Author Topic: Utilizing the P-51D's instability  (Read 7302 times)

Offline BaldEagl

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Utilizing the P-51D's instability
« Reply #60 on: January 07, 2008, 04:20:08 PM »
Quote
Originally posted by Krusty
Wind tunnel models are so folks know when the wing rips off, to simplify it.


They also use them to test fast food burgers ;)
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Offline Hoffman

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Utilizing the P-51D's instability
« Reply #61 on: January 07, 2008, 05:25:16 PM »
Krusty, if an RC aircraft can do the maneuver, and the only difference between a P-51 and an RC aircraft are size and engine power.

Then the only thing necessary for the P-51 to duplicate the maneuver is to put the same forces in the same areas of the airframe.  Which is entirely do-able.

That an RC aircraft can hover on its prop is irrelevant, engine power is irrelevant.  The only relevent thing to be tested is whether or not the same amount of forces can be placed on the same areas of the airframe.  Obviously because an RC plane is many times smaller and lighter than a P-51, it takes less of the same force to duplicate the maneuver.

But if you properly scale an RC aircraft to a P-51's size, and manage to get an engine that has the same power, torque, and ability in scale then there is nothing the real P-51 airframe can do that the RC aircraft cannot.  Or vice verca.  (Excepting of course that the P-51 has a pilot and pilot limitations over RC aircraft)

The only limitations would be gusts of wind having more effect on the RC aircraft due to less mass.

Offline gripen

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Utilizing the P-51D's instability
« Reply #62 on: January 07, 2008, 05:51:56 PM »
My Filip 400 Sport warmliner does pretty wild snap roll but I don't have good clip of that. It kills speed from around 120-150km/h to less than 30km/h in couple seconds. What separates snap roll from barrel roll is that it is a stalled maneuver and the other wing stalls before the another which causes that typical rapid flick.

Same physics apply to real planes as RC planes, however RC planes are often purposedly designed  for flick maneuvers. My favorite stalled maneuver is the climbing inverted flat spin ie it's a flat spin where the inner wing is stalled but the spinning speed is so high that the outer wing generates enough lift to stop sinking and actually makes the plane climb. I have not ever seen a real plane to do it. I'll try to take video in next indoor session because I have one plane which does it particularly well.

Offline Chalenge

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Utilizing the P-51D's instability
« Reply #63 on: January 07, 2008, 09:26:00 PM »
The scale guys have another name for this and it isnt a snap-roll but its close to one. Ive been scratching my head trying to remember the name of it but I also know its called by different names around the world. A Czech pilot used this as the spring board for his lomcevak and I keep expecting Widewing to mention that. Back in the 70s Hoover had a friend that was always doing this move as a reversal in his stunts. His name will come to me when I least expect it.

Edit: Humpty-Bump
« Last Edit: January 07, 2008, 09:29:59 PM by Chalenge »
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Offline dtango

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Utilizing the P-51D's instability
« Reply #64 on: January 08, 2008, 12:32:24 AM »
Quote
Originally posted by Krusty
Wind tunnels are used to test air flow over certain air shapes. They test for aerodynamics, drag, etc. They're definitely not being used in the same manner as a sideways-sliding P-51 going butt-first into the wind.

Wind tunnel models are so folks know when the wing rips off, to simplify it.


Sorry to have to break this to you Krusty.  That's incorrect.  They do much much more than use them to just "know when the wing rips off".  

The basic premise of scale testing of models is that if you can replicate a scale model of the aircraft (sometimes with even simulated working power-plants) and scale the reynolds and mach numbers then you have a high probability of a very reasonable estimation of how the aircraft might perform.

To give just an inkling, let's just talk about one (of many types) of wind tunnel tests- free-flighting models in wind tunnels.  This goes all the way back to the 1930's for NACA where they free-flight wind tunnel test models by literally flying a scale model with prop running in a wind tunnel like the following...

1940- Free-flight wind tunnel testing of an XF4U-1



Here's a more modern example of free-flighting an F-18



Free-flighting in wind tunnels gives us all sorts of insights into the stability and control characteristics of aircraft for many different flight situations including stalls, spins, and very high aoa studies.  Here's a sample of the type of stuff you can glean:



This happens to be some analysis of the lateral-directional flight characteristics of this baby, the X-29 thanks to free-flight wind tunnel testing:



Scale model tests in wind tunnels isn't done just for analyzing drag.  

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« Last Edit: January 08, 2008, 12:38:04 AM by dtango »
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Offline Chalenge

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Utilizing the P-51D's instability
« Reply #65 on: January 08, 2008, 08:33:59 AM »
Quote
Originally posted by Chalenge
Edit: Humpty-Bump


No thats not it either.  Its tough getting old.
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Offline MachNum

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Utilizing the P-51D's instability
« Reply #66 on: January 08, 2008, 11:21:42 AM »
Wasn't there a manuever called the "Hartmann Escape" that Luftwaffe ace Eric Hartmann espoused that involved a stick in one forward corner with opposite rudder? This sounds very similar to that.

Offline SlapShot

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Utilizing the P-51D's instability
« Reply #67 on: January 08, 2008, 11:34:50 AM »
Quote
Originally posted by Krusty
Wind tunnels are used to test air flow over certain air shapes. They test for aerodynamics, drag, etc. They're definitely not being used in the same manner as a sideways-sliding P-51 going butt-first into the wind.

Wind tunnel models are so folks know when the wing rips off, to simplify it.


Your absolutely sure about that ?

It is you that is over simplifying the realm of scale/wind tunnel testing and it isn't helping you.

Thanks to Tango for just scratching the surface of what has and can be done with wind tunnel testing ... beside seeing when wings rip off ... :rolleyes:
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Offline BaldEagl

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Utilizing the P-51D's instability
« Reply #68 on: January 08, 2008, 01:40:41 PM »
I can tell you when the wings rip off.  No need for a wind-tunnel.  :D
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Offline Krusty

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Utilizing the P-51D's instability
« Reply #69 on: January 08, 2008, 02:15:10 PM »
I realize they tell you a lot more than that, dtango. I was being super-simplistic.

They may tell you a lot about how the air flows over a perfect scale model of the real plane (NOT a non-scale flying styrofoam plane), but it's NOT going to tell you what forces of drag, inertia, torque, horsepower, and airflow, are all going to act and make a plane tumble out of the sky in full power combat manuvering.

Comparing RC planes (and diverting the topic to wind tunnels) to real P-51 mustangs is like comparing my bicycle to a F-1 race car.

To quote y'all "the same physics apply to both!!!" -- yeah, but not the same FORCES on both. Not the same interactions of forces, and not the same results.

Offline dtango

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Utilizing the P-51D's instability
« Reply #70 on: January 08, 2008, 03:01:20 PM »
Quote
Originally posted by Krusty

....They may tell you a lot about how the air flows over a perfect scale model of the real plane (NOT a non-scale flying styrofoam plane), but it's NOT going to tell you what forces of drag, inertia, torque, horsepower, and airflow, are all going to act and make a plane tumble out of the sky in full power combat manuvering.

...To quote y'all "the same physics apply to both!!!" -- yeah, but not the same FORCES on both.

Sorry Krusty, that is very incorrect.  Infact they tell you what the forces, inertia, torque, etc. are going to be - all that with scale models in wind tunnels.  That's why they do scale model testing in wind tunnels.

There are certainly differences betweeen wind-tunnel models and testing vs. RC model flight.  The differences have to do with the amount of fidelity from similarities (or lack there of) in reynolds numbers, mach numbers, mass distribution, etc. etc. compared to the full-scale aircraft in flight.

But the same physics apply to all (full-scale, wind-tunnel models, RC models) :).

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Offline SlapShot

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Utilizing the P-51D's instability
« Reply #71 on: January 08, 2008, 03:25:02 PM »
Here is a snippet from an article on the NASA X-48 Blended Wing Body (BWB)

X-48A BWB-LSV Description

The BWB-LSV is a 14%-scale version of the 450-passenger study aircraft. Built primarily of composite materials and weighing about 2,500 lb., the platform features a wide arrowhead-like body that blends into tapered wings swept aft. Flight control surfaces, or elevons, span the trailing edges of the wings while the rudders are located in winglets on each wing tip.

Three 240-lb thrust turbojet engines, from Williams International Corporation, Walled Lake, Mich., were mounted on low aerodynamic pylons across the rear portion of the center body. All three engines will operate from a single fuel tank located near the vehicle's center of gravity. The maximum speed of the BWB-LSV would be about 165 mph.

Electric actuators in the flight control system link the exterior control surfaces with a central digital fly-by-wire flight control computer carried in the center body of the aircraft. The aircraft was to be flown by a NASA research pilot sitting at a cockpit station in the remotely piloted vehicle (RPV) facility at the NASA Dryden Flight Research Center. Instruments and displays in the RPV cockpit will provide the pilot with the same systems and performance data commonly displayed in conventional research aircraft cockpits.

Two small video cameras was be installed on the BWB-LSV. One, behind the mock cockpit windscreen, presents a forward-looking view on a large video screen in the RPV cockpit station. The NASA project pilot will use this view, along with the cockpit instrument array, to fly the vehicle. The second camera was mounted atop the rearward portion of the center body, to view external areas of the vehicle during flight.

Numerous sensors installed throughout the vehicle measure aerodynamic loads, air pressures, temperatures, engine performance, and other important test and research parameters during each flight. Data would be automatically transmitted to the Dryden mission control center and monitored during flight by project engineers and other members of the test team.

A spin recovery system built into the test aircraft would allow the vehicle to be flown to its maximum angle of attack and as slow as its stall speed. The system will be used to deploy a parachute if the vehicle begins an uncontrollable descent, such as an unrecoverable spin. The parachute attach line would be cut, separating the vehicle from the canopy as soon as stabilized flight could be resumed.

Construction of the BWB-LSV began in early 2000 and was scheduled for completion in late 2002. Integration and ground testing of the vehicle was to continue through 2003, followed by the test flight program. When assembly of the BWB-LSV was completed at the Langley Research Center, it was to undergo three months of wind tunnel testing at the Old Dominion University (ODU) Full-Scale Wind Tunnel Facility in Hampton, Va.

Research in ODU's massive 30 by 60-foot wind tunnel wwould include operating the engines and the external flight control surfaces at various air speeds. Data from this research will give engineers and designers a better understanding of the aerodynamics associated with the BWB's unique design prior to flight, as well as a unique opportunity to test the same vehicle on the ground and in flight.
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Offline Krusty

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« Reply #72 on: January 08, 2008, 03:36:54 PM »
dtango, they cannot tell you the intertia, torque, or some other aspects of the real deal, because it is a model. Even if it has the scale WEIGHT, it does not have the same weight-to-surface area ratio. The same reason that most "enlarging-ray" movie plots are scientifically implausible. You can't just make something bigger and have it act the same way. The density increases while the ratio of mass to surface area decreases. There's a lot more at work than I think you give credit for.

They may tell you how the air will flow if you blow the air sideways, but they are NOT going to tell you how the combination of torque, drag, gravity, momentum/inertia and surface area into the wind are going to combine to show you how a plane will flip about.

I know wind tunnels are not my area, but even *I*'ve read enough about basic elementary school physics to piece this one together. An RC model will NOT act the same way a full-scale real warbird will. Period.


Again, unless you can show me a real P-51 that can hover on its own prop, climb, and land, flip over as if it weighed 6 ounces, and then do a barrel roll and a flip before going back to hovering, all in a box no bigger than its own wingspan.

P.S. The whole wind-tunnel side-track was Slapshot trying to divert the issue.

Offline Krusty

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« Reply #73 on: January 08, 2008, 03:43:14 PM »
Oh, and slapshot? We know that. That's what wind tunnels do. They let you know how the air will interact around a shape. It will be a big clue if the plane needs a larger stabilizer, or something. Most companies do use models to prove a design will fly. Even Leonardo used a models for his designs. It doesn't mean they are equivelant to the real thing.

Don't tell me you think the average RC pilot is as accomplished as NASA with their designs? Or are you saying NASA is so amateur that a Korean teenager can out-design them for the real deal?!?! (*gasp*)!!

By your logic, any 3-year old with a store-bought RC battery-powered plane has as much testing capability and flight capabilities as NASA and real aircraft combined. You're stretching.


EDIT: Let me try and put it in a way so simplistic even you could get my point:

Your RC plane weighs a few pounds, max. Wingspan (let's say) 2.5 feet. P-51 average wingspan, 50 feet. That's 20x the scale. Now weight... P-51 let's say 12,000lbs. Divide by 20 (the scale of the RC plane) and the little tiny 2.5-foot RC plane has to weigh 600lbs to even come close to approximating the P-51. I don't think even if it were made of solid lead it could weigh that much. I don't even know where to begin with the horsepower to weight comparisons, or surface area to weight ratios! Imagine a P-51D with a 2800-hp engine but it weighed only 1,000lbs. I'm sure such a mythical plane COULD hover on its prop and flip around like an RC plane, but the reality of physics doesn't hold up to that based on the real P-51s weight, mass, thrust, inertia, and aerodynamics.
« Last Edit: January 08, 2008, 03:50:27 PM by Krusty »

Offline BaldEagl

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Utilizing the P-51D's instability
« Reply #74 on: January 08, 2008, 03:50:01 PM »
Quote
Originally posted by Krusty
By your logic, any 3-year old with a store-bought RC battery-powered plane has as much testing capability and flight capabilities as NASA and real aircraft combined. You're stretching.


Bad analogy Krusty.  I think that NASA engineers have a lot more training, experience and comprehension than any 3 year-old.
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