Drag Reduction with Radiator Flaps

[TAW-Prof]

This might be "correct as is" as I only have 1 source. Reading "Target for Tonight" by Sqn Ldr Denys A Braithwaite DFC & Bar (ISBN: 1-84415-159-X), I came across this report of an encounter in a Mosquito (probably B IV) and a pair of FW190. In this (Appendix One, Note (c)) it states:
"... Maximum i.a.s. attained by Mosquito at rated altitude (22,000 feet) was 350 m.p.h. i.a.s. in gentle dive. (Note: approximately 15 m.p.h. gined by closing radiator shutters)."

Assuming that the 350 was with the shutters closed this would be 335 mph open. This equates to an 8% reduction in drag (TAS open = 461.2 mph, TAS closed = 480.7 mph & drag reduction = 1 - [TAS open]^2/[TAS Closed]^2)

This is equivalent to an IAS of 240 mph shutters closed and 250 mph open at sea level.

As said, this is a single source so SMEs may have better information but I thought it might help tune to flight model.

Keep up the great work.

Edited February 5, 2022 by TAW-Prof

[grafspee]

350mph ias at 22k is 550mph. It had to be hell of the dive. 

350 tas i would believe in this.

Edited February 5, 2022 by grafspee

[TAW-Prof]

The quote is lifted from the report. I guess they used IAS as that was what the pilot could see. Whether you believe this is credible is a good and worthy question. I'm just putting this up as evidence for the difference in drag with shutters open/closed. There may be better evidence out there. Given that this was written in a WW2 Mosquito squadron it can't have been too outlandish. In Mosquito Intruder (I believe also published as "Terror in the Starboard Seat") Dave McIntosh says that he saw 350 "on the clock" when diving from 10,000 ft after a V1 and that they leveled out doing 400 mph but "[the wing] was flapping up and down like a seagull working in a huricane."

According to this: https://aerotoolbox.com/airspeed-conversions/
350 mph IAS @22,000 ft is 480 mph TAS (M 0.685), my original numbers were TAS in knots (mea culpa and now corrected/editted)

Even if these are TAS rather than IAS the drag difference is essentially the same (7.95% if IAS and 8.39% if TAS), so call it 8% either way.

 

[grafspee]

In reports speed was often TAS.

What does "rated altitude " mean in this report, if it mean critical alt, in thhat case this is not a marlin 25 powered mosquito, merlin 25 is rated at around 14k

Just tested it, at 22k maxed out 8-9 punds of boost in shallow dive i managed to get 260mph at 20k, you really thinks that ED overshoot performance by 100mph  Comon.

mosquito with merlin 72/73 yea i can believe this but not with merlin 25

 

Edited February 5, 2022 by grafspee

[TAW-Prof]

"Rated Altitude", sorry I'm not sure what this means either - I'm just reproducing what was in the report.
Mosquito B Mk IV has either a Merlin 21 or 23 engine, according to the Pilots Notes (page 6) here: https://www.avialogs.com/aircraft-d/de-havilland/item/104130-a-p-2019d-pilot-s-notes-for-mosquito-biv-merlin-21-or-23-engines. This seesm to tally with the Wikiepedia enter here: https://en.wikipedia.org/wiki/List_of_Rolls-Royce_Merlin_variants . Granted this is hardly a definative source!
Given that the report is 1942 it can't be using one of the later merlin engines.

I a 5-10 degree dive (50% fuel) I can get 350 mph ias with full revs and throttle forward to detent. Boost is about +12. Still accellerating when diving from 25,000 to 20,000. I suppose it all depends on what you class as a "shallow dive" it looks like my shallow is a lot steeper than your shallow.

Anyway much of this is academic. IF, and it is an IF, the report is credible then the impact of closing the shutters is larger then currently modelled in DCS. This is NOT to say that its wrong, the source that I am quoting may well be wrong. If you have any other sources that give any indication as to the drag change on opening/closing the radiator shutter then please share it.

Thanks

Prof
 

[grafspee]

Manual which i have mosquito mk 6 used 23 or 25 merlin engine.

And merlin 25 is modeled in DCS it allow 18lbs boost merlin 23 was limited tp 14lbs for take off

Edited February 6, 2022 by grafspee