Author Topic: SpitfireV speed at +12lbs. (Read 5279 times)

VooDoo · « **Reply #45 on:** February 18, 2004, 11:41:15 PM »

Sounds about right, I don't know weight that is at, and don't remember what weight I was using for my figures.
That is why I want to look at them

. So Im waiting till you'll have enough time...

Flyboy · « **Reply #46 on:** February 19, 2004, 05:43:45 AM »

thanks alot henning

really cleared the issue for me

btw, is the supercharger a part of the engine or a completely different part?

for example, can i load varios kinds of superchargers to the same engine\ plane ?

MiloMorai · « **Reply #47 on:** February 19, 2004, 06:01:42 AM »

A description of the Merlin

http://www.unlimitedexcitement.com/Pride%20of%20Pay%20n%20Pak/Rolls-Royce%20Merlin%20V-1650%20Engine.htm

and some pics

http://www.cjbalm.com/auto-aero/aitem60.htm

The opposite end from the prop shaft is the supercharger case with the carb below.

MiloMorai · « **Reply #48 on:** February 19, 2004, 06:10:06 AM »

An interesting chart (Source: Mercedes-Benz AG, Archives, Stuttgart, Germany) on the DB605 engines

http://w1.1861.telia.com/~u186104874/db605.htm#appraisal

Angus · « **Reply #49 on:** February 19, 2004, 08:10:36 AM »

Speaking of acceleration and power, does not wing loading (and therefore A of A) influence initial acceleration from the stall towards a certain level?
That is, low wingloading will sooner reduce the aircrafts angle of attack, but as lift also creates induced drag, at higher speeds the lift becomes a negative factor, since it is creating lift that no longer is necessary?

gwshaw · « **Reply #50 on:** February 19, 2004, 08:28:38 AM »

Quote

Originally posted by Angus
Speaking of acceleration and power, does not wing loading (and therefore A of A) influence initial acceleration from the stall towards a certain level?
That is, low wingloading will sooner reduce the aircrafts angle of attack, but as lift also creates induced drag, at higher speeds the lift becomes a negative factor, since it is creating lift that no longer is necessary?

Yes, wing loading and aspect ratio both effect the drag curve, and indirectly effect acceleration.

The drag curve is kind of U or V shaped. At low speeds there is a lot of induced drag due to the wing being at a high AOA. It falls off pretty quickly with speed, and the lower the wing loading and higher the aspect ratio the quicker it falls off.That causes the high left side of the V. After the lift curves cross the induced drag quickly becomes a very minimal part of the total drag.

Profile or pressure drag (I can never remember the correct terminology) increases with the square of the speed. So at low speeds it is minimal, but at high speeds it makes up probably 90%+ of total drag. That is the high right hand side of the V.

If you graph both curves they will cross at some speed, typically 150-180 mph IAS for WW II fighters. That speed is the optimum speed for that aircraft. That is theoretically where the best climb and best cruise would be, although propellor efficiency, control/stability and cooling limitations often require higher climb speeds.

Hope that helped answer your question.

Greg Shaw

Angus · « **Reply #51 on:** February 19, 2004, 08:52:45 AM »

Lovely! Excellent!
So, initial acceleration is in favour by low wingloaded aircraft, but the true speedmonsters are aircraft with higher wingloading, or more correctly, less lift, which corresponds nicely with top speeds pr. hp of ww2 fighters. More or less I guess.....

HoHun · « **Reply #52 on:** February 19, 2004, 03:50:37 PM »

Hi Flyboy,

>btw, is the supercharger a part of the engine or a completely different part?

>for example, can i load varios kinds of superchargers to the same engine\ plane ?

The supercharger is mounted neatly to the engine.

(At the back in the case of the Merlin, on the left side in the case of the DB601/605. That's why the Merlin could easily accomodate a large two-stage supercharger, but never an engine cannon. It's also the reason why the Messerschmitts all have the supercharger air intake asymmetrically on the left side.)

As the supercharger's performance has to be closely matched to the engine's characteristics, there's usually only one supercharger to be used for each engine variant, and if you want a different supercharger, you have to create a new engine variant. (In fact, the main difference between engine variants often is the supercharger :-)

Regards,

Henning (HoHun)

HoHun · « **Reply #53 on:** February 19, 2004, 04:07:46 PM »

Hi Angus,

>So, initial acceleration is in favour by low wingloaded aircraft, but the true speedmonsters are aircraft with higher wingloading, or more correctly, less lift, which corresponds nicely with top speeds pr. hp of ww2 fighters. More or less I guess.....

Actually, acceleration is mathematically tied to climb rate.

The aircraft with the superior climb rate at a certain speed will out-accelerate the one with the inferior climb rate at the same speed.

Of course, climb rates are given at best climb speed, which varies from plane to plane, so using published climb rate figures is not perfect for a comparison.

It also gives only one data point for comparison. At high speed, the aircraft that accelerated worse at low speed might actually accelerate away from its rival.

So at sea level and starting at best climb speed, a Spitfire XIV at +18 lbs/sqin and 8500 lbs with a climb rate of 23.9 m/s will probably out-accelerate a Me 109K-4 at 1.8 ata and 3400 kg by a very small margin because the latter only climbs at 23.3 m/s.

Assuming the Me 109K-4 has a higher top speed at sea level for the sake of the example, there'll be a speed between best climb speed and top speed where both aircraft accelerate equally well, and above that speed, the Me 109K-4 will out-accelerate the Spitfire XIV.

(Of course, at high speed acceleration is much less rapid anyway, and so the difference will be much smaller in absolute terms, too.)

Regards,

Henning (HoHun)

gwshaw · « **Reply #54 on:** February 19, 2004, 04:13:54 PM »

Quote

Originally posted by Flyboy
thanks alot henning really cleared the issue for me

~~btw, is the supercharger a part of the engine or a completely different part?~~

[/b]
Yes, it is either a part of the engine or a separate part, depending on the engine.

In the Merlin it was more or less bolted on to the back of the engine, which made it easier for Rolls-Royce to change it around. The two-stage engines just stuck another stage on back to back with the original with a small intercooler between them.

On the Allison it was more integrated with the engine, and when they went to two stage setups it was an aux blower completely separate from the main engine.

The DB and Junkers inlines stuck them on the sides, and like the RR setup they could be changed around without effecting the main engine assembly.
Quote

for example, can i load varios kinds of superchargers to the same engine\ plane ?
[/b]

To some extent. Most started off with a single-speed/single-stage blower, went to a single-stage/two-speed blower for better altitude performance without harming low altitude performance. Then went to two-stage/two-speed or variable speed setups to further improve altitude performance.

The DB engines had a single-stage/variable speed blower that was probably better than any other single-stage engine. Later models with the enlarged compressor were pretty competitive with the two-stage Merlins and Allisons. Mostly because the German engines operated at much lower manifold pressures and didn't need to compress the charge air as much to make power. So they didn't need the intercooling and aftercooling systems the Allied engines required.

Greg Shaw

Angus · « **Reply #55 on:** February 19, 2004, 08:19:04 PM »

HoHun:
"It also gives only one data point for comparison. At high speed, the aircraft that accelerated worse at low speed might actually accelerate away from its rival. "
Exactly then! ?
Take a Spit I and a 109E (given the blessed same torque/(power) to a race where they open up side-by-side at 80 knots.
The 109 will be flying at a higher angle of attack, slots even deployed.
They open up, Spit will gain the lead. It will keep so for quite a bit, then eventually at some certain high speed the 109 will be faster. They might be level at 300 mph or so?
Yes?

HoHun · « **Reply #56 on:** February 20, 2004, 01:36:14 PM »

Hi Angus,

>They open up, Spit will gain the lead. It will keep so for quite a bit, then eventually at some certain high speed the 109 will be faster. They might be level at 300 mph or so?

Staying with the Spitfire XIV/Me 109K-4 example, that's about what would happen.

In a drag race, the plane with the better low-speed acceleration would soon get a lead with regard to speed, so that you're actually comparing two aircraft at different speeds most of the time.

That makes your example a bit tricky :-)

Regards,

Henning (HoHun)

VO101_Isegrim · « **Reply #57 on:** February 20, 2004, 02:21:12 PM »

Greg, can you calculate approx. speed at given (variable) powers at altitude, if the several historical speeds and their associated powers are given ?

I wonder this because I want to make range diagram where cruise speed is presented vs. fuel consumption and thus would give a range@speed diagram, in which the best cruise speed with the largest range/endurance can be found. Quite complex, but would be interesting, I think my simple empirical formula is not accurate enough for that task (10 km/h error would give already 20-50 km range error or more for planes with large fuel capacity).

gripen · « **Reply #58 on:** February 20, 2004, 11:19:40 PM »

gwshaw,
As for additional information, "Vee's for Victory" by D. Whitney is a pretty good source and highly recommened for anyone interested on the V-1710. Following part (p. 325) explains pretty well why the single stage Merlins were superior altitude engines if compared to the single stage V-1710:

"Reviewing Table 15-3 , it is not immediately clear why the single stage Merlin could develop both more power and more manifild pressure, and achieve higher critical altitudes than the V-1710. The combinations of rotor diameter and supercharger drive step-up ratios gave almost identical tip speeds to boyhy engines, meaning that the supercharger pressure ratios should be quite similar. Yet there was a difference in the outlet or resulting manifold pressure produced by the superchargers. This means that it must have been the pressure at the supercharger inlet, just downstream from the wide open throttle and ahead of the supercharger impeller, where the differences were occuring. An investigation of the differences in the flow areas at the single-stage supercharger inlets shows that the Allison area was about 25 percent less than that of the Merlin. When flowing the quantity of air needed to match the Merlin XX, the Allison would have had over 1 inHg of extra pressure loss, not including any losses upstream of the throttle."

In addition Whitney quotes a Wright field report which confirms his conclusions. This also explains why Spitfire Vs with +16 and +18 lbs boost had unlogical FTHs on climb; inlet size of the Spitfire V was too small (not designed such MAPs) therefore at low speeds these did not reach unrammed FTH (the Spitfire IX results are logical in this respect).

gripen

HoHun · « **Reply #59 on:** February 21, 2004, 08:44:12 AM »

Hi Gripen,

>This also explains why Spitfire Vs with +16 and +18 lbs boost had unlogical FTHs on climb; inlet size of the Spitfire V was too small (not designed such MAPs) therefore at low speeds these did not reach unrammed FTH (the Spitfire IX results are logical in this respect).

Hm, I'm not sure I understand that. Do you mean the Spitfire V didn't reach rated power at full throttle height?

Regards,

Henning (HoHun)