mustango656

Hi folks,
My club, Sacramento Area Modelers, has a group of about 25 people that get together every first Sunday of the month to pylon race Model Tech P-51D mustangs with same engine, prop, fuel etc. The P-51D is discontinued but my LHS, R/C Country, can still order lots from the manufacturer just for us in RTC form. I thought it would be fun to convert two of these models into a P-82. The P-51 kit comes with about 3 to 5 degrees incidence in the wing. My question is if we modify our P-51s by putting zero incidence on wing engine and horizontal stab ether by trimming the wing saddle or adding a shim to the horizontal stab for more speed, should I leave the factory incidence on my P-82? Also, what degree of right engine thrust, if any, do you recommend for a twin? I have some scale drawings of both planes and intend to lengthen the tail like the original P-82 for more one engine out stability. Any ideas, comments, appreciated. I already have all the stuff I need for the conversion (about 700 bucks worth).

Mark C.

Tall Paul

What power you want to use?
The Dynaflight P-82 flew reasonably well with a minimal amount of right thrust and 2 .40s.
The long nose on the P-82 creates a problem anyway, the motor(s) are really far ahead of the c.g.
Single-engine performance at full-power is poor!
Recognizing which engine has failed is the key to surviving such a thing.
Power back and land!
There's no real need to cut the incidence for the first plane.
Going fast though will require some change there, do it.
Remember to keep the speed up.
The wing isn't that much larger, while you're doubling the fuselage weight.

FHHuber

Start out with minimal changes. Leave the incidence and downthrust alone... give a little left thrust or 0 side thrust to the left engine.

Downthrust plus dihedral = a safety margin for single-engine-out. FF modelers use hevy downthrust on twin engine types (usually rubber power) so the prop that produces thrust longest doesn't pull the model into a spin. The side that sddenly has no downthrust wants to rise as if you had fed in aileron opposing the imballanced thrust.

As you learn the single-engine out and asymetric thrust (such as when you leave one engine full throttle and the other at 50% to 75%...)characteristics... you can adjust the thrustlines to help.

Rule one of keepng a twin in one piece is PRACTICE asymetryc thrust probelms. If you don't know how it will respond on loss of one engine... you won't know what to do to prevent the crash.

mustango656

Hi Paul, I'm using two OS .25FXs

Mark

mustango656

The P-82 was made of two P-51g or h as I recall. They didn't have the fillets on the leading edge near the fuselage and the outer wings were shorter and had a bit more chord. I plan on using two unmodified wings from the kits and fabricating a center wing that continues from each wing with the additional chord of the fillet. It'll be a little different then the full size P-82, having longer wingspan and I can also play with the width of the center wing moving the fuselages closer or farther apart. Scale is roughly 12" to 13" center to center. I can also adjust the incedence with the horizontal stabilizer.

Wouldn't you want to have some right thrust in the right engine and about zero in the left?

FHHuber

Actually... the P-82 was only dimly related to the P-51. The fuselage of the P-82 is longeer and taller. the span of the wing outboard of the fuselage is considerbly arger than the same portion of a P-51 wing. The ruder and fin are larger... (its not 2 mustangs joined... its a compltely different airframe)

They simply BASED the design on something that had worked.
*********

Re the left engine... on some twins the left engine has as much as 9 deg LEFT thrust angle. (that same design would have 3 deg MORE right thrust on the right engine... 12 deg!) They found that downthrust can markedly reduce the requirement for side thrust on twins for single-engine stability. (it really does work... 1 deg downthrust can reduce side thrust required by 2 to 3 deg)

Get good 3-views of some twins... you will find almost all use both down and side thrust on both engines. I'll look for my book that has the info in the F-82... and other twins. Has good pictures showing thrust angles.

mustango656

Hi Huber, I have scale drawings of both and it seems they added length to the tail behind the oil cooler. The P-82 1s 10 feet longer but I think the nose is longer also to accommodate gearboxes for the counter rotating props. I intend to lengthen my fuselage about 4 inches to include taller and longer tails. When built the P-51 kit is about 40 inches long. So 44' +/- P-82 model.

Tall Paul

Mustang, don't get misled or sidetracked..
I've seen a P-82 about the size you're looking for, made from two HOB 44" P-51s. With two piped .25s it was a screamer!
I looked at the GP P-38 profile today.... they recommend 4 degrees of outthrust on each motor... left on the left side, right on the right side.. The nose on the P-82 is a lot longer though, and there's probably no way to put enough sidethrust in each motor.
My normal HOB P-51s were seriously affected by torque when taking off, despite right rudder, they would swing left due to that long nose.
They'd fly best when there was some wind so there'd be a fair amount of slipstream over the vertical from the start.
You might make the verticals 10% larger in span and chord. Won't be too noticeable, but every bit helps back there.
I've flown a HOB T-12 (2 T-6s joined ala the P-82) with two OS 26 4-strokes. Wasn't mine, the owner was reluctant to try it.
It went well, but I didn't make any attempt to wring it or see what would happen if (when) an engine died.
Your P-82 should be OK as long as you don't push the motor run too far.
It will be heavy (relatively) so remember to keep the speed up, and cut the power if (when) either motor dies.
You'll a better chance gliding than with asymetric power.

FHHuber

"The complete Book of Fighters" on page 448-449 has minimal info in the F--82 just notes it as a gap-filler design with inadequateinfo on it to be useful. The 3-views of the F-82 and P-51 are useful though when scaled for the canopy bubbles. (they did use the same bubble as the P-51D)

Unfortunately... the book won't fit on my scanner.

http://www.wpafb.af.mil/museum/air_power/ap34.htm http://www.wpafb.af.mil/museum/research/fighter/f82.htm For some variants... photos and desciptions.

AND....
http://www.warplanes.com/prop_powered/AF82.cfm

Tall Paul

The things you don't learn here...
"Re the left engine... on some twins the left engine has as much as 9 deg LEFT thrust angle"
North American had it wrong all those years...

Tall Paul

All spoofing aside, the F-82 was quite a larger plane than the P-51.
The poor lateral dynamics of the P-51 were fixed by the more "modern" sized vertical on the F-82.

mustango656

The P-82 had counter rotating props so the common pull to the left on takeoff was eliminated. My P-82 won't have this feature, unless they make a counter rotating OS 25FX . Thats why I thought it would be good to move the left engine to 0 and keep the stock P-51 5 degrees right in the right engine. I'll also incorporate a couple of down thrust degrees as well. All for better single engine engine stability. Ill keep you all updated on its progress.

Tall Paul

The problem isn't torque... it's the serious mismatch in thrust between an engine running on one side with the other dead!
With the long nose moment on the P-82, such a mismatch regardless of thrust offset is likely to be fatal, unless it occurs at a high enough altitude to permit some chance of recovery.
I doubt any thrust offset would help at all.
If the plane is sufficiently overpowered, as I expect yours to be, takeoffs should be OK, with the much longer tail moment and larger verticals. Expect to hold some right rudder when the plane begins to accelerate, and ease off as it picks up speed.
Downthrust.. HA!

mustango656

I don't know how accurate a flight model the GP flight simulator is but when I added those thrust figures to the P-38 it had better one engine stability in all flight conditions except near stall. I believe the engine torque would help counteract left pull with the right engine out and right thrust in the right engine would help with the left out. A slight down thrust should reduce the amount of yaw an engine out situation would induce. Of course you still have to take emergency procedures if it should occur.

Tall Paul

I'll be awaiting the flight test comparison to the simulator.
Can you fail an engine in the simulator?

mustango656

Yes but its random left or right.

Tall Paul

"random" as in which or when?
The critical time is takeoff, and when slowing down to land.
At speed, the plane should be controllable, as the stabiliziing forces are pretty high.
The P-38 has shorter noses on the motors, but worse single-engine flight characteristics I'd bet, due to the really small verticals.
Models of the P-38 usually end up as land-fill not too long into their flying careers.
Bungee launching would help the takeoff situation, and ease the landing since a wheel-less approach should be faster and flatter.
Speed helps!

FHHuber

I don't have anything that shows the true thrust angles used on the F-82...

There were a couple of WWII and between the wars full scale bombers using 9 deg outward thrust offsets both sides (with almost no downthrust) The Me 262 used downthrust (hard to see on the pictures but I have a detail 3-view set that lists the angles...) and a small outward thrust angle on both engines.

mustango656

which. I can trigger a flame out though it's hard to do on takeoff so I tried some touch and go's with either engine out. I haven't yet but it's possible to change the flight model of the P-38 to similar measurments as the P-82.