More power will always produce a better rate of climb and a higher top speed, with little gain in weight. The effect on rate of climb and top speed depends on the aircraft. A ww1 aircraft was light with a lot of drag, more power would affect rate of climb much more than top speed. "Compression ratio" applies more to early un supercharged engines, with a supercharger it is boost pressure increase that matters, the increase in octane value is just to allow that increase in boost without detonation.
Discussion about video - "P-51 Mustang vs. Fw 190 D-9"
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This not quite true.
There are two ways to exploit a higher octane fuel.
Method 1. Increase the compression ratio. The higher octane fuel resists the danger of preignition. The higher compression ratio essentially gives more power for the same fuel.
If you increase compression ratio from say 8:1 to 9:1 you are expanding the ignited combusted mixture 12.5% greater distance and can extract more shaft horse power.
This will increase power at any altitude and therefore speed at any altitude and it is more or less what Daimler Benz did depending on engine sub model. There are caveats.
Method 2: Force in more air using a supercharger. More air means more fuel means more power. There is and increase in shaft horsepower but not increase in efficiency.
However Method 2 wastes energy in the exhaust but in reality this could be recovered as Jet Thrust. The Jet thrusts of a Merlin 60 series was about 300lbs. The Jet thrust of a Jumo 213E was about 448lbs.
It would be more common to use method 2.
Here is how it works.
Imagine an Me 109K4 whose supercharger pressurises the manifold at 1.8 atmospheres at 24,000ft. The engine can not take any more and the supercharger can not produce more pressure at this altitude though the engine is producing near full power. Therefore the Me 109K4 achieves its maximum speed of 441 mph because it has near to full power and the air at 24,000ft is thin and produces little drag.
Now lets imagine technical and fuel advances have allowed a increase in allowable boost pressure of 2.0 atmospheres.
The speed cant increase at 24,000ft because the supercharger can't produce more than 1.8 ata.
However the speed can increase at altitudes lower than this because the pressure can go up.
Using Method 1 may be more efficient at producing shaft horsepower but it actually reduces jet thrust. For a fighter this jet thrust is more valuable than shaft horsepower which needs a bigger propeller anyway.
Deleted member 68059
Staff Sergeant
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A big benefit from increasing CR, is that it can also enable you to slightly lower boost pressure to achieve the same power. Hence, can give you increased service ceiling as you can fully open the throttle at a slightly higher altitude. It is VERY difficult to look at something like CR in isolation and make comments about the balance between exhaust thrust and prop power in purely "general terms", because its massively influenced by valve overlap, which is often radically different between engine families (especially between Allied and German engines, which is due to the use of direct injection). The impact on level speed will be very complex, and is nearly impossible to put into an "all other things being equal" sort of description. Increased CR also significantly lowers fuel consumption.
GregP
Major
Hi Mike Williams,
Questions on post #22. I do not dispute the chart, but how is it possible thaht the rate of climb has a discontinuity in it? Shouldn't it be one, continuous curve?
Just curious. Cheers.
Questions on post #22. I do not dispute the chart, but how is it possible thaht the rate of climb has a discontinuity in it? Shouldn't it be one, continuous curve?
Just curious. Cheers.
No doubt, the operationals D9s were badly outperformed on the western front. But if we want to compare airframe to airframe, then we should consider reasonable production facilities for the D9 and availability of fuel.
Currently we compare the P51, built in ideal conditions which resulted in exceptional airframe quality, with an aircraft ,D9, that was not possible to recieve even simple design futures like main wheel doors or engine cowling without huge gaps.
Even in the range sectors , with C3 availability, the rear fuselage MW50 tank , would be used to boost range without performance reduction. Also the wing tanks were designed but impossible to be produced
The rear disadvantage of the D9 versus the P51d was the wing profile and the lack of 2 stage supercharger.
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It depends on when the supercharger gear switched from 'low' to 'high'. In the test it was changed at 15,000 feet, and nothing is really gained by connecting the two lines.
Though, if the test pilot had changed over at 8,000 feet -- it would be one continuous line (at those specific engine settings).
- Thread starter
- #48
Care to paste the comments here? I'm not able to find anything beyond:
Corrections and Additions: North Korea uses meters for altitude, I simply forgot about that country
Carb heat kills performance. I was speaking from the top of my head in general terms and not in reference to a specific fighter. For Merlin or Allison powered planes it's not really a factor. That said, fuel injected engines have an advantage in that they don't need carb heat, either as something selectable by the pilot or a source of heat that runs all the time to prevent icing.
- Thread starter
- #50
Thanks.
Unfortunately, Greg didn't address the claims - that I find badly wrong - forwarded via your question to him:
- hi-oct fuel increasing speed at high altitudes vs. use of low-oct fuel at high altitudes
- remotely-installed S/C stage on P&W radials
- UK/Merlin being dependant on US-supplied 100 oct fuel before 1941, with Roosevelt breaking the US law in process
- Merlin was 'sucking tit' before the Americans gotten involved in Merlin development and production
- 2-stage S/C on Merlin was at least 'US derived', if not outright 'US-made'
You can take a look on Calum's comment at post #2 in this thread for another.
Mike Williams
Senior Airman
- 572
- Oct 19, 2006
Geez, that is bad...
He uses your site to justify his claims about 100 Octane but wont post here lol.
I listened to this again, my, he does like the sound of his own voice doesn't he? At 46 minutes he argues that he flies all over the world and only the Chinese (but not all of China) use meters as a measure of altitude. Having worked in China and Russia and Japan and Mexico and many other countries in Europe and the middle east they have no concept of what a "foot" is, if required in a contract they will convert from their normal measurement which is meters to feet. Of the many howlers in his propaganda this is perhaps the most outrageous, why mention China and the Chinese and why imply they are somehow backward for not using feet as a measurement, the meter is the ISO standard measurement for length, why pretend otherwise?
Greg from Greg's Planes and Automobiles is right about more than he is given credit for.
1 High Octane fuel can improve the high altitude performance of an engine but it requires a mechanical change in the engine (increased compression ratio). Daimler Benz did this on several of its engines.
2 US Refineries did ship 100 octane fuel to the UK. It's true that the USAAF specification for 100 octane was different to the RAF specification for 100 octane. Both fuels shared the same lean rating but the RAF fuel specified a rich mixture Performance Number (% power increase) thusly 100/130. US refineries that shipped fuel to the UK adjusted their cuts to match the UK specification as much as possible. For a few months the fuel may have been 100/125 instead of 100/130. After that the US refineries had the equipment to meet the RAF specification most notably test engines that could measure a P/N performance number.
The UK was able to make its own 100/130 fuel by its own refineries and technology (mostly in the Caribbean) , most notably acid alkylation but its clear that the US was shipping vast amounts of this fuel as well.
1 High Octane fuel can improve the high altitude performance of an engine but it requires a mechanical change in the engine (increased compression ratio). Daimler Benz did this on several of its engines.
2 US Refineries did ship 100 octane fuel to the UK. It's true that the USAAF specification for 100 octane was different to the RAF specification for 100 octane. Both fuels shared the same lean rating but the RAF fuel specified a rich mixture Performance Number (% power increase) thusly 100/130. US refineries that shipped fuel to the UK adjusted their cuts to match the UK specification as much as possible. For a few months the fuel may have been 100/125 instead of 100/130. After that the US refineries had the equipment to meet the RAF specification most notably test engines that could measure a P/N performance number.
The UK was able to make its own 100/130 fuel by its own refineries and technology (mostly in the Caribbean) , most notably acid alkylation but its clear that the US was shipping vast amounts of this fuel as well.
Deleted member 68059
Staff Sergeant
- 1,058
- Dec 28, 2015
I have a really simple rule which really helps if you are publishing material on a historical channel, printed or video. If you do not have it written in a primary source archival document, don't say it; that gets you out of so much trouble.
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Clayton Magnet
Staff Sergeant
- 854
- Feb 16, 2013
Unfortunately the aviation industry worldwide (almost) uses feet for altitude readings.
He said a few Chinese in a few parts of China. All of China uses Metric or their traditional measures many of which are standardised to metric, as do the Russians. That is more people than North America and Europe combined. The reason they dont and wont use "feet" is because they have their own traditional measure Chi which means foot and is 13.12 inches because it is standardised on metric being exactly 1/3 of a meter 33 1/3 cm. Outside of aviation control measurements of the atmosphere are all standardised on metric ISO. The discussion is about superchargers, the people involved actually use pressure in their calculations which an altimeter converts to whatever the maker wants for the client. Only the wildly eccentric would defend using thousands of feet in one direction and miles, nautical miles mph and kts in the others.
You post under your real name and have an academic and professional reputation to uphold. I'm an electrical engineer by profession but I often don't have time to find primary documents in aviation topics out of my area so a touch of anonymity allows a few mistakes that I eventually get called upon. Greg from Greg's Planes and Automobiles is an airline pilot by profession I believe. Usually I get in to trouble from reading something that has incomplete information. My brain then makes inferences to 'fill in' the missing information and fact and inference become conflated.
Anyway, if all three of us ended up in the same hotel from a flight cancellation we'd probably end up in the bar over a few pints talking engines.
