SOH P-61 project

[PRB]

... Also for reference, on Navy aircraft this angle is around 25 degrees or more, since the Navy parks their aircraft with the tails hanging over the side of the ship, they don't want them tipping and falling off before they're tied down in rolling seas. This is also why navy aircraft can't aerobrake while landing on runways like air force aircraft; i.e., they can't hold the nose up as long as ground based aircraft can. This is one of the reasons why it's always easier to make a naval aircraft land based than it is to make a land based aircraft work on a carrier. It was also one of the problems with the F-111B, which is why the production version would have had it's main landing gear moved back, among other fixes.

The extent to which the physical defferences between navy and air force planes affect tactics between the two services is always interesting to me. For example, in WW-II, the short noses of navy fighters, made that way to allow better visibility over the nose when trying to land on aircraft carriers, also allowed the navy to practice 90 degree deflection shots from fighters, something our air force shipmates had trouble with because the amount of lead required for a 90 degree deflection shot places the target under the long nose of a P-40, and out of sight. Ok, sorry for the OT. Back to the P-61!

[warchild]

Tom:
Let me upload the FM i have at the moment, and when you have a chance, fly it and see what you think about the roll, rate.. I find it terribly difficult to judge roll rate while i'm flying it because my mind somehow always stays a little ahead of the plane.. I dont know if i have it exact ( unlikely ) but i think i'm pretty close and bordering towards the non ho hum side of things..
I guess part of my philosophy is that, somehow, it not only has to be real, but at least perform the way she could perform up against other planes but in FSX. That means theres a whole big can of worms out there to be tackled though, and i know it.. I dont know if the roll rate on say the mossie is accurate ( or even on many of the aircraft i myself have worked on ). But this Lady is definitely a scrapper, and she seems to me to want to dogfight. I was even trying to figure out last night how to maybe get it into CFS3 someday.

Regarding flairing and holding the nose up.. 15 degrees is possible physically, but, any more than that and we'd need training wheels under the vertical stabilizer.. this girl has one very long tail, but over 3/4s of the weight is in front of the main gear ( engines, half the turret, radar, pilot, gunner, armor plating, canons). She was designed to fly extremely well, but i aint so sure about ground handling. Aerodynamically, she reminds me of a gooney bird ( the feathered ones ) Incredible to watch in the air, but funny as hell to watch them try to land.. Sadly, i havent been able to find any videos or movies with a p-61 landing in it.. but let me try a littl trick here and see what i can come up with before i upload tonight..

[warchild]

OH.. movies rates ..man i'm slow tonight.. in general, when a movie was converted between 18 and 24 fps what had to change was the sprocket hole interval and the timing mechanism in the camera and projector heads. That meant re-gearing everything and invariably at first, sometimes a frame would stop half way across the gate and yoy would have th top of one fram and the bottom of another showing for a split second. That caused the invention of the frame adjustment wheel that classic images always show projectionists playing with. The over all effect of trying to convert 18 fps to 24 fps however was painful enough that it was easier for everyone to switch to 24 fps equipment.. I dont understand the physics entirely, but i'm pretty sure it made the flicker worse.. The secret is in the gearing however.. You can slow a 24fps camera down to 18 fps, and then play it back at 24 fps because the gearing and sprocket alignment is identical. it just makes it fairly obvious that the film has been sped up.. Thats part of why silent movies played today seem to be done in hyper speed, because to get them to run with minimal flickering each frame needed to be copied to a new inter-negative and a whole new film made from that. Since the movie was originally filmed slower, the over all appearance of silent movies is slightly sped up..
If the roll of the P-61 had been done at 18fps, it would look more comical than anything else..

[fliger747]

It is a little difficult to perfectly judge the angle of repose since many photos are of parked aircraft thta have been sitting, sometimes for a very long time. Nose struts particularly seem to be prone to loosing charge.

Fifteen degrees would surprise me as I can't think of any taildraggers that have that attitude. my Supercub with big tires and extra extended gear probably doesn't exceed 10-12 degrees. All of the Boeings I have flown actually sit level or with a 1-1.5 degree nose down attitude on the ground. The F7F has a 5.9 deg static pitch, with a pretty long nose gear. Remember most airfoils are near stalling at 15 degrees, and would certainly have to land very slowly to avoid hitting on the nose gear first.

Cheers: T

[warchild]

2.34 doesnt see a lot of changes, but a couple of important ones.. The role rate has been reduced. and the way the nose should stay in the air a few moments on touchdown has been greatly improved..
I inserted lines in the config file allowing the CG to slide back and forth a bit, and activated variable CG in the air file.. I also changed the pitch from the engines and lowered the pitch from the flaps. This has resulted in a more responsive, yet easier to control behaviour, as well as letting the nose wheel float naturally till gravity and friction on the main wheels take over..

I think your gonna like it i sure do..

[warchild]

It is a little difficult to perfectly judge the angle of repose since many photos are of parked aircraft thta have been sitting, sometimes for a very long time. Nose struts particularly seem to be prone to loosing charge.

Fifteen degrees would surprise me as I can't think of any taildraggers that have that attitude. my Supercub with big tires and extra extended gear probably doesn't exceed 10-12 degrees. All of the Boeings I have flown actually sit level or with a 1-1.5 degree nose down attitude on the ground. The F7F has a 5.9 deg static pitch, with a pretty long nose gear. Remember most airfoils are near stalling at 15 degrees, and would certainly have to land very slowly to avoid hitting on the nose gear first.

Cheers: T

::chuckles:: if i flair past seven degrees, it means i'm drunk or running a fever.. i really think i got it though with this last fde i just uploaded.. On the other hand, I've got more time in this plane than most of the real pilots that flew her.. i'm partly on automatic with everything, so i may very easily be missing something..

Wasnt the F7F a navy plane though??? They dont flair. they land flat.. so there would have been no worry about flairing or the nose being in the air.. Unless i'm thinking of a different plane..

[N2056]

One last pic from the weekend work :salute:

Attachment 19966

[Kiwikat]

Very impressive work Robert! :ernae:

[SkippyBing]

Wasnt the F7F a navy plane though??? They dont flair. they land flat.. so there would have been no worry about flairing or the nose being in the air..

There's a difference between not flaring and landing flat. Ideally a carrier aircraft will touch rear wheels first as the subsequent rotation around the contact point will result in a reduction in alpha and therefore lift, stopping you from bouncing over the wires.

[A26]

Its starting to sound like a tigercat forum!

a26

[Sundog]

It is a little difficult to perfectly judge the angle of repose since many photos are of parked aircraft thta have been sitting, sometimes for a very long time. Nose struts particularly seem to be prone to loosing charge.

Fifteen degrees would surprise me as I can't think of any taildraggers that have that attitude. my Supercub with big tires and extra extended gear probably doesn't exceed 10-12 degrees. All of the Boeings I have flown actually sit level or with a 1-1.5 degree nose down attitude on the ground. The F7F has a 5.9 deg static pitch, with a pretty long nose gear. Remember most airfoils are near stalling at 15 degrees, and would certainly have to land very slowly to avoid hitting on the nose gear first.

Cheers: T

I think you're confusing tip back angle with tail scrape angle and angle of attack. The tip back angle on a tricycle landing gear has to be higher than the tail scrape angle to keep it from becoming a tail dragger.

It's merely a reference to where the cg is located wrt the main gear and it has to do with tail sizing as well. If we move the cg further forward, hence a larger tip back angle, we need a bigger tail to rotate and that adds more weight and drag. If we move it too far back towards the main gear, a lower tip back angle, and it becomes less than the tail scrape angle, the plane could rotate down to it's tail and end up stuck with the nose up in the air. We don't want that either.

Also, this wouldn't be based off of what you see in photos, it would be based off of the engineering drawings, or a company side view that shows where the engineers designed the static reference to be. of course, if you have those, you probably already would have the cg range on the drawing as well.

Here, I put this together, since a picture is worth a thousand posts.

[warchild]

Its starting to sound like a tigercat forum!

a26

::roflmao:: it does doesnt it??
I guess what it boils down too, is that we're working wuth a plane of which there are no flyable examples: where the official documentation is practically none existent, and what documentation there is is mostly pilot chatter or opionins from people who watched her fly. Neither are there any good videos of her beyond one army training film, mostly filmed against a backdrop as the camera would have been sliced in half if the prop was rotating. So we analyze, and analyze and discuss and analyze some more; every nuance and aspect of this bird till we get a consensus and understanding of exactly how this bird performed. Now, Fliger and I both know enough about flight models that if you want a plane that does this this or that, we can make it do it because we both know exactly which numbers to change ( though i admit he knows a bit more than me ), so at least for me, the fun isnt in making her fly, but in duplicating her behavior. Thats why every aspect of the plane gets re-hashed time and time again.
You can tell me that exact isnt achievable, that perfect is out of reach, but i wont believe you. :;chuckles:: Not on this plane. I will not settle for 98%. I can show you picture after pucture of mothers sons; brothers in arms: connected to me by tradition, history, war, and the uniform we wore: that never made it home. I personally owe it to them, their memory, and theur spirit, to give this plane everything i am made of to make it right. .. Call me a nut case ..
Besides, everything we need to know is sitting right there in front of us in the form of the planes design and construction. Interpreting that design is a whole nuther story. Understanding it and translating it can be quite the challenge.
Now, its true that the P-61 was a design done by committee. You only need to look at the cockpit to see that. But at least one person in the right place knew exactly what he was doing, because she flies like you would never believe. Personally, i'm no longer creating the plane. The plane is creating itself. I'm just learning to watch and listen, and understand. The plane is building itself. I just monkey handle the keyboard.
Always

[warchild]

Also, this wouldn't be based off of what you see in photos, it would be based off of the engineering drawings, or a company side view that shows where the engineers designed the static reference to be. of course, if you have those, you probably already would have the cg range on the drawing as well.

We dont have those, but the ones we got weren't too shabby. i was able to transpose measurments from a few different versions of the three views, including one used for crating, shipping and lifting the plane ( by crane ) and Fligers CG is maybe two inches off at most. MAYBE.. Thats why i havent wanted to mess with it.
Thrust pitch was part of the answer. It was one degree off, perhaps a tiny bit more. Dont know, i'm still working with it, and learning from it.. The massive size of the propellers caused a very slight downward pitch. Barely enough to be otherwise noticeable, but that downward pitch makes all the difference between the nose naturally coming down on landing, and slamming dawn like a jack hammer. At the moment, when you land, the nose will remain in the air, and glide down over a three second period when flaps are used. If no flaps are used, the float time on the nose is somewhat longer.

BTW, your drawing is wonderful.. I'm chuckling a little because your CG is exactly where we figured it should be, and is. Its nice sometimes to get confirmation from another person .. Thanks

[warchild]

Robert was kind enough to upload the latest VC to the file browser. This is the version just before the one with the switches on the right hand switch box. The file name is 26sep.zip. get it while its hot ..

[fliger747]

Yes the F7F was a Navy aircraft, but the first to operationally deploy it on Okinawa (on the last day of the war) were the Marines. It was designed to be used on the larger Midway Class Carriers. Unfortunatly, and unusually for a Grumman aircraft, it experienced some structrual issues from repeated deck landings. A wing root seperation.... ouch....

As such the idea was to land on the mains, those spindly nose gears were always a bit on the fragile side!

Can't wait to get the latest updates! The project is looking good in the raw, hard to imagine how good it will look all textured and painted!

Cheers: T

[warchild]

Hard to wait aint it?? Roberts doing an awesome job..
i increased power scalar to 1.0, increased wheel drag again. increased the prop moi back to 419 but ot still stops windmilling between 10 and 15 seconds on shutdown. i left beta max at 57 which seems high to me compared to curtiss electric's other offerings, but it works so i aint touching it. I cant find any exact data on that prop anyways, outside of it being a paddle style prop. Increased minimum throttle to 0.015 to keep the engines running during idle.
I was almost able to clone John Myers demo flight, but that requires a better pilot than me to do.. I also crashed several times.. Full inside loops from ground level are hard to do. its weight and the engines pull it into an egg shaped loop that takes you below the altitude where you started.. In flight chatacteristics are untouched. the higher power settings and prop MOI have changed roll rate downward towards that 50 degrees per second.. Out of my own insecurity and paranoia, i want to wait till others report back in on the flight characteristics before i change them any more. i think we're very very close.. Maybe closer than that..
If our takeoff distance is right but the nose slams down again because of the raise in the wheel drag, i'll start looking at all the other contributing factors..

[fliger747]

Table 509, Friction vrs RPM is the best control over the engine spool down rate.

To have her stand on her hind legs better, it is usually necessary to move the CG further aft to closer to the main wheel line. Lowering the CG can also help.

Of course, change one thing.....

Cheers: T

[warchild]

Of course, change one thing.....

Cheers: T

Gods aint that the truth!! ::LOL::..
I'm not sure why, but just changing the pitch resulting from thrust helped a great deal. we went from never gonna happen to three seconds hang time.. Other factors that would effect it are the negative lift on the horizontal stabilizer and the elevator angle of deflection..
At most right now, i'll move the CG back one half inch.. it's the half inch we measured as being possibly off when we first started on the plane... From there we can figure the rest out. we have to anyway to be certain the plane is right..

[fliger747]

From a brief test flight; Roll moment might be too high, tends to wallow on approach, even without wing external fuel.

Another thing that might help the nose wheel is adjusting the lower AOA lift and pitch. Very small adjustments. Found these necessary with the A26 to be able to get it to rotate at the proper speed. Just don't get the elevator forces replicated in FS thta you might find in real aircraft with a significant prop blast effect.

T

[warchild]

Yeahhh, thats true. For me its one of the less discouraging things about FS, but every now and then it bugs me some..
I was thinking about that roll moment earlier when i was using full flaps for take off.. The plane wont go over a hundred with full flaps, and its rocking back and forth like cradle at that speed. They wouldnt have built it like that. I was thinking though that if i take the roll moment closer to zero, then i would be making it easier for the plan to roll. The further away from zero i move it, the more the plan had to overcome before it rolls, and would therefore be steadier.. Am i incorrect in my logic??

[fliger747]

From what I can gather, even though the roll rate at approach speeds was fairly anemic, the plane was quite stable and inspired confidence.

I once landed a 747-400 in San Fransisco on a nice day, with 117,000 Kg of fuel... A lot of roll inertia out there compared to a normal landing that might have only 13,000 Kg of fuel. Get it rolling and it keeps rolling and needs a correction to stop the roll. At some point most aircraft will have enough aerodynamic roll dampening to more or less tend to stay were you stick them. Exceptions are overbanking tendency and planes with excessive dihedral.

Reducing the roll MOI somewhat will increase the transient manuverability and probably tend to make the plane feel much more sprightly without changing the overall roll rate parameters.

Prop MOI... I am experiencing my head slamming back and forth in the cockpit on approach. This seems to be associated with a serious power surging, as much as 20% with a cyclic rate of 1/2 second or less. Better to adjust the per cyl friction table in the .air file.

Not having the prop animation fully showing yet I can't evaluate the spool down rate. he sound will not have too much to do with the actual engine rotation. Accel and SP2 treat this engine regiem differently. SP2 will develop more low RPM/low throttle shaft power.

I may be mostly out of the loop for 10 days or so.

Cheers: T

[warchild]

Not having the prop animation fully showing yet I can't evaluate the spool down rate. he sound will not have too much to do with the actual engine rotation. Accel and SP2 treat this engine regiem differently. SP2 will develop more low RPM/low throttle shaft power.

I may be mostly out of the loop for 10 days or so.

Cheers: T

No worries Tom.. Real life comes first.. Always..
I think you and i have two completely different styles for short field landings, because i'm not experiencing any head banging. But i'm coming into a 4000 foot runway at 100 mph from a steeper than normal approach with full flaps. I use about 10 percent power just to keep the plane from stalling during the approach. We need to duplicate your approach and hopefully the headbanging, so we can eliminate it. When you have time, if you could give me your information, i'd be grateful. What i need is approach speed, approach angle, flaps settings, manifold pressure and throttle pecentage. That should give us enough to work on..
In the mean time, let me go recalculate the roll moi. no doubt i made an error somewhere doing it the first time..


PS:: on the headbanging. In the meantime while we figure out what your experiencing and its cause, might i recommend that on landing, you reduce the prop pitch to its lowest setting ( 15 degrees ) and if you can, set the mixture control to auto?? it might help a little..

[warchild]

I've encountered the surging.. Playing around in table 509 and doing a bang up jib of screwing up as usual :;chuckles::.
The surging reminds me of an engine with a stick butterfly valve, or a clogged fuel line/ fuel filter.. I once drove my old cutlass across country and had that same surging happening all the way from denver to san jose. all i needed was a little grade and it would start surging..

Another thing I'm looking at on it is that according to Curtiss in their Pamphlet ( http://www.enginehistory.org/Convent...20Electric.pdf ) the paddle style Curtiss Electric prop only had 30 degrees of total pitch movement. I'm going to drop max pitch back to 47 degrees with 17 being the lowest pitch, and then start looking for the culprits responsible for keeping us from reaching optimal performance.. ::chuckles:; back to being a mechanic .. seems i will never escape wrenching on an engine of one type or another..

[warchild]

the culprit is in the time constant on the propeller.. 0.038 for a tc is incorrect. What IS correct however, i'm gonna have to figure out and make adjustments across the board to compensate and correct with regards to the engine itself.. Isnt wrenching FUN?? ( oy vay )


EDIT:: I think i fixed it, BUT, there is still one little moment in there. I'll explain..

As you know, the props initial pitch into the wind is almost flat. but as you gain speed on takeoff it rotates through to its maximum pitch. With a paddle type propeller ( as seen in the first pic of curtiss electrics brochure mentioned above ) there is a specific angle of pitch where the paddle bites hard into the air and surges the plane forwards. With our plane, it's happening at about 110 mph. It will definitely set you back in your seat. I want to leave that in, because it reflects something that i believe is quite realistic.. just needs to be tuned..

[warchild]

theres enough changes in this FDe that i couldnt call it a step. This is a major step up from 2.33.
THAT being said. There are now a dozen or so areas that will need to be carefully re-tuned.. But..

by changing the prop tc i was able to eliminate the headbanging.
Engine friction mas and min have both been significantly lowered to reflect a well lubed engine.
drag has been increased, allowing the plane to slow down naturally without the need of chandelles or other tricks..
CG has been moved AFT 0.5 inches, center of lift has been moved aft 2 inches.
theres lots more i simply dont remember at this moment.. Upshot of it is that, besides the above, takeoff with full flaps now generates a 2300 fpm climb rate. a moderate dive from 2300 feet will produce 300 mph (i'm working on the prop table at this moment, attempting to adjust for the paddle style shape and hollow core of the propeller ), Stability is greatly increased at low speed, but that will need to be retuned.. not sure what else..
enjoy
Pam