Fisher P-75 versus Davis Manta

[KJ_Lesnick]

Caveman

"Elaborate convergence"

Well I meant a lot of it

That is far simpler than designing guns/engine-gearbox combos to fire through hubs or interrupter gears.

However a straight line of fire is way better at shorter and longer range than the sweet spot...

Greater diameter of propellor =! More power.

Well, to absolutely honest -- my interest was simply making it as big as the XP-75

If you want a sweeping generalisation large props are better for slow heavy lifting whilst small props are better for speed.

True enough, but the Eagle was fast and had a bigger prop...

[KJ_Lesnick]

Flyer

And different aerodynamic shape and probably weight too. The difference of prop size could even be down to something as simple as the designs used different gearbox ratios, or had differing prop ground clearance.

The prop to ground clearance would be accomplished by a landing-gear that tilts the nose up a little (P-39/P-63)

[wuzak]

Certainly would be possible, the degree of difficulty would depend on the scale of the changes. What prompted you to ask about those specific changes?

Simple

• The longer nose gear would make possible a bigger propeller
  
• The propeller arc on the Manta seemed small, compared to the XP-75 Eagle: Considering both had the same engines, a bigger propeller would provide more power.
  
• The cannon/guns on the Manta were far spaced apart requiring an elaborate convergence scheme; it seems simpler to put one of the following: 1 x 20mm cannon in the propeller hub + 4 x 12.7mm in the nose with a synch gear; 6 x 12.7mm in the nose with a synch gear.
  

The Manta was an earlier project than the Eagle, so may have specified an earlier, less powerful, version of the engine.

Also, as the Manta was never actually built you don't know what they would have ended up using.

Consider that the different prop diameters are run at different speeds. So you may still get the same thrust as a larger, slower rotating prop.

[jcf]

The only engine ever directly associated with the known actual Manta project was
a single 1,100hp Allison V-1710. After wind-tunnel tests Davis claimed that the model performance
would scale to a top speed of 430mph on the full-size aircraft with a landing speed of 60mph.
Unfortunately, or conveniently, the sole wind-tunnel model was reported stolen before anyone else
could test his claims. 

His high-speed claims are extremely dubious given that the maximum thickness of the wing was
42 inches.

-ref: September 1941 and January 1942 Air Trails magazines. The latter also has full details
of the design of the Davis airfoil; ordinates, offsets, the whole works, so model designers could
try it out on their next creation to see if the benefits of the design would improve the performance
of their next masterpiece.

[KJ_Lesnick]

Wuzak

Consider that the different prop diameters are run at different speeds. So you may still get the same thrust as a larger, slower rotating prop.

I figured you'd ultimately get more with a bigger prop.  Could be wrong, but the F4U did it


joncarrfarrelly

The only engine ever directly associated with the known actual Manta project was a single 1,100hp Allison V-1710.

Then where did the stuff about the V-3420 come from?

After wind-tunnel tests Davis claimed that the model performance would scale to a top speed of 430mph on the full-size aircraft with a landing speed of 60mph.

With a V-1710?

September 1941 and January 1942 Air Trails magazines. The latter also has full details of the design of the Davis airfoil; ordinates, offsets, the whole works, so model designers could try it out on their next creation to see if the benefits of the design would improve the performance of their next masterpiece.

Do you have a link to it?

[jcf]

The V-3420 stuff comes from drawings by Justo Miranda of supposed 1st (twin-tail) and 2nd (single tail)
'Manta' designs. Perhaps like others he conflated the Manta with the McDonnell Model 1, which was to be
3420 powered. There is no evidence of any twin-tail Manta, just some mis-identified advertising art
that has a 'Manta-ish' wing.

Read through the Secretprojects thread and you'll see the problems:
http://www.secretprojects.co.uk/forum/index.php/topic,3510.0/all.html

No I don't have a link to the Air Trails pieces, I have the actual magazines and I've not bothered
doing scans as the images are the same ones available from Popular Science, Flight etc.
It is amusing how the top speed went from 430mph in the Sept. 1941 Air Trails, to 445mph in the
Jan. 29th, 1942 edition of Flight. Still with a single V-1710. Magic. 

http://www.flightglobal.com/pdfarchive/view/1942/1942%20-%200234.html

[KJ_Lesnick]

The V-3420 stuff comes from drawings by Justo Miranda of supposed 1st (twin-tail) and 2nd (single tail) 'Manta' designs. Perhaps like others he conflated the Manta with the McDonnell Model 1, which was to be
3420 powered.

Interesting

There is no evidence of any twin-tail Manta, just some mis-identified advertising art that has a 'Manta-ish' wing.

Oh

No I don't have a link to the Air Trails pieces, I have the actual magazines and I've not bothered
doing scans as the images are the same ones available from Popular Science, Flight etc.
It is amusing how the top speed went from 430mph in the Sept. 1941 Air Trails, to 445mph in the
Jan. 29th, 1942 edition of Flight. Still with a single V-1710. Magic. 

Why did he believe he could do such speed?

[jcf]

Davis wanted money to develop his ideas, so he exaggerated. Nothing new, nothing unique.

[wuzak]

Davis wanted money to develop his ideas, so he exaggerated. Nothing new, nothing unique.

Sounds like Bell....

[KJ_Lesnick]

I'm curious if the Davis Manta could have been evolved into something practical provided

• V-1710 substituted for a V-3420
  
• Propeller modified for double horsepower
  

By 1939-41?


joncarrfarrelly

His high-speed claims are extremely dubious given that the maximum thickness of the wing was 42 inches.

I did some calculations and (providing Justo Miranda's drawings are accurate other than the engine) the thickness to chord is around 16.239% - 16.428% at the wingroot.  That's not much   different than the P-51...

BTW: Regarding the air-trails magazine.  If you don't have a link to it, can you either scan it or at least list the full details of the Davis foil?

[wuzak]

His high-speed claims are extremely dubious given that the maximum thickness of the wing was 42 inches.

I did some calculations and (providing Justo Miranda's drawings are accurate other than the engine) the thickness to chord is around 16.239% - 16.428% at the wingroot.  That's not much   different than the P-51...

But the P-51 wing was laminar flow, having the maximum thickness further to the rear of the chord.

So I wouldn'y hold out hopes for it being a low drag shape or having a high critical Mach.

[KJ_Lesnick]

But the P-51 wing was laminar flow, having the maximum thickness further to the rear of the chord.

I think the maximum point of thickness on the P-51 was around 40% in some areas, unsure about the root; the Manta at the root was around 35.989% to 37.363% depending on the analysis of Justo Miranda's drawing at the roots

So I wouldn'y hold out hopes for it being a low drag shape or having a high critical Mach.

Wasn't the P-47 around 16 percent T/C?  This design was around 12.2% at the tip.

[jcf]

Davis


Corrected Davis (B-24 airfoil)

From here (which link I've posted in other Kendra threads):
http://m-selig.ae.illinois.edu/ads/coord_database.html#D

... and a bunch more poo on the Davis:
http://airfoiltools.com/airfoil/details?airfoil=davis-il

[KJ_Lesnick]

joncarrfarrelly

What does a basic and corrected airfoil mean?  It might sound stupid, but it's a good question for a person who is not an engineer...

[jcf]

The reality vs. what Davis claimed.

http://m-selig.ae.illinois.edu/ads/ref/davis_corrected_ref.html