Author Topic: Some New Data Carts to chew on (Read 3271 times)

F4UDOA · « **Reply #75 on:** January 07, 2002, 05:08:00 PM »

Question: Where on the internet can I purchase 10 more points for my IQ?

Anyway

Badboy!!

Great to hear from you again!

I will edit my spreadsheet ASAP.

Anyway for all parties involved this is the root of my question. Take a F4U-1 in comparison with a P-38J. I know that the P-38 could outclimb the Ubird. But what I do not understand is how a P-38 can outstrip an F4U in acceleration when the Power to weight ratio favors the P-38 but not significantly and the Cdo heavily favors the F4U as well as top speed at sea level.

Based on this I can only believe that the Aspect Ratio of the P-38 assist in climb. But the larger wing and increased drag would counter any power to weight advantage.

What factors make an A/C superior in climb/Acceleration?

[ 01-07-2002: Message edited by: F4UDOA ]

Dwarf · « **Reply #76 on:** January 07, 2002, 05:31:00 PM »

Quote

Originally posted by F4UDOA:
Anyway for all parties involved this is the root of my question. Take a F4U-1 in comparison with a P-38J. I know that the P-38 could outclimb the Ubird. But what I do not understand is how a P-38 can outstrip an F4U in acceleration when the Power to weight ratio favors the P-38 but not significantly and the Cdo heavily favors the F4U as well as top speed at sea level.

Based on this I can only believe that the Aspect Ratio of the P-38 assist in climb. But the larger wing and increased drag would counter any power to weight advantage.

What factors make an A/C superior in climb/Acceleration?

[ 01-07-2002: Message edited by: F4UDOA ]

I'll take a stab.

Higher aspect = less power needed at any AoA because the wing generates less drag at that AoA. Thus, more of the potential power the engine(s) can create is available to both climb and accel. Other things being equal, the aircraft with the higher aspect wing will climb faster and accelerate faster.

However, at the other end of the spectrum, the high aspect wing will also run out of acceleration potential sooner.

Ignoring critical mach for the moment, there is only so far you can reduce AoA before you start creeping up the back side of the drag curve. Because the low aspect wing must operate at a greater effective AoA at all speeds in order to produce enough lift to maintain level flight at that speed, it will still have accel potential left when the high aspect wing runs out of gas.

Thus, the 38 climbs and accels better, but the Hog is capable of a higher top speed.

Dwarf

wells · « **Reply #77 on:** January 07, 2002, 06:26:00 PM »

DOA, the drag coefficient is not a constant. It includes the induced drag, which varies with speed and lift coefficient. Those drag coefficients are for maximum speed, where acceleration is 0 and induced drag might be < 10% of the total. At 150 mph, the drag coefficient of the F4u might be 0.04, with induced drag being 50%. Dwarf, those prop efficiency formulas are very accurate, even with the 20% estimate of losses. I'll show you as soon as I put some graphs up.

Dwarf · « **Reply #78 on:** January 07, 2002, 06:32:00 PM »

Quote

Originally posted by Badboy:

Acceleration and Climb rate are directly related, they only differ by a factor that includes (v/g) and so it is possible to make direct comparisons. Trust me on this, HoHun is correct.

Badboy

I've never said the two problems aren't related or similar. What I have said is they are not equivalent. Equivalence implying point for point identity.

You yourself say they "differ by a factor that includes (v/g)". 1 != 1.0001.

What I've tried to emphasize is that climb and acceleration differ most markedly at the margins. That is where you need to be most wary of the Ps value. Ps is a value which is most accurate and believable for both problems precisely when it is needed least and most unreliable when it is needed most - at the margins.

At low speed you can safely believe it with respect to acceleration, but not climb. At high speed you can safely believe it with respect to climb but not accel. In between, it doesn't matter, because inertia alone will carry you through either transition state. In fact it's inertia alone which allows you to recover from a terminal velocity dive. At terminal velocity, Ps is most likely negative. If that truly meant what it implies recovery would not be possible.

Where I was in error, was early on, before I'd really dug into the problem, believing that climb and accel were most similar at the margins and most different everywhere in between.

Believe me, I wish Ps was more useful. I'd love for it to be. It would make performance modeling and prediction so much easier. I'd love for wells' equations to have been dead accurate at calculating prop efficiency. Or even mostly accurate. I really want something that is better than simply assuming an efficiency of 80%.

I'd also like equations that didn't require graphs I don't have. Purely mathematical means, if accurate, are much more useful to me. BTW, my source does include generic graphs for a "typical 3 blade propellor". I'm unwilling to try to extrapolate much from those for any specific 3 blade propellor or any 2 or 4 blade prop.

Dwarf

[ 01-07-2002: Message edited by: Dwarf ]

wells · « **Reply #79 on:** January 07, 2002, 07:21:00 PM »

Dwarf, the number of blades matters not, only the diameter. The number of blades determines the diameter (or vise-versa). The formulas I gave you for prop efficiency should give you this graph for the F4u-1.

Then, you can figure out thrust and drag

From there, you can figure out climb rates, glide ratios, sustained turning speeds, whatever you want. Those formulas predict the F4u to have a best climb speed of 70 m/s (136 knots), which is what? 1 knot error from what's in the manual?

Dwarf · « **Reply #80 on:** January 07, 2002, 08:35:00 PM »

Quote

Originally posted by wells:
Dwarf, the number of blades matters not, only the diameter. The number of blades determines the diameter (or vise-versa). The formulas I gave you for prop efficiency should give you this graph for the F4u-1.
...
From there, you can figure out climb rates, glide ratios, sustained turning speeds, whatever you want. Those formulas predict the F4u to have a best climb speed of 70 m/s (136 knots), which is what? 1 knot error from what's in the manual?

Here's what my source has to say.

"For a two-bladed propeller, the forward-flight efficiencies are about 3% better than shown... but static thrust is about 5% less than shown... The reverse trends are true for a four-bladed propeller. Also, a wooden propeller has an efficiency about 10% lower due to its greater thickness."

It's the "about" that troubles me. Fudge factors, especially approximate ones, don't make for very reliable calculations.

The definition I have for prop efficiency says that it is "the ratio of thrust power obtained to energy expended". In fact, another equation for prop efficiency listed in conjunction with that definition is:

n(PE) = Pt/delta(E) = 2 / ( u/u0 +1)

u = propwash velocity
u0 = freestream velocity

The problem with this one, for me, is it requires you to assign values to u and u0. Thus, by adjusting those values you can skew the result to anything you desire whether any actual prop could operate at that efficiency or not.

Make 'em the same and you've got a 100% efficient prop and no thrust. Go the other way and you've got a Hollywood wind machine. But, nowhere, do you have any assurance that your efficiency number corresponds to reality.

Dwarf
[ Hope Badz didn't read this as originally posted. That WAS pretty erroneous.

]

[ 01-07-2002: Message edited by: Dwarf ]

wells · « **Reply #81 on:** January 07, 2002, 09:23:00 PM »

Take a look at this site:
http://beadec1.ea.bs.dlr.de/Airfoils/propuls4.htm

Dwarf · « **Reply #82 on:** January 07, 2002, 09:38:00 PM »

Quote

Originally posted by wells:
Take a look at this site:
http://beadec1.ea.bs.dlr.de/Airfoils/propuls4.htm

Thanks. Excellent site. Just wish some of his equations didn't come with caveats.

Dwarf

Dwarf · « **Reply #83 on:** January 07, 2002, 10:57:00 PM »

Quote

Originally posted by F4UDOA:
1. The P-38J has two 1600HP engines and has a VMAX of only 415MPH.

6. The P-38J has a relatively short range in comparison to other A/C. Why? I thought the P-38 had longer legs. Looking at the P-38 Manual it would seem that it consumed a great deal of fuel compared to the F4U and only carried marginally more fuel.

Any comments?

V Max for the 38 was due to compressibility. IIRC, one early model was really capable of 437, but they were all restricted to lower speeds due to the fear of losing aircraft and pilot at higher speeds.

I believe the listed ranges (radius of action) are predicated on operation at approx 2/3 throttle, auto rich, and the aircraft's best cruising speed. At lower power settings and leaned out mixtures they were all capable of considerably greater range than "the book" figures. On occasion, with drop tanks, there are documented 38 missions of over 2,000 miles.

They used the 38's to get Yamamoto because nothing else in the theater had the legs.

38 compares well with the F7F, which if I'm making out the numbers correctly carried the same 360 gallons.

Even in those days, the government wasn't about to document or encourage modes of operation that had been deemed unsafe. It's at least probable that all US aircraft could exceed their book numbers in some respect. you always need to look at whether the data originated with the manufacturer or relevant service. Manufacturers tend to be optimistic while the services tended to be conservative.

Dwarf

[ 01-07-2002: Message edited by: Dwarf ]

F4UDOA · « **Reply #84 on:** January 08, 2002, 10:51:00 AM »

Gents,

Zigrat has just reared his head from the depths of Aeronautic Engineering school to send me his latest Performance Calculator.

New Calculator

I think the results of these calculations somewhat prove my point. If you look at the difference in calculated performance no where in the climb numbers does the P-38 exceed the F4U by more than 100FPM.

This is not saying that the F4U should be able to climb with a P-38. Mainly because when a radial engine A/C climbs the clowl flaps are opened created extra drag. However in straight line acceleration this condition does not exist. And the numbers are almost identical.

P-38J
Weight= 16500LBS
Wingspan= 52FT
Wing Area= 327Sq ft
Max Speed @ Sea level= 338MPH
1G stall @ gross weight= 106MPH

F4U-1
Weight= 12000LBS
Wingspan= 41FT
Wing Area= 314 Sq FT
Max Speed @ sea level= 355MPH
1G Stall @ Gross Weight =100MPH

F4UDOA · « **Reply #85 on:** January 08, 2002, 11:08:00 AM »

Dwarf,

Two quick things.

1. How does the P-38 compare well with F7F? The F7F is nearly 60MPH faster at sea level where there is no danger of compressabilty. I'm not sure if I understand what you are comapring?

2. I know the P-38 has a reputation for long range but when the Navy's range calculator is applied it is not nearly as impressive. 450 miles in combat range with no DT's. I know during the course of the war many changes were developed to run lean to get longer range. However this calculator gives a baseline for range for all A/C with out leaning out the mixture. I would just expect more based on reputation.

Also in regard to the Yamamoto mission. I wouldn't put to much stock in "It was the only A/C for the Job". More decisions where based on interservice rivalies and politics than on what was the best for the Job.

HoHun · « **Reply #86 on:** January 08, 2002, 11:46:00 AM »

Hi Dwarf,

>V Max for the 38 was due to compressibility.

Though the P-38 had a lower critical Mach number than most contemporary fighters, it still was unable to reach it in level flight.

Regards,

Henning (HoHun)

HoHun · « **Reply #87 on:** January 08, 2002, 12:22:00 PM »

Hi Dwarf,

>At low speed you can safely believe it with respect to acceleration, but not climb. At high speed you can safely believe it with respect to climb but not accel.

Your low speed example is misleading because you implicitely assume that initially, the plane is in level flight and has to pull up to enter a climb. You're right it can't pull up while flying at 1 G stall speed without stalling. However, if the initial situation is a climb, this is not necessary, and aircraft can climb quite well at 1 G stall speed.

Your high speed example is slightly flawed, I'm afraid. At top speed, an aircraft uses all of its power to overcome drag, and there's no power left to climb. Only by going slower, the aircraft can begin to climb - just like it could accelerate by descending.

The math isn't that complicated:

Ps=Pe/(m*g)=(Pt-Pd)/(m*g)

=> acceleration: a=(Ps*g)/v, climb rate: Vv=Pe/W=Pe/(m*g)=Ps

The conclusion:

a=Vv*g/v

Acceleration and climb are directly and linearly interdependend.

The derivation of the above formula requires no knowledge of aerodynamics at all, it's a fairly simple application of Newton's axioms.

Regards,

Henning (HoHun)

Zigrat · « **Reply #88 on:** January 08, 2002, 01:03:00 PM »

my sheet will not work well for the p38 since it is kind of stacked against it. the fact that the nacelles block a large portion of the spanwise flow will mean that the p38 generates less induced drag but the sheet does not take this into account.

as for aspect ratio, yes in general a larger aspect ratio would lead to higher climbrates given equal area, especially at higher altitudes or low speeds. What you must remember though is that high aspect ratio is costly in terms of wing areal weight, so there is a point of diminishing returns where the increased structural weight penalty eliminates the benefits due to reduced induced drag. Its a similar thing to winglets... sure they may decrease spanwise flow, but they add weight and parasitic drag so they cannot be used indiscriminately.

Zigrat · « **Reply #89 on:** January 08, 2002, 01:30:00 PM »

also wells uhmm those equations you have up there dont work. they always equal unity.

as for number of blades not mattering thats not true. a single blade propeller would be the most efficient actually -- as number of blades is increased efficiency decreases. of course you may able to be absorb more power, but efficiency is still lower.

other things matter too, like design lift coefficient of the blades, taper ratrio of the blades and millions of otehr things. its just that where the hell can you find the activity factor of a bf-109 propeller? damned if i know.

the hamilton standard red book has great data for propeller performance calculation, its just that you cannot find data on the propellers themselves anywhere.

anyways for anyone who wants to learn about aircraft performance the best text book i have found is Jan Roskams Airplane Aerodynamics and Performance (ISBN: 1884885446). It is pricey but is the best book i have found on the subject because anyone can understand it but it still has all the information you need.

all in all tho these little algebraic equations we use have to be taken with a grain of salt. the stuff i am using now in my graduate work for analysis takes weeks to run on sun workstations and still makes assumptions. they are still valuable as estimation tools, but really cant be used for much more than that.