SOH P-61 project

[warchild]

Tom,can you tell me where that data is?? I'm sitting here with a calculator trying to do the math and frankly, the math is giving me a headache.. I'm not saying that you, I or the data is wrong, but i just dont trust the data.. Time and time again Meyers would take fighter pilots up in one of these things and scare the living hell out of them with what it could do..
The P-40 was another example of dumbed down data. originally classified as able to only take 42 pounds of manifold pressure, it wasnt until the British airforce in africa contested it that Allison detroit finally admitted it could be safely pushed to 60 pounds.. things like that make me extremely mistrusting of data.. for now, lets stick with your figures..
One thing i noticed in the test flight is that raising the flaps does nothing to the flight path. The plane should drop some as more than a few crashes were caused by the pilot raising the flaps too soon. I'll compare our two pieces of work and make that adjustment..
great work over all.. I'm liking it.. i dont know how in bloody blue blazes we're gonna get this up to 360 mph. The best i can achieve so far in level flight is 350 mph, and thats using 459 slugs on the prop moi.. but if we can get it to 360, lets do..

[warchild]

P:PS

Still about 15 mph fast at altitude, usual way to deal with this is to lower the ramp threshold of Mach drag. Supercharged engines used more power to run the high blower, so shaft power decreased, this is not reflected in FS as it models a supercharger. So power is a bit high at altitude. Next itteratrion will take a look at speed at critical altitude.

T

in the end result, 15 mph isnt that much to be off by, but if you can make it correct, i'm all for it. Just dont beat yourself up if we cant get it right ok??

[warchild]

One last thing..
I've noticed that regarding specific aspects of maneuvering, we're comparing this plane with planes that use ailerons, and i'm just not certain that thats a valid comparrison.. The operational theory behind each type pf control is completely different even though they are accomplishing the same thing.. Ailerons use force to rotate the wing around the longitudinal axis of the plane, the spoilers use lift and gravity..
i think as engineers we should be doing the math ourselves on this one ( irregardless of how many headaches i get ). I know thats asking a lot, but, i now that for myself, i deserve to give myself the satisfaction of knowing that i've done the absolute best job i can do on this.. Even if it takes an extra mile or two..

[fliger747]

Pam:

Not at home, but the roll data is published in Hundred Thousand, as is the roll data for many of the planes he examines in each chapter. Makes some sense as the weight of both engine components and wing fuel make for a bunch of roll inertia. From what I can glean the transient manuverability was probably not great as exemlified by the comments by the evaluative pilots at the 1944 Fighter conference (I have that book too) but the continuous manuverability was fairly good (due to the flaps).

Found it... was in some of the pages I photographed before leaving.

Cheers: T

[fliger747]

From Hunner't Thou:

These comments are from the 1944 Fighter Conf "flyoff".

T

[rohan]

Soo, we have a 33 foot wing falling a distance of 51 feet through its arc of motion, accelerating at 23ft per second, per second.

Pam,
I'm not sure whether this was just a mistype, but I think my elementary physics taught me that acceleration due to Earth's gravity was around 32 ft / sec / sec ...

Hope that helps,
Ro

[fliger747]

On the airspeed:

I use the aerodynamics tab of the AFSD utility (By Herve Sors) to measure true airspeed, mach, mach drag and much more. The engine tab monitors all of the important outputs and parameters, MP, RPM, HP, friction, prop efficency, and on and on.

On Mil power at 21,000 ft I got 363 mph (in this case 2032 hp). On WEP (2224 HP) I get at 21,000 ft I get 370 MPH. Just a snitch fast. The measured CDm (mach drag) is .0096. This can be increased to make up for the fact we aren't loosing any power at high blower.

Speeds just a bit fast, but cureable. The CDm can be increased to reduce the increase of speed with altitude to about match what it should be.

Attached the aerodynamics tab, ISA conditions, 21,000 ft WEP.

Cheers: T

[heywooood]

in the end result, 15 mph isnt that much to be off by, but if you can make it correct, i'm all for it. Just dont beat yourself up if we cant get it right ok??

don't want to be too anal about specific flight data - not all airframes are identical...even if every P-61 was built using the same jigs, the same material, by the same people - you will have variables
both in the airframe and engines...use the published data as a guideline but don't get caught up in exact detail..life is too short

[fliger747]

Flaps:

I didn'tchange anything on the flaps directly, but did move the normal takeoff CG %MAC from about 10 to 27 or so and adjusted the downforce on the tail to keep the trim about the same. Adjusting to get both a nominal 25% MAC CG and more or less neutral trim (for a selected speed) is sort of a cut and paste deal. However moving the CG probably reduced the moment arm for any pitch effect that moving the flaps would cause. Increasing the pitch moment change for flap extension/retraction would probably cure this.

Cheers: T

[fliger747]

Flown a lot of planes and yes, generally they have differences! However we do have some good data and the ability to get pretty close to it, 1-2%. That's what Pam and I do, keep knocking all the rough edges off till we get something pretty smooth. And yes, the data from the era also has some variation in it due to the way it was collected, but it is what we have.

Cheers: T

[heywooood]

believe me when I say I respect that

but at 15mph indicated over the max rated speed at altitude I wanted to agree with warchild - thats pretty fine...that and when I hear other forumite complaints about similarly close results as if the difference were sacrilegious - my dander goes up a touch

the books and charts are an outstanding source of information - and the data should be adhered to..but I promise I wont die of a broken heart if the simulation doesn't match the data exactly note for note

[fliger747]

Pam:

(REC) 1101

Try CL_Lift-Flaps to 1.7505 and Cm_df Pitch Moment Flaps to 0.010 and see if you like that for "settling".

Cheers: Tom

[warchild]

Pam:

(REC) 1101

Try CL_Lift-Flaps to 1.7505 and Cm_df Pitch Moment Flaps to 0.010 and see if you like that for "settling".

Cheers: Tom

Will do Tom.. thanks.. .. That'll give us our 90 mph takeoff speed..

[warchild]

believe me when I say I respect that

but at 15mph indicated over the max rated speed at altitude I wanted to agree with warchild - thats pretty fine...that and when I hear other forumite complaints about similarly close results as if the difference were sacrilegious - my dander goes up a touch

the books and charts are an outstanding source of information - and the data should be adhered to..but I promise I wont die of a broken heart if the simulation doesn't match the data exactly note for note

I think the 15 mph difference is more for out personal challenge than it is for the customers. We know we can do it. Its a matter of finding the right data path to follow and maximizing it then making the adjustments..
What Tom said is very true. its what we flight dynamics engineers do.. I think Tom must be better organized at managing his personal time than i am, because for me, its not just what i do, but outside of what i do for the peer center, its all that i do.. i dont fly just to enjoy it any more because i cant. I dont have the time. So my fun comes in the form of challenging myself and seeing how far i can push my abilities. Tom uses tools i've only ever heard of. i use nothing but a 3 view, an architects scale, a calculator and sometimes plain old dumb luck. Whats amazing is that we both end up with a very similar number set. That continually delights me for several reasons, And its because we take it beyond the engineering part. It's a piece of our souls.
I'm afraid people like Tom, I, Milton and others are responsible for the nit pickers. The work we're driven to do, has made people knowledgeable and expectant of the very best.. yes, they can get on my nerves too, just like they'd get on anyone elses, but whats worse is not being able to say that i've done my absolute very best when i go to bed at night..

[warchild]

From Hunner't Thou:

These comments are from the 1944 Fighter Conf "flyoff".

T

Hmm, yeahh. So we got a plane that can out turn an F6F and therefore a Zero, and is better at rolling than a B-25 but worse than seven other fighter planes they considered as tops. So technically, if you could bank this thing, there wasnt much out there it couldnt get behind, but banking it was so slow you may as well not even try.. Its too bad this plane came so late in the war ( 1944 ). Roll rate could have been increased by clipping its wings.
When i was in Viet nam, the infantry had quad fifties mounted on the back of a duesanhalf and GI legend has it that it was used for cutting roadways through dense forests as well as what it was deigned for. Those were only 4 50's. i dont know what the fire rate was because the 50 cal is a variable/ selectable rate weapon.. I also got to see Shadow breath its fire with four mini guns. it was the predecessor to spectre.
this plane reminds me very much of those things. and too, its a bit like the A-26 as well.. With its 45lbs/ft sq wing loading, this plane could take anything except an idiot, and it would deliver and deliver and deliver.
She herself wasnt hated so much by the brass, as much as many of her crews were.. One crew was tracked on radar during a ground attack mission over china, and for the entire length of the mission time, she never left safe territory. She dropped her load over a friendly territory. Things like that gave P-61's a bad name and reputation.. it wasnt until Army recon came up with a picture of a locomotive that had been completely cut in half with a single burst of the widows guns that anyone started getting any real clue about what this bird was best at..
She isnt as pretty as some planes, in fact, she looks a great deal like her name sake, but she did the job, irregardless of what the job was, which is quite a bit more than you can say for most of the planes that were flying back then. there wasnt a lot of romance surrounding her like there was a spit or mustang or fortress, but she was a good friend. She really didnt get a fair shake out of life..