tempest to do the job the Typhoon failed at.. interception.
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As mentioned, that IS the reason. The North Korean Air Force.. all 10 planes was destroyed in the first week of the war. There was no need for fighter -interceptors in the Far East, but the Soviets -still- had a very positive in Europe. When the Soviets and Chinese interfered in the Korean "police action" with the MiG-15, then a suitable opponent was introduced.
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Must have been quite a thrill.

It's true that the Typhoon wing was thick like the Jug's & quite unlike the Tempest & Fury wings. For some reason the Jug was comparitively better that the Typhoon at altitude. The Tiffy was not only pretty fast on the deck, unlike its performance at altitude, it was a respectable dog-fighter down there, whereas the Jug was best fighting higher up, rather than on the deck.

I know what VNE is. But what is the VNE for each plane? I would bet some planes can reach terminal velocity and not be above VNE. Power off, dirty config.

So we fly along at VNE some one yells go, we nose over still maintaining VNE, who wins? Well no one, IF VNE is the same for both planes, otherwise the plane with the highest VNE wins.

Or we hang nose down on the side of a building, and someone yells go and we release the planes at the same time now who wins? Obviously who ever reaches VNE first. So at idle who will win. If both planes are set to full power who will win. If the planes are only 4" off the ground will the time interval between release and impact be measurable with a 1920's pocket watch?

To many unknowns.....