Merlin negative G cutout too quick?

[Moggy]

Originally Posted by klem

I work at Tangmere as a volunteer, I'll go through our library again. I'll also ask around and talk to a friend there that maintained the Grace Spitfire (not a MkI/II) but I think IvanK has already covered the 'AFAIK' aspect. What I really need is a Hurricane MkI veteran to walk through the door

On a semi related issue (very semi!), I need to have a chat with you sometime about the museum klem.

[klem]

Originally Posted by Moggy

On a semi related issue (very semi!), I need to have a chat with you sometime about the museum klem.

Your "Wing's" name arose a week ago - a telephone call?
pm me.

[Viper2000]

Originally Posted by cheesehawk

However, I'm a bit taken back by the above post, sounds to me like you're setting up a scenario that "Even if a vet says I'm wrong, I'm going to insist that I'm right?"

That's a very over-simplistic attitude to take.

Firstly, people tend to have pretty imperfect memories. That's why we write things down. Very few people will remember precise details about aeroplanes that they flew 70 odd years ago.

I last flew a Bulldog about 10 years ago. I can remember the numbers which were most important to me; climb at 80 knots, loop entry at 140, stall turn entry at 120, VNE was about 200 knots, but you'd only get that going downhill very steeply (at which point, if you're 17 and will live forever, puling as hard as you can will get you about 6 g - "do what you want - it's going to the scrapyard tomorrow!").

I can remember that it used to be happier to stall turn one way than the other, but I couldn't honestly tell you which way was the easy way.

I couldn't tell you the engine limits off hand; there's a tendency to just remember that green is good and red is bad, so if it's green it's good and that's it. I know it'll handle negative g without complaint, because I did it.

The stall was well mannered, but I couldn't tell you the stall speed because I wasn't interested in stalling it; I wanted aerobatics. Anyway, there was plenty of buffet to warn you if you were close to the edge of the envelope.

I landed it quite a lot of times, but I can't even remember the speeds for that - 70 knots on approach? Probably a bit less over the fence and then you're not looking at the ASI anyway...

That's 10 years ago - ask me again in 60 and it'll be a wonder if I can remember what a Bulldog even was!

Secondly, it's not even as though you can just talk about "a Hurricane". Even in 1940 there were a heck of a lot of potential mod states knocking around.

The sim is seeking to model the performance of a couple of Hurricanes in defined mod states. Information relating to a different mark or mod state won't necessarily read across.

Of course, this observation equally reads across to current flight experience. Nobody operates Hurricane Mk I aeroplanes in their 1940 mod state anymore.

The average warbird knocking around has a civil Merlin (500 series for the single stage engines, 700 series IIRC for the two stage engines, though quite often you'll also see single stage engines retrofitted into aeroplanes that would originally have had two stage engines), or a transport command merlin (T.25) rather than an authentic fighter Merlin, because the latter offer considerably longer overhaul intervals. Most of the time they have FS gear disabled because they're only interested in low level work at airshows; hence the unfortunate tendency to retrofit single stage engines into aeroplanes which would originally have had two stage engines*.

It can actually be very difficult to work out exactly what engines and mod states current warbirds operate in, because the pilots often don't much care, the PR people haven't the slightest idea, and the engineers are generally too busy. In fairness, it's not the sort of question that they'll generally be asked...

///

In other news, I have been experimenting with the Rotol Hurricane this evening using the latest patch, and it's crazy. I get black smoke and misbehaviour at even slightly reduced positive (like 0.9ish g I guess; not much point getting out the stopwatch and calculating g from pitch rate and TAS; the rates would be too low for the results to be sensible). It's set off by turbulence, and you'd probably also get it from a gently phugoid (which would probably be the most repeatable way to test & quantify it; trim for say 200 mph IAS at 10,000 feet and then pull to x mph IAS slower than the trimmed speed and release the stick - naturally this this will generate a stick-fixed phugoid given the nature of the sim's modelling methodology - anyway, a given value of x should correspond to a fixed g load for the first cycle, and this is probably repeatable to smaller g increments than directly hand-flying a pushover).

The cut behaviour is obviously wrong, quite apart from the fact that it's on a hair trigger, because we're straight into rich cut with no preceding lean cut.

*Of course, a single stage engine will give more bhp at any given boost level because less power is consumed driving a single supercharger stage than driving two. Single stage engines are also lighter. So if you only want up to about +18 psi at low level then a single stage engine will deliver more performance, especially since single stage engines are obviously lighter, and the lack of an intercooler/aftercooler means that you've instantly gained a load of extra radiator if you're prepared to plumb the intercooler/aftercooler radiator into the main loop...

Viper,don't forget Packard Merlins on Mustangs!

You'd be surprised to see how many warbirds owners and operators tend to go for an accurate engine selection: the third generation of warbirds owners do any possible effort to have their machines to wartime specs,down to wirings,radios and equipment (working gunsights are a must!).

Same goes for superchargers: the era of non working superchargers is gone,pretty much everyone is going for working ones.

It's all down to how deep the pockets are and availability of parts.

[Viper2000]

Originally Posted by Sternjaeger

Viper,don't forget Packard Merlins on Mustangs!

You'd be surprised to see how many warbirds owners and operators tend to go for an accurate engine selection: the third generation of warbirds owners do any possible effort to have their machines to wartime specs,down to wirings,radios and equipment (working gunsights are a must!).

Same goes for superchargers: the era of non working superchargers is gone,pretty much everyone is going for working ones.

It's all down to how deep the pockets are and availability of parts.

Times have obviously moved on! In that case, mine's a Dh-103 with real Merlin 130/1s please (since Eric Brown gave it such a glowing review). Or an MB.5, perhaps modded with spring tab ailerons...

I'd also love to see a Tempest V with Napier Sabre, because despite the huge reliability issues it would be such a wonderfully different sound! But my true love is and remains the English Electric Lightning... *sigh*...

As for the whole Packard thing, I rather like the original Mustang X with Merlin 65; just a pure scrapheap challenge job by the installation department, but it gave quite a good showing against the vastly more elaborate & expensive P-51B/V-1650-3 combination; AFAIK a few examples even flew on ops. But that's a really obscure machine. You know they also generated performance curves for Mustang + Merlin XX series? It would have arrived earlier and presumably served as a Hurricane replacement (since there would then have been no supply of XX series engines for the Hurricane line). But then Packard made Ford's refusal to play ball irrelevant and the idea got parked.

Did anybody ever compare the behaviour at low g of the Spit and the Hurri? My impression is that the Hurri is less sensitive though still quite sensitive.

I too think it is a bit overdone. The slightest dip will lead to a significant performance loss.

I do understand that when you carefully push the stick this will reduce the lateral g from 1g to something like let's say 0.9g or 0.8 g which will lead to reduced hydraulic pressure at the fuel outlet of the fuel tanks hence reducing the fuel flow hence leading to a leaner mixture. Cut-out only should happen when the total pressure at tank outlet (including hence the hydraulic pressure) is equal or less to the necessary pressure for combustion in the cylinder heads plus the pressure losses in the feed lines. Pressure losses are a function of fuel flow and decrease with decreasing mass flow rate as the flow velocity decreases. I though do not know at which g this could happen.

Up to now I just wonder why there was no mechanical blocking of the forward movement of the stick as even levelling out from climb is extremely tricky.

Just to get me right: If it was historically this sensitive I wish to keep the effect. Currently I have some slight doubts.

[jdbecks]

Originally Posted by 41Sqn_Stormcrow

Did anybody ever compare the behaviour at low g of the Spit and the Hurri? My impression is that the Hurri is less sensitive though still quite sensitive.

I too think it is a bit overdone. The slightest dip will lead to a significant performance loss.

I do understand that when you carefully push the stick this will reduce the lateral g from 1g to something like let's say 0.9g or 0.8 g which will lead to reduced hydraulic pressure at the fuel outlet of the fuel tanks hence reducing the fuel flow hence leading to a leaner mixture. Cut-out only should happen when the total pressure at tank outlet (including hence the hydraulic pressure) is equal or less to the necessary pressure for combustion in the cylinder heads plus the pressure losses in the feed lines. Pressure losses are a function of fuel flow and decrease with decreasing mass flow rate as the flow velocity decreases. I though do not know at which g this could happen.

Up to now I just wonder why there was no mechanical blocking of the forward movement of the stick as even levelling out from climb is extremely tricky.

Just to get me right: If it was historically this sensitive I wish to keep the effect. Currently I have some slight doubts.

Your misunderstanding as to why the engine cuts out during negative G maneuvers.

Naw naw. I understand why the engine cuts out under NEGATIVE g manoeuvers.

The thing is I don't understand why any slight push of the stick should lead to negative g

You only get negative g when the total acceleration acting upward exceeds gravitational acceleration in horizontal flight. I don't see why this should happen at any slightest forward move of stick even in horizontal flight ... but perhaps you can enlighten me.

[klem]

I hope to get into the library this week but please don't hold out too much hope on that as if there were a publication I'm sure someone would have popped up with it by now. The chances of bumping into a Hurricane pilot are extremely thin!

However, on this general subject, a small point for the devs:

To confirm the reversed Rich/Lean problem, if you open an info Window you can see that the Hurricane lever in the rear 'Rich' position is giving 0% Rich and in the forward 'Lean' position is giving 100% Rich. You get the same results whether you are using an Axis or Keys for your mixture control. I assume that is what is feeding into the simulation and its not just showing my Axis position.

On the Rotol prop Hurri FT-N, when you close the throttle it pushes the mixture lever to the rearward opsition, i.e. Lean not Rich (remember its backwards) which is not what you want when starting the engine, you want Rich, and I have to move the Throttle forward to around halfway up the Rich-Lean gate so that I can move the mixture lever out of fully Lean to start the engine. My point for the devs: Strangely, when I then close the throttle the info window shows the mixture doesn't change until I give my axis a slight tweak when it goes to 0%. Just another tweak needed when they can.

Rich for the Spit should be forward and Lean backwards. It just reads the wrong way in the cockpit model. Physically it is correctly implemented.