Same engine for P51 and Spit- low alt speed difference?

[ICDP]

1 hour ago, Silavite said:

There were many factors that influenced the difference in speed.

• The P-51's more efficient radiator design when compared to the Spitfire Mk IX.
• Although the P-51 had more guns than the Spitfire, the barrels of the Hispano cannons protruded quite prominently from the wings and the breech/feed of the cannons required blisters on the wings.
• The P-51 had a retractable tailwheel. The Spitfire Mk IX did not.
• The P-51 had doors for the main gear, which fully enclosed the it from the airstream. The Spitfire Mk IX did not.
• The P-51 had a lower drag intake for the engine/supercharger (the Spitfire's intake juts out from the fuselage/cowling, while the P-51's is tucked snugly below the engine).
• The P-51 had a slightly smaller wing area to begin with (22.5 m² for the Spitfire with the non-clipped wing, 21.8 m² for the P-51).
• Canopy design(?)
• Airfoil section(?)
• And doubtless more that I missed!

It was a combination of many factors, but for the comparison between the Spitfire and the P-51, I'd opine that the radiator design was the biggest single difference (as others here have already stated).

 

So you are saying, the Mustang is using magic?  Why didn't someone say this earlier. 

[[DBS]Browning]

4 hours ago, Silavite said:

•  

It was a combination of many factors, but for the comparison between the Spitfire and the P-51, I'd opine that the radiator design was the biggest single difference (as others here have already stated).

 

Perhaps the radiator is the biggest difference. That's certainly easy to imagine and I think it likely. 

But if we simply transplanted a 51's radiator onto the body of a spit and put the Spitfire's radiator on the body of a 51, I'd bet any money that the 51 would still be faster. 

As you say, there are many other factors. 

 

I compleatly agree with your post. 

Edited August 1, 2019 by [DBS]Browning

[Kurfurst]

The huge and somewhat old fashioned radiator design, or its turbulent front canopy design on the Spit certainly does not help, but given the data I have seen I am fairly sure that the main culprit for the difference in speed lies with the uncovered undercarriage and the wing cannon design. The non-retractable tailwheel and uncovered mainwheels on the Spitfire, and large protunding Hispano installation  added quite significant amount of drag. 

[[DBS]Browning]

Where the props the same? That can certainly make a huge difference.

[JtD]

No, they weren't, but this does not influence drag. Thrust production would be different, and could also lead to different speeds, but in this case the margins would be small.

[ZachariasX]

12 hours ago, ICDP said:

As you can see from the specs above the Spitfire Mk IX was very similar in size to a P51D. 

Size and (aerodynamically) wetted area are not the same.

[unreasonable]

1 hour ago, VO101Kurfurst said:

The huge and somewhat old fashioned radiator design, or its turbulent front canopy design on the Spit certainly does not help, but given the data I have seen I am fairly sure that the main culprit for the difference in speed lies with the uncovered undercarriage and the wing cannon design. The non-retractable tailwheel and uncovered mainwheels on the Spitfire, and large protunding Hispano installation  added quite significant amount of drag. 

 

The Spitfire PR Mk IX had no armament or wing bulges and a rather smoother windshield. Is there any direct speed comparison available?  I have not seen any, but I suppose most of the concern there was ceiling and range anyway.  

 

From an article: https://www.aerosociety.com/news/escort-spitfire-a-missed-opportunity-for-longer-reach/

 

 Incidentally, Supermarine produced a design proposal involving moving the radiators from under the wings to the fuselage underside just aft of the cockpit; clearly the Mustang had been an object lesson. A company report in December 1942 claimed a 30 mph speed increase would accrue from that and certain other modifications but the scheme was taken no further.

 

Sadly a reference for this report is not given: would have been interesting to see a breakdown of what Supermarine though the effects would have been. 

[ZachariasX]

35 minutes ago, unreasonable said:

The Spitfire PR Mk IX had no armament or wing bulges and a rather smoother windshield. Is there any direct speed comparison available?

The speed of the Mk.IX went up from ~650 km/h to about 670 km/h in the PR XI after cleaning up some obvious imperfections like the steep windscreen.

 

There are many little things that were cleaned up and made the PR Spit versions significantly faster than any corresponding 109 types. But it takes a lot more excess speed to make interception easy than it takes to make hauling a** a viable strategy to be safely home for tea. Changing that small shopping trolley for a retractable tail wheel implies enough to make it a difficult production issue.

[JtD]

P.R. XI (comparable to F IX and HF IX fighter versions):

 

http://www.wwiiaircraftperformance.org/Spitfire_PR_XI.html

 

P.R: XIX (comparable to XIV fighter version)

 

http://www.wwiiaircraftperformance.org/Spitfire_PRXIX_aircraft-data-sheet.jpg

 

The speed improvements in the best comparisons I found are 6-9 mph. Thanks for bringing that up, never thought a ballpark would be that simple.

[ICDP]

4 hours ago, ZachariasX said:

Size and (aerodynamically) wetted area are not the same.

 

I was correcting a post stating the Spitfire was smaller than the Mustang and that this gave it an aerodynamic advantage.  My premise being they are similar in size so that can't be true.  Is this inaccurate or not?

Edited August 1, 2019 by ICDP

[ZachariasX]

38 minutes ago, ICDP said:

 

I was correcting a post stating the Spitfire was smaller than the Mustang and that this gave it an aerodynamic advantage.  My premise being they are similar in size so that can't be true.  Is this inaccurate or not?

That was not what I said (or intended to say). Drag or friction with airflow is largely agnostic to the dimensions you mentioned. What matters is how much area there is that the air has to flow around. Using that metric, the Mustang is a very large aircraft. It is not as thin as the Spit or the 109, it has more volume, giving it much more skin. Air flowing along and around the skin causes drag. The more skin you have, the more drag.

 

This was very much known to the designers back then. When tasked with making a fast aircraft, they had two priorities: small frontal section and small wetted area. The 109 and the Spitfire are prime example of this, and they came up with basically the same solution: an engine with the pilot stapled to its back holding a mass/kettle worth of fuel. Basta. You also then add wings that are good for producing them/taking off from a golf course.

 

Both frontal section and the amount of skin exposed to airflow (wetted area) are hard metrics. The latter took a long time to be less of a hassle with improved clearances and flush riveting. But to get your design out, you have to run a shop first that is proficient in such, hence, you have what you have. And if one thing you have but must is suboptimal, you try to use as little of it as possible. Hence, the Mustang in terms of a design displays more than anything a superior building quality. They dared giving it much more surface. That is the magic. If you have a superior shop, your design will not suffer where the competition does, no matter how much the competitors fudge with windscreen slope or tail wheel configurations. If you have the good skin, you just need to shape it wisely and you still have less drag than the competition.

 

Hence, your assumption is not true, as different skins and production clearances have a drastic impact on parasitic drag. This is why designers love composite materials for aircraft besides weight and strength. You just can make these perfect skins. Sailplanes had a dramatic performance increase once you made them of plastic instead of cloth and wood or aluminum.

[Venturi]

20 hours ago, JtD said:

No, they weren't, but this does not influence drag. Thrust production would be different, and could also lead to different speeds, but in this case the margins would be small.

 

Dude, props definitely induce drag.

 

Let's not forget, the tested 375mph on the deck is WITH WING PYLONS...

Also, the car was named after the aircraft, but the horse was the symbol used by the marketing guys.

 

Cause, you know, a lot of people don't know what a P-51 is...

[Frequent_Flyer]

On 7/26/2019 at 9:22 PM, Bilbo_Baggins said:

 

 

On 7/26/2019 at 9:22 PM, Bilbo_Baggins said:

That is a truly incredible difference given the same motor. 542kmh Spit airframe and 603-610kmh for mustang.

 

Didn’t know how amazing that airframe really was.

To truly appreciate how superior the P-51 design was consider the weight difference:

                                    P-51-25      Spit. LF mk.IX          BF-109G-6

Empty:                        7,125                5,800                         5,900

Normal loaded:         10,100              7,500                         6,950

 

Edited August 3, 2019 by Frequent_Flyer

[Bilbo_Baggins]

4 hours ago, Frequent_Flyer said:

 

To truly appreciate how superior the P-51 design was consider the weight difference:

                                    P-51-25      Spit. LF mk.IX          BF-109G-6

Empty:                        7,125                5,800                         5,900

 Normal loaded:         10,100              7,500                         6,950

 

 

Thanks for that, interesting to see the weights. She's a heavy! Weight doesn't really mean anything for speed though- climb and acceleration are where weight comes in. Maximum level speed is a power vs drag relationship. 

Edited August 3, 2019 by Bilbo_Baggins