Fwd: Reno Air Race - Probable conclusion to fatal crash

The elevator - and the trim tab - do a LOT of work on a race plane at

500mph. And a lot at 200 too.
Reply to
clare
Loading thread data ...

Not exactly a taildragger, is it? Well, actually it sorta is - skids rather than wheels. LOTS o drag on that there tail.

Minimum drag, sure.

Zero drag, no.

It's still drag.

And you can calculate how much will be there under any given conditions.

Reply to
Richard

Absolutely.

Add to that there is a lot of speed changes as the aircraft hits the turns.

TANSTAAFL...

Reply to
Richard

Explain! I haven't seen any explanations here, just claims. Richard claims the nose must forcibly be held down at 500 mph.

The airplane is nose-heavy already, it must be held up. If it takes 200 lbs of downforce at the tail to hold the nose up at 200 mph, it also takes 200 lbs of downforce to hold the nose up at 500 mph.

We want that 200 lbs of force to come with as little drag as possible at

500 mph. Can anyone explain how full nose-down trim tab deflection + pushing the stick forward will produce the necessaty 200 lbs of downforce at the tail?

It will do it because the stabilizer incidence is all wrong for 500 mph. The stock stabilizer rigging is a compromise, to work acceptably through the whole speed range. At 500 mph it's pushing the tail down too hard, so Richard has to trim, and push the stick, to help hold the tail back up. The tail surfaces are working against each other. Pitched up/down crap hanging out in the wind.

Reply to
Beryl

Richard wrote: ...

The winning pilot minimizes speed loss and G forces through the turns.

That's why to optimize the airplane for top speed, the slow speed performance suffers.

Reply to
Beryl

You're the only one who specified Zero, wasn't me.

Well then, why ever point the nose up to climb? Just lower the flaps and add power, it's only drag.

Really?

Reply to
Beryl

Bye!

Wrong. Lift equals weight PLUS tail downforce. You can't "forget nose heavy" as you suggest.

Wrong. That trick is lessening the tail surfaces' angle of attack, and letting increased airspeed assume the task. There must be downforce at the tail, always. You only think you're "pushing the nose down" because you're fighting the stabilizer angle of attack, which, at 500 mph, is far overdoing its job of holding the nose up. Maybe you need to think Stabilator to make the light bulb come on.

Reply to
Beryl

Not on this planet...

Reply to
Richard

You actually think the airplanes mass balance changes with speed?!!

Reply to
Beryl

I think that might be me and I was quoting from the acceptance test done by the AAF for the P-51H to verify the manufacturer's figures.If I remember correctly it stated that at top speed the trim was such that a great deal of force was necessary to hold the plane in straight and level flight. (I'm not at home and accessing the Web through a very slow modem or I'd look it up)

You are forgetting that as speed increases so does lift. As the main source of lift is forward of the elevators as lift increases so does the nose up tendency. Thus the full down trim and high stick forces to hold it level at very high speeds.

I believe that you are forgetting that wind pressure goes up at something like the square of the speed. wind pressure against the control surfaces will be substantially more at, say 400 MPH, then it was at take off, say 75 MPH. Thus control surfaces produce more force at high speeds then they do at low.

But, if you set the incidence of the horizontal stabilizer at sufficiently high an angle, i.e., build in nose down trim, will you be able to get the nose up high enough at low speed to fly?

Reply to
John B.

No an aircraft's mass changes as fuel is consumed or something falls off. But you seem to be forgetting that lift changes. Note also that the CL moves as lift changes. You seem to be thinking of a static device when you talk about 200 lb. to hold the tail down at 200 mph then it only takes 200 lbs at 500. That is wrong, the forces acting on an aircraft change, rather radically, with changes in speed; among other variables. If your thesis was correct there would be no need of trim tabs at all. Just built it in and away we go.

Reply to
John B.

John B. fired this volley in news: snipped-for-privacy@4ax.com:

Stop for a moment and think about what you wrote. CL is aft of CG (must be for it to be "nose heavy") -- and besides, CG aft of CL loading is unstable and unsafe.

As the lift increases with airspeed, the CG doesn't change, so the nose- DOWN tendency increases with increased lift. That's just the opposite of what you wrote.

Also, if nothing else changed, a tendency to point nose-down would depress the effective angle of attack of the tail, creating more down- force at the H-stab. So what would really happen is that the nose would tend to depress a little with increased speed, until it was counteracted by the more negative angle of attack of the horizontal stabilizer; It would reach a point where the forces were balanced.

For maximum "slippery-ness", you want the surfaces set up so everything is more or less neutral at the speed you're intending to go. To repeat my old saw, anything sticking out in the wind is just an air brake.

For a given target speed, these high-end racing guys aren't going to fly anything less than a fully optimized aircraft. They'll settle for less- than-optimum at approach speed to get the extra knots. That's a given.

Lloyd

Reply to
Lloyd E. Sponenburgh

formatting link
The Mustang's wing is symmetrical top and bottom, as you can see by sighting down it from the wing tip.
formatting link
jsw

Reply to
Jim Wilkins

Yes, the center of lift moves aft as speed increases. Which makes the nose become even heavier, calling for more nose-up trim, not nose-down. Which counters Richard's got-to-push-the-nose-down reasoning.

I'm ignoring other variables. There could be thousands, can't discuss what they all may be doing without losing focus on what the tail feathers do. Pressure on the canopy may force the nose down, while pressure on the cowl forces the nose up, while pressure somewhere else forces the nose down, while... up... down... up... etc.

That's Richard's thesis. He just said I should forget about the airplane being nose-heavy. Forget that, and, as you say, the whole trim problem dosappears, at any speed.

Reply to
Beryl

I hesitate to call it nose-down trim (or incidence). I still call it nose-up trim (or incidence, rigging, whatever) at any speed, but less of it at 500 mph.

When you can't get the nose high enough at low speed to fly, then you need to abandon your old "low speed" and accept a higher one.

I have not much more than 200 hrs in my logbook, almost all of it in

152s. And I haven't flown in 25 years. Richard has far more experience, I'm sure, and is more current. So what?
Reply to
Beryl

"Jim Wilkins" wrote

Isn't symmetrical when you can fold it in half along one plane, and two sides match?

Steve

Reply to
Steve B

"Steve B" fired this volley in news:L1Kgq.2427$ snipped-for-privacy@news.usenetserver.com:

And where does it say in that site that the Mustang's airfoil was symmetrical? Symmetrical airfoils are used predominantly for aerobatics where extended inverted flight or strong negative Gs need to be pulled, but they aren't very effective for range and speed. One has to maintain a significant angle of attack to develop lift (either way, up or down).

The site you quoted was talking about NACA-designed lamilar flow airfoils. That doesn't translate directly to "symmetrical".

LLoyd

Reply to
Lloyd E. Sponenburgh

Some interesting Mustang trivia from a UK site:

formatting link

It would have been more complete if you mentioned that the P51 was designed by North American Aviation, and production was started in California. The P51A had two problems: First was that Allison promised NAA and Larry Bell a 1,150 hp supercharged V12 power plant for both the P51 and the Bell P39 Aerocobra but tried and failed to copy the front mounted gear driven supercharger that Rolls Royce had designed into the Merlin Engine.

Both companies were forced at the start of production to use the naturally aspirated 750 Hp version of the same engine, which was great on fuel and reliability, but was too weak for both planes.

The second problem with the P51 was the wing air foil design which was a modification of a 1933 design. The air foil actually created drag at speeds over 200 mph that require tremendous increases in horsepower to overcome, and the faster the wing flew, the worse the problem became. The solution came from Cal Tech or U. of Southern California with a new air foil called lamilar flow air foil which allowed the air behind the wing to "knit" back together without creating excessive drag. It also allowed the centre of lift to be set to the centre of gravity of the plane, and the two stayed together as speed increased unlike the original air foil where the two centrelines separated making handling of a tail heavy plane at high speed nearly impossible. Boeing was given the air foil design and used it on the B29 with great success.

(note: NACA 66 series airfoil and a slightly thinner wing than that used by earlier Mustangs)

As for the engine in the Mustang, all but the P51A engines were made by Packard Motor Car Company in Detroit, Michigan as Rolls did not have the engine building capacity to supply the needs of their own planes, much less the Mustang and the Bell King Cobra. A real fight broke out between Packard and Rolls as at the time, the Merlin was only 1350 hp. Packard interviewed British and American pilots who had flown the engine who repeatedly told Packard that the engine was not even "trying" when at full power.

Packard made small modifications to the fuel system and produced 2,000 hp on their first try. Rolls said no-way were they going to have their name on that engine as it would not hold together. Packard had collected info that the average British fighter plane was shot down with only 97 hours on the engine. Rolls demanded 2,000 hours with only normal oil, fuel, and air filter changes and valve adjustments. Finally the War Department picked a number of 1,650 Hp and that was what went into production at Packard. Spare engines were sent to England to support the P51B and C. Spitfire pilots got hold of a few then demanded that Rolls at least match the Americans engines, which they finally did.

I grew up in Chicago and two of my neighbours flew Mustangs as bomber escort in Europe and in Korea against Yaks and Migs. The other fellow flew his against Japan from March 1945 on until the end of the war in the pacific. Both men loved their Mustangs. As Chuck Yeager said; it is not an airplane, it is more like a well tailored suit that you put on it fits so well you can?t believe it! It goes where you point it. Just fly it fast and use the see-kill-go combat approach.

I have flown a Mustang back in 1964 after I first got my licence and fell in love with it. This one had a 2,000 HP Rolls post war engine and could screw itself right into the sky.

The Mustangs only rival was the Bell P63 King Cobra which used the same engine but mounted it mid ship allowing faster turns with less wing area, and it used the lamilar flow air foil also. While it had almost the same profile as the much smaller P39, it had over 40% more wing area and over 200% more horsepower. Since the P39 was such a failure (under powered and wing loading too high), the War Department promised 100% of the production of the P63 to the USSR before even seeing it. I worked for a man who flew them over to Russia as part of the lend / lease program. He said it was the best plane he had ever flown.

Most of the Mustangs were built in Texas near Dallas.

The Mustang I flew had been converted to have two seats. A second fully functional seat had been added after removing the big radio and the 85 gallon fuel tank behind the pilot. I had learned to fly in a 1947 Piper Cub J3. After take off and climb to 8000 feet in the Mustang, the pilot offered me the controls. My Cub required about 6" of stick to the right or left turn the airplane. Using the same on the right side of the Mustang stick caused the view above my head to turn from sky blue to green corn fields with no more effort that it takes to wink your eye.

There I sat hanging from my belts as amazed as the instructor was. Finally he asked if I intended to continue inverted as we were not cleared for aerobatics. The plane rolled back to level.

One problems with the P51D was that on take-off with a full load of fuel (with drop tanks and ammo) the plane at maximum weight AND was tail heavy.

Instructors in the US trained the new pilots to burn off their drop tanks FIRST, then begin burning off fuel from the tank behind the pilot in order to get maximum range.

The problem was that if a problem came up that meant returning to the field to land, the plane could not be landed in the tail heavy condition: it would flip upside down on its tail on approach. Many green pilots were killed.

The experienced pilots quickly retrained the green kids to take off on the wing tanks, then at about 2000 feet switch the tank behind the pilot to burn off the 85 gallons that was making the plane tail heavy during the remaining time it took to climb to 30,000 ft plus. That way if they did have to drop the wing tanks to go after BF 109s for FW 190, the Mustang would not have to fight in a tail heavy configuration, which would mean sure death.

Landing the Mustang had some Do's and Don'ts. The plane required itself to be flown onto the runway with ample power. Too many green pilots would find themselves "short" of the runway and at just above stall speed, trying to add a big burst of power from the Merlin. The Merlin is not a high rev engine, but it IS an extremely high torque engine.

Opening the throttle would cause an immediate increase of torque to be applied to the massive 4 bladed propeller which reacted slowly causing reaction torque causing the plane to roll in the opposite direction of the propeller rotation, usually causing a stall and crash since there was no time to apply opposite stick to correct. Most experienced Mustang drivers landed well above stall speed and slightly long to assure that they would not be caught with this problem.

Reply to
Richard

Yes, for a paper rib pattern.

formatting link
A real one:
formatting link
jsw

Reply to
Jim Wilkins

The trim TAB is a servo mechanism that causes a small amount of force to control a large amount of force. The little trim tab is what makes the control input neutral. At 500mph, the force produced by a couple degrees of "angle of incidence" on a tab 20 inches wide and 2 inches long - just as an example is VERY SIGNIFICANT - Stick your hand out the window at 50mph and change the angle - feel the force. Now remember aerodynamic drag increases at the cube of speed increse. The lift and drag work directly in concert.Double the speed - 4X the force. You are going to go 10 times as fast. What does that do to the forces? And that tab is aerodynamically a lot cleaner than your hand. It is also SIGNIFICANTLY more area - A few degrees of tab trim will input a lot of force - particularly at the trailing edge - up to several feet from the pivot. That trim makes the elevator (in this case) follow along at the correct angle of incidence for straight and level flight with no control input force (stick pressure). Now, let that trim tab come loose at one end and start flapping in the breeze, 2 feet farther back from the pivot than where it should be

- or simply 15 degrees or more off from where it should be - and all of a sudden LARGE AMOUNTS of control input are required to hold the elevator at the right position for level flight. Several hundred pounds of force on the stick would be required INSTANTLY to correct for the separation - and if that correction is not made INSTANTLY, the quick movement of the elevator control surface through a significant degree of movement causes a dangerously violent change in attitude - forcing the tail surface down - and on a LONG lever - the down force a LONG way back from the center of lift - which acts as the fulcrum. It does not take a lot of force that far back to really toss the aircraft out of straight and level flight. The up-pitch of the plane cuased by the quick drop of the tail in this case caused well over 12 G's of force on the plane- and the pilot - making it virtually impossible for him to correct and control the plane - particularly when that close to the ground. The probability is VERY high that the 12 Gs of force caused the (average)20 lb human head to weigh 240 lbs plus - instantly snapping the pilot's neck in the process.

Anyone who doubts the effect of a trim tab at speed has never looked seriously at aerodynamics or the flight characteristics of an airplane. (and has likely never been at the controls of an airplane)

Reply to
clare

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.