What determines an airplane's roll rate?
John F Carr
about something. What about an airplane gives makes it roll quickly
or slowly? Ailerons on nearly all WWII fighters were unboosted, so
the force available to move the control surfaces is more or less
constant. Yet among planes of similar size, weight, wing span, and
wing loading some roll much faster than others.
--
John Carr (j...@mit.edu)
Charles K. Scott
This is a good question which I'd be interested in learning more about
myself. I have my speculations, for what they're worth. First, not
all fighters had the same wing loading, the P-47 for instance had the
highest wing loading of any single engine aircraft to see combat during
WWII. Some sources of information allude to it's roll rate as being
the highest of any fighter. This is VERY hard to verify because combat
does not occur at one specific speed where roll rate can be measured.
SPORT AVIATION had a fly off a few years ago in which they tried to
match up a Spitfire, P-47 Thunderbolt, and a P-51 Mustang. They used
various maneuvers and pulled various G's all over the sky in an attempt
to once and for all put to rest the hangar combat between boosters of
each aircraft. What was the result? Mustang lovers were pretty angry,
or so it seemed from all the mail that came in afterward. Was anything
settled? Far from it because the aircraft were not configured as they
flew during WWII. All of the airplanes had been modified. None of
them had their armor or guns and ammo, the P-47 was missing it's
supercharger and the Mustang did not have the rear fuselage tank as
well as missing it's armor and ammo. Does this make a difference?
Well lot's of readers thought so. For instance the Mustang was
castigated for having an exceptionally high stick force, requiring two
hands for most maneuvers but editorials from ex Mustang pilots recalled
gentle stick forces, one guy remembering doing slow rolls with two
fingers on top of the stick. Also, maneuvers that were supposed to
explore the flight characteristics of the aircraft at high G forces and
steep turns revealed a truly treacherous high speed stall in the
Mustang.
As I recall, the maneuver was a high speed vertical climbing turn and
at the proper angle of attack, the Mustang suddenly stalled and blew
out of the turn tumbling in the opposite direction; not a nice thing to
have happen during combat. This was no fluke as the maneuver was
repeated with the same results.
So what aircraft was rated best? The P-47 seemed to be the steadiest
in tracking a target, making it the most stable gun platform but it's
gargantuan nose viewing the target difficult. So in the end, as I
recall each aircraft was noted as having some favorable properties and
was probably superior in one maneuver or another but that none of them
seemed to be notably better overall from one another.
So we're back to hangar flying.
Corky Scott
John Caraher
Charles...@dartmouth.edu (Charles K. Scott) wrote:
> In article <32s0bo$3...@senator-bedfellow.MIT.EDU>
> j...@ATHENA.MIT.EDU (John F Carr) writes:
>
> > I've been reading about WWII era airplanes recently and I'm curious
> > about something. What about an airplane gives makes it roll quickly
> > or slowly? Ailerons on nearly all WWII fighters were unboosted, so
> > the force available to move the control surfaces is more or less
> > constant. Yet among planes of similar size, weight, wing span, and
> > wing loading some roll much faster than others.
The cheap answer is that the aircraft that has the largest ratio of
"aileron torque" to roll moment will have the best rolling performance. But
of course, this is just freshman physics.
Most of this is guesswork based on the physics of the situation but I think
in principle there are two types of roll performance one might want to
measure. One is the "roll response," by which I mean how fast an aircraft
can reach its maximum roll rate, the other would be that maximum roll rate
itself. I'd imagine that in general it is roll response that is most
important; the important point is that there are slightly different
considerations contributing to each.
The top roll rate for a given speed would be achieved when the torque
available from the control surfaces (adding or subtracting whatever the
engine contributes, in a propeller-driven aircraft) equals the torque
generated by drag forces along the wings and stabilizers. The weight of the
aircraft, its inertia about the roll axis, wing loading, etc. shouldn't
matter here, since the aerodynamic forces will be more a function of wing
shape and size than anything else. The top roll rate may be limited by some
structural or other considerations. I also suspect that the effectiveness
of ailerons might decrease as roll rate increases, so it is possible that
the limiting factor might be the rate at which aileron effectivess
decreases.
But this is probably not the most important measure of roll performance; a
good initial roll rate probably matters more in combat. So what you want is
to maximize angular acceleration about that axis, and at first glance you
might think the way to do that is to have short, stubby wings (reduces
moment of inertia) and light weight (same benefit). But you have to trade
this off against the fact that short wingspan also means that for the same
aileron control force the torque is less, since it operates on a shorter
moment arm. So you must make some compromises on aspect ratio, and other
considerations (such as induced drag, skin friction, wing loading, etc.)
might override roll performance in wing design. Finally, how well-designed
and balanced the control surfaces are can be critical. For unboosted
controls the fundamental limit might be how much force the pilot can put on
the stick, but aircraft might vary in how efficiently that effort on the
part of the pilot is converted into aerodynamic forces. This sort of
information would be especially difficult to find in standard references
that concentrate on the types of stats you mention. Apparently the FW-190
had particularly excellent control response.
(And John, I know you're wondering this but no, I don't know how JD
generates his roll stats for Over the Reich...)
<P-51 stall stuff deleted>
From what I've heard the P-51s deservedly glorious reputation was built
less on its excellence 1 vs. 1 in mock dogfights than on its ability to
take the war to the Germans. Of course all the Mustang drivers believe
their plane was the best, but most pilots are pretty loyal to their own
aircraft. If range is not a consideration I'd say the late-model Spitfires
were markedly superior in performance to the Mustang, and the pilots who
flew for Israel, many of whom flew US fighters in WWII, considered their
Spitfires to be their most valuable aircraft over their Mustangs. The P-47
is also a much tougher aircraft than the other two (something many people
hangar-flying fail to factor in - they're so focused on outflying the other
guy that they forget that in real combat a survivable aircraft is also
important, especially given the dangers of the bounce). All in all the
results of the tests described sound pretty fair to me; while removal of
much of the equipment probably does affect the results somewhat it's hard
to see how that would be to the detriment of the P-51, especially on the
stick forces.
If I need to escort bombers or have other long-range requirements give me
the P-51. If range doesn't matter and the mission is close support I'll
take the Jug. If I'm fighting a 1 vs. 1 from an even start I'll take the
Spitfire. All three aircraft were winners, but for all their virtues the
Thunderbolt and Spitfire could not make it to Berlin, making the P-51
infinitely more valuable to B-17 crews...
> So what aircraft was rated best? The P-47 seemed to be the steadiest
> in tracking a target, making it the most stable gun platform but it's
> gargantuan nose viewing the target difficult. So in the end, as I
> recall each aircraft was noted as having some favorable properties and
> was probably superior in one maneuver or another but that none of them
> seemed to be notably better overall from one another.
> So we're back to hangar flying.
There's no hope of ever ending hangar flying anyway - and it's a lot more
fun to argue, especially when it isn't a life-and-death procurment issue...
--
John Caraher
car...@umich.edu
Nicholas Strauss
I thought it deserved its own subject...
I've got a reprint of the Sport Aviation "flight test" myself and
had wondered the same thing about the Mustang's very high
stick forces (something like 20lb per G!).
I think the removal of the fusulage tank and radios must explain
it:
With the fusulage tank full, the stick forces were very light, and
could reverse at high G. (This means that you will start to
pitch up on your own...you have to PUSH out of the turn/dive).
With all the mass of the radios (remember, these are 1940s radios!)
and the fuel tank, even just its structure and plumbing, sitting
there aft of the CG, it would make a big difference to how the
aircraft handeled.
Later build Mustangs had a bobweight added to the control collum
(whatever a bobweight is) to raise stick forces. With all the
equipment being added, they'd gotten too light (not to mention
the reversal). Its possible the test aircraft had these modified
controls...so with the lighter aft weight, you'd have doubly high
pitch forces.
Dunno, just some thoughts...
--Nick
--
______________________________________
\ \ _ ______ |
\ NICHOLAS STRAUSS \ / \___-=O`/|O`/__|
\ Silicon Graphics-Network Operations \_______\ / | / )
/ nstr...@netmare.corp.sgi.com / `/-==__ _/__|/__=-|
/ pi...@leland.stanford.edu / * \ | |
/_____________________________________/ (o)
"What, are you kidding?? We're on a spaceship! This place is *crawling*
with toothpicks!" -- the silly quote
"Who dares, wins." -- the serious quote
Paul L. Suh
(John F Carr) wrote:
One often ignored factor is airframe stiffness. The F/A-18 had a problem
with this during its initial development. It was so bad that the
prototype had a rate of roll of less than 50% of the design spec (100
deg/sec vs. 240 deg/sec at some specific altitude and speed if my hazy
memory is right). This happened because the force on the wing from the
ailerons actually warped the wing in the opposite direction:
Side views:
level flight
----- ----------------------
aileron wing
in roll
\ -------
\ -------
\ -------
This partially counteracted the effect of the aileron, reducing the force
rolling the plane. This was fixed by extensively strengthening the wing
structure. McDD/Northrop never did get the rate of roll fully up to spec;
I think the figure is 220 deg/sec on production Hornets today.
--Paul
jk...@ragtime.vnet.net
Hmm.. interesting; I don't suppose you've heard of AirWarrior; a realtime
WWII flightsim in the GEnie network ... Basically, you have upwards of 100
pilots in the arena, flying late '30s, mid '40s vintage warbirds..
A friend of mine went to the NASM, paged through ALL the available
docs, test reports, etc for these planes, and turned them over to the
programmers who run AirWarrior ... the result? Well, the mustang lovers were
WAY pissed ... it exibited the EXACT razor-edge stall characteristics that
you report here ... not exactly "Cadillac of the Skies".
When fed the test data, the CPUs at AW central came to the same conclusion
that your mag reporters did.
BTW, I heartily recommend that you NOT try AirWarrior; it's the most
addictive experience you can encounter, while sitting down. RCAF pilots
used to have their own squad ... it's TOUGH fighting vs REAL fighter
jocks.
It truly ruined me for ALL flight sims.
Jkay
John F Carr
Paul L. Suh <pl...@econ.sas.upenn.edu> wrote:
>One often ignored factor is airframe stiffness. The F/A-18 had a problem
>with this during its initial development. It was so bad that the
>prototype had a rate of roll of less than 50% of the design spec (100
>deg/sec vs. 240 deg/sec at some specific altitude and speed if my hazy
>memory is right). This happened because the force on the wing from the
>ailerons actually warped the wing in the opposite direction:
This happened with WWII fighters too, but from the few descriptions
I have read it seems it was not a serious problem below about 350-400 mph.
--
John Carr (j...@mit.edu)
Earl Kaye Dille
a group of us flight students were standing outside the main hangar at North
Whiting Field, Pensacola, when a Mustang landed and taxied over to us. An
Air Force light colonel jumped out and asked where LCDR Rhodes was.
Apparently the leader of the Blue Angels, Dusty Rhodes, had had a few
drinks at the Eglin Field officers' club the night before, and had bragged
about the superiority of the F8F Bearcat. One of us went and got Rhodes,
and he soon got there. He jumped into his Bearcat, and we were treated to
a great display over the field. Some dogfight, what we could see of it. It
was generally thought a tie. The F8F could outclimb and outturn the F51, but
the F51 could dive faster. Who knows which pilot was really the best.
The best "all round" prop fighter probably was the FW190!
--
Earl K. Dille
ekd...@artsci.wustl.edu
John F Carr
John Caraher <car...@umich.edu> wrote:
>But this is probably not the most important measure of roll performance; a
>good initial roll rate probably matters more in combat.
I read some WWII-era NACA reports on aircraft control. For most aircraft
roll rate approaches peak rate exponentially with a timescale of about .2 to
.5 seconds. NACA TN1670 says "If the ailerons are suddenly deflected an
airplane ordinarily reaches its steady rolling velocity very rapidly."
According to NACA report 715 the peak roll rate typically satisfies the
equation pb/2V > .07 for aircraft considered to have good handling qualities
(p = roll rate, b = span, V = speed). At V = 120 m/s and b/2 = 6m, p >= 1.4
rad/sec. (Compare an actual figure from an independent source: a late model
Spitfire rolls about 2 rad/sec at 130 m/s).
What the reports I've read don't explain is why aircraft designed for similar
missions don't all have similar rolling performance.
--
John Carr (j...@mit.edu)
John Caraher
F Carr) wrote:
There's a brief discussion of rolling performance in the appendix to Robert
Shaw's _Fighter_Combat:_Tactics_and_Maneuvering_ in which he lists
considerations related to roll performance, most of which have been
mentioned in this thread already. In addition, he says that peak roll rate
is not as important as roll rate acceleration, on the grounds that it is
rarely necessary to roll more than 180 degrees to perform a maneuver.
--
John Caraher
car...@umich.edu