F8F Bearcat Questions

[MrMojok]

Hello All,

Question 1- This may sound a little weird. It might just be that I'm crazy.

But I could *swear* that I read somewhere, at some point, that the vertical stabilizer on the Bearcat was actually canted one or two degrees to the starboard. That is to say, the leading edge of the vertical stabilizer was angled slightly that way. The way I remember reading it, this was to offset the monstrous torque generated by having the P&W R2800 mounted in what was really a pretty small airplane, and this contributed in part to the Bearcat being a really easy, pleasant aircraft to fly.

It sounds crazy but I just feel sure I read this somewhere. If I did, I am sure not able to find it now. Has anyone ever heard anything like this? Or am I getting senile?

Question 2, for GregP- I understand a Bearcat is going to be at the Chino airshow at the end of the month. Is this flying in from somewhere else, or does Chino Planes of Fame actually have one of their own?

 

[soulezoo]

I could not respond to the poll because it did not have my requisite response:

Great plane... most awesome? meh....

 

[MrMojok]

Fair enough!

(poll is a joke of course )

 

[grampi]

I could not respond to the poll because it did not have my requisite response:

Great plane... most awesome? meh....

I would easily rank it in the top 5 propeller driven planes of all time performance wise...

 

[MrMojok]

By the way, for those interested, here is a very interesting article about flying the Bearcat: Pelican's Perch #39:<br>Bearcat! - AVweb Features Article

One highlight:

Very few pilots, new to the airplane, will even be able to get full power on before liftoff, because things happen VERY fast. It feels like the throttle travels about three feet (it's maybe 8 inches), and once it's about half throttle, it feels like more than enough, more power than anyone would ever want. A quick glance at the manifold pressure might show a mere 35 inches, or maybe 40. That "10 more inches" is a LOT of additional power, and the fleeting thought for many will be, "Gee, do I really want to do this?" Then, "Ooops, too late, we're already 100 feet in the air, and accelerating, and we don't even have full power on it, yet!" So much for that CAF regulation. The noise is just incredible, even inside a padded helmet, with insulated earcups.

 

[T Bolt]

The vertical stabilizer being offset by a few degrees is not unusual for World War II era fighters

 

[MrMojok]

Is that right? For some reason I thought I remembered my possibly-imaginary source speaking of it like it was unusual. If it was a common thing, I apologize. I did not know that.

 

[Airframes]

Yep, even some 'early' monoplane fighters, for example the Hurricane, had an offset fin to counteract the torque of the (early) Merlin engine - at that time very powerful, but a baby in comparison to the much later Bearcat.

 

[MrMojok]

Ok then, I'm not crazy or senile, just ignorant LOL

 

[Airframes]

No, not ignorant - just learning a bit more !
And remember, the only 'stupid' question, is the one you don't ask !

 

[wuzak]

The Spitfire was one aircraft that the fin/rudder wasn't angled with respect to the centreline.

Which no doubt pilots were thankful for when they put the opposite turning Griffon in place of the Merlin.

I believe Bf 109s had the fin angled in this way, which led to handling issues with the derived Hispano-Suiza HA-1112 with Merlin engine, which turned in the opposite direction to the original DB engines.

 

[MrMojok]

Wow, the more you know! That's what's so great about this forum

 

[GregP]

Hi MrMoJoke,

Yes, we (the museum) will have a Bearcat or Bearcats flying in. They are semi-regular visitors at the Museum. We have one, but it hasn't been completed yet. Every time we complete one for Steve Hinton, some guy comes in and buys it out from under him. Hopefully, he'll get to keep the next one.

When I say "we" just above, I mean Fighter Rebuilders, not the volunteers.

We have commitments from some 40 WWII aircraft. Should be great show. Why not come see it?

I do not believe the Bf 109 had an angled fin, though I could be wrong there. I think, rather, that if had a very asymmetric airfoil, with the high-lift side being set to help counteract the torque of the DB engine.

The P-51D has some downthrust and some sidethrust in the engine mount as well as an offset vertical fin. Some of the "speed mods" for Strega and Voodoo include removing and or decreasing some of the offsets since their primary operating mode is at the high-speed end. Since tghey are racers, you might expect some aerodynamic "tweaks." For instance, a stock P-51D is out of nose-down trim at about 490 mph or so. Neither Strega nor Voodoo are out of available trim at that speed, as you might expect, but stock WWII fighters need to have safety built-in when they approach the limits of the flight envelope.

 

[MrMojok]

Greg, under normal circumstances I would definitely be there. I'm not sure if I can make this one unfortunately but if I don't I'm going to just come to Chino another time, soon. To check everything out, and maybe also for the 475th and F4U weekends.

 

[GregP]

Most any Saturday is good. Ask for me in the restoration hangar and someone will find me.

Look forward to it.

 

[drgondog]

Greg - when you volunteer labor and time to rebuilding an a/c, you are definitely part of the Team.

Per the Mustang - yes to down thrust on thrust line to reference line and yes to left offset for DFF but IIRC no to side thrust line of engine relative to CL. I know that the angle of incidence of the wing to the reference line was brought back to zero for Voodoo and Strega (maybe the Baron also) which may have squeezed elevator trim envelope at top end but I suspect the Ghost took the approach of adjusting elevator trim? You probably know the answer to that one.

 

[tomo pauk]

There was a story about the P-47 with contra-rotating props, the aircraft nicknamed as 'Double twister'. The fin/rudder assembly was canted couple of degrees on the usual P-47s, and somehoe people forget to installa the rudder with 0 deg inclination. So the Double Twister almost crashed on the 1st flight due to, now symetric thrust being applied with a non-symetric angle of the rudder.

The 'Super Corsair' have had a part of rudder that is automatically deployed (20 deg IIRC) when wheels are down, in order to counter-act the amazing thrust provided by powerplant at low speed, when the classic rudder is inefficient.

 

[stona]

If we're talking monstrous power in a small air frame than you'd have to go a long way to beat the Supermarine Spiteful and Seafang.

With 3,300+ hp in a 9,070 lb air frame, the Griffon 101 powered version of the Spiteful (RB518) had a maximum speed of 494 mph (Full Supercharger (FS) gear at 27,500 ft, 2750 rpm at +25lb boost). It had a rate of climb of 4,750 ft/min at that take off weight.

If the 'Bearcat' was the 'most awesome piston engine aircraft ever', then we'll need another word for the Spiteful

Cheers

Steve

 

[soulezoo]

I would easily rank it in the top 5 propeller driven planes of all time performance wise...

I offer this in the spirit of friendliness...
I don't necessarily disagree with what you just said about performance either... however, the premise was most awesome piston engine plane of all time. So...

I really think the Sopwith Triplane was awesome. The Constellation was awesome. The Ford tri-motor was awesome... ok, maybe not that. But you get my point.

 

[GregP]

Hi Bill,

See below and note the unusual absence of the belly scoop. Wonder what CD0 is for THIS buggy? The radiator boil-off exits are just to the left of the base of "1" in the "177." Note the flat plates on the wingtip and horizontal tail tips. They were definitely trying to eliminate leaks, as the Aluminum tape on the panel join lines shows near the boil-off exits and around the vertical and horizontal tail panels. The finish was excellent on race day and we all expected him to go very fast. In the event, he did.

The Ghost had a lot of modifications that were never FAA documented. The oil cooler was a boil-off as you know, the wings were shortened, the horizontal tail was shortened, the belly scoop was gone and the radiator was also a boil-off. But consider the next mods.

The elevator bobweight was removed ... the one right about at the wing trailing edge inside the fuselage, and the entire trim load was put on one trim tab, I believe the port tab. The starboard tab was locked in trail with a fixed pushrod. So, he doubled the trim load on the functioning tab and never modified the tab or the actuating system to handle the extra stress. Proper safety might also might require mods to the elevator when the trim load at 500 mph is double on one tab, I don't know. I am not aware whether or not the Ghost had the wing and tail incidence modifications, or whether it had the fin offset removed.

When the Ghost hit the turbulence, the aircraft experienced a several G bump of unknown (to me) level, and somewhere about the next 1 - 2 seconds the working trim tab lost some 80% of it's surface due to being slightly loose. The tab that remained was the bit where the pushrod was attached and was not fluttering but was also insufficient to help the trim. At that point, the aircraft pulled up at some 17 G according to the motion analysis. When it did the tailwheel broke loose from the uplock and went down all on it's own. The pilot was unconscious for the rest of the ride.

I noticed that on the first two laps of the Gold race, and also in preceding heats, the Ghost seemed to want to tighten up in any turn where sudden gust load happened, and Jimmy was doing a great job of catching it. I have never seen that behavior before in a P-51. Most of the P-51 pilots I know think the tendency to tighten up was due to removal of the elevator bobweight. If you watch the last race, particularly in slow motion, on lap 2 you can see the tendency to tighten up as Jimmy goes around pylon 8 and turns away from the grandstands. He hits wake and the plane wants to snap inward, and he catches it quickly and continues gaining on the front runners as he goes by Rare Bear.

I can say this, had the accident not happened, I don't know if he could have won, but he was gaining at a pace that would have forced the front runners to step it up or be passed. I have no idea of the amount of speed they had left, but none of the Merlin engine racers can go full throttle for 8 laps. At some point they get heat soaked and have to back off or trash the engine. Perhaps Jimmy would have gotten heat-soaked himself and might have had to back off, too, but his Merlin was nowhere near as tweaked as Voodoo and Strega's engines are. He was playing the extreme drag reduction game as opposed to the add more power game (not that Voodoo and Strega haven't done drag reduction, too, they have). His engine WAS a race engine, but not quite at the same level as Voodoo and Strega's engine.

As for the P-51 racers, there are other mods than the ones mentioned above, but a complete list of the mods is somewhat of a team secret. I watched some of them being developed on Voodoo, but was asked not to discuss them. I can say that a lot of attention was paid to drag reduction, in-flight trim loads, and cooling airflow for both oil and radiator. That much seems obvious to a casual observer. There is a rather special wing mod that Dago Red had that nobody else is using, but they all know about it since it was published in aerodynamic journals some years back. It consists of adding a small bump along the top of the leading edge at about the 50° point from the leading edge radius center. I have no idea why even after reading the paper, but it appears to lower the overall drag. The sticky point is the amount of precision required to get the bump shaped just right. I know for a fact that the Dago Red airframe has had that particular mod removed since it was taken back to bare metal when it was being restored to flying condition by the new owners. As far as I know, it is no longer a racer, but I have not heard if it is flying again yet. Seems like a good thing to ask about after the airshow.

I mixed up the side thrust above as you caught. I used to add that as indicated in my old RC pattern models. I flew pattern for about 12 years and after doing so for some 3 - 4 of them, began to correct side thrust in the community-recommended manner. The first requirement is to get the airframe straight, including the fin. There are probably a few full-scale aircraft that could use some side thrust, but engine mounts are WAY more expensive in full scale and there's a definite ease of correcting an under-5 hp RC plane as opposed to a 200+ hp aerobat! At least in the RC plane, there are no structural issues doing it. I have never and will never do any analysis on full scale side thrust, but I'd bet someone flying something like a Zivko Edge 540 or MSX in aerobatic competition might really love a plane that goes straight up without much use of rudder to hold it there once the upline is established.

If you don't correct it in RC pattern, you have to hold in varying rudder as speed changes to keep on a straight vertical line. That is sure to result in downgrading your score if you are even slightly off. In racing full-scale circles I bet they'd like to able to go around Reno at race speeds without straight-line rudder offset, not that they go in a straight line often. I really don't know if they have achieved that or even attempted it as I have never asked or investigated at all. If you aren't flying precision aerobatics for score, there is no incentive to pursue it.

The correction would be similar to RC pattern corrections, but would result in a lot of headaches like moving the spinner as side thrust was added, resulting in cowling mods. I seriously doubt anyone wants to go to that trouble. They are flying pretty fast now and are capable of record speeds even if the averages don't show it since they all throttle back if nobody is catching them. No sense running the very expensive hybrid Merlin-based engines any harder than necessary. You can easily verify that by looking at the lap speeds. One year, Stevo Hinton managed a lap of 504+ mph in heats but his last 2 Gold laps were in the 470 mph range since nobody was catching him. The race average does NOT indicate the speed potential as a result. All it tells you is how fast they had to run in order to win that year. Usually, nobody shows his real speed at Reno until Sunday afternoon, and then only on the first 2 - 3 - 4 laps. They just run hard enough to qualify for the Gold start position they want in qualifying. The only reason to run a fast lap in a heat race is to check the systems. All you really have to do is go fast enough to advance to the next heat.

I'm quite sure you're probably as or more aware of all this than this than I am.