Mosquito KA114 Take Off Handling Questions

Archer;cavitation; look at the photos of any heavy prop job on a moist atmospheric day on take-off…

No, that’s condensation, a sudden drop in pressure which causes moisture in the air to change phase from gaseous to liquid.

Or listen to a Harvard/Texan!

And that is the result of local supersonic flow over the prop blades, which because of the sudden change in pressure can cause condensation.

Cavitation is the situation where the local pressure in a liquid drops enough to cause vapour bubbles to form, which then collapse.

Archer;cavitation; look at the photos of any heavy prop job on a moist atmospheric day on take-off…

Or listen to a Harvard/Texan!

Just to add a little which may help those understand a bit more about engines/props boost.etc; at idle ,as an example,6-800 ERPM,the prop rpm would be about 300-350 ; the boost would be say,-8/9 (10 to 12″Hg),as the throttle butterflies are almost closed,and there is a lot of `suck` through the inlet. Assuming a C/S prop the blades are on the fine pitch (low blade angle) stops ;as the throttle is opened the manifold pressure will increase ie (-5/20″),and the governor of the C/S prop could now begin to operate,if the control was set to 1500 rpm,for example.However,it will be set to max .for T/O,so any further increase in throttle will increase RPM to the maximum setting,as the `boost/manifold` pressure reaches `static` boost of `0lbs/30″.Normally one would check the magnetos,and cycle the RPM levers thru` their range a couple of times,to bring fresh warm oil to the CSU.On a multi-engined a/c one would also cycle /operate the feathering buttons/lever briefly to check that the blades move towards the feather position,can be seen/heard,and observed as a drop in RPM.
From now on any advance of the throttles will open the `butterflies and allow manifold pressure to rise , say ,up to +12lbs/54″ Hg.; but the ERPM will remain at 3000/prop RPM about 1430; The blades will automatically start to increase pitch as speed increases.If you leave everything there( one will need to open the throttle to maintain the selected boost,unless it has the altitude compensator) as height is gained,the boost will reduce,,until at some altitude,there will need to be a `gear change` on the supercharger,either automatically or manually by the pilot/engineer.This will resore the boost,until again one reaches an altitude where the Throttle is wide open,just like a normal aspirated engine.
If you now roll over and pull-thru`,and dive to Vmax or beyond,the CSU may now reach the allowable blade angle that it can control,and so the propeller is now`fixed` pitch,and any speed increase can have serious effects on the engine,as it will probably overspeed,given the reduction ratio of engine to prop.
Do not take the numbers I used as gospel,purely as illustration…

Archer;cavitation; look at the photos of any heavy prop job on a moist atmospheric day on take-off…

Torque is proportional to the power being put into the props. The more power the engine puts into turning the props in a clockwise direction, the more the engine wants to turn the other way (and, as it is attached, the aircraft). This is more pronounced on single engine airplanes as this torque acts around the CG. The prop doesn’t let you know that it is in its governing range, but you will notice as the rpm remains constant. Below a certain power setting the governor is trying to increase the prop rpm but is unable as the blades are at the fine pitch setting, as the power increases the prop gets into the set rpm range and then the governor keeps it there by increasing the blade pitch. As from that point on the rpm stays the same (and the mass of the prop as well) the amount of torque also stays more or less the same.

So with the engines stabilised before brake release you are keeping the number of variables during take off to a minimum. There may still be a bit of swing from P-effect as the aircraft gets its tail up but that’s another thing again.

Ie the pitch becomes coarser as the boost is increased indicating higher altitude and thinner air? But this would then be cavitating at GL… with poor performance but high torque reaction.

Cavitation only happens in liquids, so we don’t get that with props. As altitude increases the prop meets less resistance from the air but also delivers less thrust as the mass of air it can accelerate decreases.

Does a constant speed prop indicate that it is variable pitch? Ie the pitch becomes coarser as the boost is increased indicating higher altitude and thinner air? But this would then be cavitating at GL… with poor performance but high torque reaction.

The torque reaction is that related to the forces generated at the prop, so less torque reaction with a feathered prop but more efficiency at GL with the higher density air and no boost – is this right? Sorry I am an engineer but not in aviation and I am guessing about the operation of the prop and related controls. At least it may generate some more discussion and a better understanding for everyone.

Thanks Sycamore. If the props are rotating at zero boost, the propellor rpm is fast. I expect the torque effect on brake release to be great, but from what you’re writing it’s a high throttle setting which can cause this.

So is it that high throttle setting produces faster movement of working parts in the engine freslting in more torque, but boost does not? To me, high rpm = faster moving parts = high torque…

Thanks for that Sycamore. It makes a lot of sense

Having only ever flown ‘handed’ tricycle twins, it has never been a problem I have had to think about at any great length.

Moggy

If you set `0` boost,(which equates to 30″Hg) ,the props are at the recommended RPM(ie 3000ERPM)and are constant-speeding.You now have a significant amount of airflow over the tailplane and fin and rudder,to counter the yaw when the brakes are released.You have also brought the engines through the range where one may have a sluggish prop.governor,which will cause slower response and create an asymmetric power surge.This would be highly undesirable commencing the t/o roll,especially if one was to open the throttles from idling. The Mosquito I personally think is `underfinned`,and one possible reason for it`s relatively high `safety speed`.At high operational weights,even using full rated boost( +12/54″ ?) acceleration would not be brisk,and the undercarriage takes a little time to get cleaned-up,as well.
Anyone who operates any large/powerful piston engined aircraft will usually `open -up` to `static boost ,before brakes-off,otherwise you`ll need some wild and fancy footwork to stop going off the side.
I would guess that the present Mossie pilots are perhaps not using more than about +4-6 lbs boost,as the a/c will not be near an operational weight anyway,but it is quite adequate.

I thought you were here to maintain standards? :p

9) Please post in comprehensible English, commonly used abbreviations are acceptable however refrain from using ‘mobile’ text as not everyone understands it.

Don’t you mean…’..not everyone understands it…innit’?

I thought you were here to maintain standards? :p

Odd innit as unlike the later Hornet, the Mosquito has even handed engines so you will get swing.

9) Please post in comprehensible English, commonly used abbreviations are acceptable however refrain from using ‘mobile’ text as not everyone understands it.

Odd innit as unlike the later Hornet, the Mosquito has even handed engines so you will get swing.