True Air Speed Vs Indicated Air Speed

JDCAVE · Oct 20, 2023

MIflyer said:
Later, when they had P-51's, they would send out Weather Scouts, a flight of four Mustangs, to report on the WX in the target area and presumably unforecast winds. Of course, the WX Scouts were not adverse to bagging a few Luftwaffe aircraft that they ran across during the course of their primary mission. I think I have an article on that somewhere.

I wonder what they did for radio communications for the WX Scout missions. The SCR-522 radios the Mustangs in the ETO were equipped with normally could not reach GB from Germany, but they did stage radio relay aircraft between GB and the mainland, usually P-47's, I think.

Yes. Bomber Command dispatched a Meteorological Mosquitoe to the target area in advance of the Main Force. It communicated with the Master Bomber prior to the attack. It reported cloud conditions and winds to inform changes to marking methods as well as winds for adjustments to bomb sights.

Jim

JDCAVE · Oct 20, 2023

MIflyer said:
Later, when they had P-51's, they would send out Weather Scouts, a flight of four Mustangs, to report on the WX in the target area and presumably unforecast winds. Of course, the WX Scouts were not adverse to bagging a few Luftwaffe aircraft that they ran across during the course of their primary mission. I think I have an article on that somewhere.

I wonder what they did for radio communications for the WX Scout missions. The SCR-522 radios the Mustangs in the ETO were equipped with normally could not reach GB from Germany, but they did stage radio relay aircraft between GB and the mainland, usually P-47's, I think.

I think the shortcoming of using a S/E fighter for WX would be that the pilot would/could not provide wind finding. The mosquito crew with a navigator could possibly have done wind finding calculations during the route to the target area. I say possibly, as I am uncertain if this was actually performed.

Jim

JDCAVE · Oct 21, 2023

This interesting. Evidently the USAAF used mosquitoes for meteorological observations.

When the US Army Air Forces flew de Havilland Mosquitos

That comment by one unit commander sums up the views of many in the US Army Air Forces on the de Havilland Mosquito, an aircraft that served the AAF in relatively small numbers, but was arguably superior to anything American makers offered

www.key.aero

drgondog · Oct 21, 2023

MIflyer said:
Later, when they had P-51's, they would send out Weather Scouts, a flight of four Mustangs, to report on the WX in the target area and presumably unforecast winds. Of course, the WX Scouts were not adverse to bagging a few Luftwaffe aircraft that they ran across during the course of their primary mission. I think I have an article on that somewhere.

I wonder what they did for radio communications for the WX Scout missions. The SCR-522 radios the Mustangs in the ETO were equipped with normally could not reach GB from Germany, but they did stage radio relay aircraft between GB and the mainland, usually P-47's, I think.

The first Scout force was Scout Force (exp) based at Steeple Morden. Col Budd Peaslee brought 1st BD volunteer pilots and the 355th recruited volunteers to provide experienced fighter pilots as wingman. The ground crews and Mustangs were maintained s E Flight from each squadron. IIRC July 16 was first mission.

AT-6s from 355th were used to familiarize the B-17 pilots but several fatal accidents occurred through September. Additionlly one was shot down air to air.

The Experimental Scouts were the core that morphed into 1st, 2nd and 3rd SF in September. At that time the 2nd SF formed and my father was both acting 354FS CO and Deputy Gp CO to fill Clay Kinnard's move to 4th FG.

As part of his responsibilities he was also chief of the accidents board at 355th and also took on the informal responsibilities as IP for all the fresh B-24 drivers in the 2nd SF pool. AT-6 and Clobber College Mustang two seater (single control). He washed out a couple, primarily because he couldn't quite get that 'roll and pull that stick into your gut' thangy. He also intrduced mandatory Link Trainer time for all 355th/2nd SF replacement pilots. The ex-bomber pilots rarely required the IFR type experiences - but stateside replacement pilots were. in his opinion, 'pretty sad' on instruments.

The 2nd SF scored 13-3-5 air, 7-4 ground including two Me 262 air. The 3rd SF scored 3-0-0 air. the 1st SF did not score.

Uniquely, not ony scoring 4X over the other two SF combined, ex-B-24 drivers accounted for 6 of the 13.

The original July/August missions were a flight of four. All they needed was SCR-522 because they were transmittng 50-100 miles on the Bomb Division Command freq and the 355th Command freq. No relays necessary. 2nd SF adopted full sections of two flights. Don't know about the other SF doctrine.

I have written several articles on Scout Force. I think Mike Williams still carries on spitfireperformance.com

THE SCOUTING FORCE

drgondog · Oct 21, 2023

JDCAVE said:
This interesting. Evidently the USAAF used mosquitoes for meteorological observations.

When the US Army Air Forces flew de Havilland Mosquitos

That comment by one unit commander sums up the views of many in the US Army Air Forces on the de Havilland Mosquito, an aircraft that served the AAF in relatively small numbers, but was arguably superior to anything American makers offered

www.key.aero

The original Scout Force (Experimental) Budd Peaslee had Mosquitos as the Plan scouts when he presented to 8th AF Hq in May 1944, but they were 'unavailable' and P-51Ds were acceptable. Although the phrase 'weather scouts' was used, they were focused on two factors - 1.) weather/visibility conditons over the Primary - then Secondary targets if Primary obscured, and 2.) major weather threats such as T-Storms blocking planned route to target.

They 'din't care nuttin' about winds aloft so much as route conditions and target conditions. I suppose the primary attitude was 'if the path is clear' the bomb group lead navigators will figure it out'

MIflyer · Oct 21, 2023

drgondog said:
I have written several articles on Scout Force. I think Mike Williams still carries on spitfireperformance.com

Okay so:
1. Scout Force was formed up as a special unit.
2. They did not get to the target areas before the bombers took off but were just ahead of them.

Question I have wondered about for some time. The USAAF fighters in the ETO usually were not equipped with either the BC-453 LF receiver from the SCR-274-N or the BC-1206 LF receiver (often called the Detrola but in fact there were at least two manufacturers of at least two different designs). Artwork often shows the wire antenna extending through the bubble canopy to the tail but usually it was not there. In the US it was there because of the AN Ranges and the use of LF by control towers.
I have read repeatedly of fighter pilots homing in on their home field beacon in GB. What were they homing in with?

Barrett · Oct 23, 2023

Laurelix said:
I wanted to showcase the difference between TAS (True Air Speed) compared to IAS (Indicated Air Speed)

P-51D
View attachment 591938

Stall Speed at 9611lb (4360kg) loaded weight - 110.7mph (178km/h) at Sea Level
(No flaps, level flight)

That's Wing Lift Coefficient of 1.31 CL_Max which is standard for a laminar flow wing.

With this info I can calculate the stall speed at different altitudes by changing the air density to match different altitudes.

This is the TAS it needs so it doesn't stall at altitudes with different air density.
IAS = TAS at Sea Level, however TAS has to increase to maintain same IAS stall speed due to decreasing amount of air as altitude increases.

ALTITUDE / SPEED / AIR DENSITY
Sea Level - 178km/h - 1.225kg/m3
1000m - 187km/h - 1.112kg/m3
2000m - 196km/h - 1.007kg/m3
3000m - 207km/h - 0.9093kg/m3
4000m - 218km/h - 0.8194kg/m3
5000m - 230km/h - 0.7364kg/m3
6000m - 242km/h - 0.6601kg/m3
7000m - 256km/h - 0.5900kg/m3
8000m - 272km/h - 0.5258kg/m3
9000m - 288km/h - 0.4671kg/m3
10,000m - 306km/h - 0.4135kg/m3

This is why stall speed is measured in IAS.
P-51D has 178km/h IAS stall speed but it needs to at least travel at 306km/h at 10,000m to maintain 178km/h IAS and avoid stalling. As a result of higher stall speed in TAS for all planes, as you get higher the more sluggish planes turn. Their radius of turn increases drastically and also their power output is also reduced at higher altitudes making the power to weight ratio worse and worsening the sustained turn capability.

FUN FACT:
To get into Space and leave gravitational pull of earth you need to go beyond 100,000m altitude.
If P-51D was to reach 80,000m altitude, it would have to travel at 45,800km/h TAS (Mach 37.1) to maintain 178km/h IAS in order to not stall.
This is why even jets like SR-71 Blackbird have service ceiling of 26,000m altitude.

Conclusion:
TAS = How fast you travel from point A to point B. Basically how much distance you move in set amount of time.

IAS = The airflow over your wings. You need to go faster to maintain same airflow due to decreasing amount of air as you go higher in altitude. All the TAS speeds at different altitudes = 178km/h IAS

———

Another Fact:
Every plane has a structural limit and there is a maximum limit at every altitude where the plane will fall apart at certain TAS due to the extremely high IAS.
The newer next generation jets which are suppose to reach Mach 6 will still be limited to same speed at low altitudes to earlier jets, the difference is the capability of going even higher which allows the jet to achieve greater TAS before reaching structural IAS limit of an airframe. Jet technology today is at its Apex level right now that increasing max speed is the same as saying - Increasing Service Ceiling. It's a matter of creating engines which can still operate at altitudes with ridiculously low air density which would increase service ceiling.

Jumping in late here
but
One of my aviation author colleagues engaged in an Air Progress dispute (fun on both sides) with a columnist regarding the danger (or not) of downwind turns. That was when AP was still Air Progress, sorta like the days when Flying was still Flying, 1970s or so.

But I digress.

The dispute centered on whether the perceived increase in downwind groundspeed might prove dangerous in relation to TAS. Of course, much depended on altitude but as I recall, the editor finally blew the whistle and moved on to another author duel...

yosimitesam · Oct 23, 2023

No matter how you slice or dice it, airspeed measurements are ultimately governed by the notorious Navier-Stokes differential equation. It is hideously non-linear and complex and governs all fluid flow in nature (liquid or gas). Proving that bounded solutions to the equation exist in general is an open mathematical problem and one of the Clay Institute's 1-million dollar prize problems, if you can solve it. Even in the simplest cases, there aren't really any useful "spherical cows" that can give you approximate analytic solutions. (Google 'spherical cow' for those that don't know what that term means.) In short, ANYTHING dealing with fluids in relative motion (such as air) is going to be horribly complex, and there's nothing you can do about it but sigh. "Sigh..."

SaparotRob · Oct 23, 2023

Spherical cows is one of the few engineering concepts I have heard of.

ThomasP · Oct 25, 2023

Hey MIFlyer,

re "I have read repeatedly of fighter pilots homing in on their home field beacon in GB. What were they homing in with?"

The SCR-522 had 4 channels - A,B,C,D. In the ETO channel C was generally reserved for homing on your home base.
The airbases multiple ground set equivalents of the SCR-522, but with higher power and mast antennae.

Channel A was normally used for air-air with local squadrons and GCI
Channel B was normally used for air-ground with local tower and other-than-local towers, and air-air with other-than-local squadrons
Channel D was generally used for air-ground with DF stations and GCI when fitted with 'pip-squeak' IFF

Snautzer01 · Oct 25, 2023

SaparotRob said:
Spherical cows is one of the few engineering concepts I have heard of.

But only when you are in a vacuüm.

drgondog · Oct 25, 2023

ThomasP said:
Hey MIFlyer,

re "I have read repeatedly of fighter pilots homing in on their home field beacon in GB. What were they homing in with?"

The SCR-522 had 4 channels - A,B,C,D. In the ETO channel C was generally reserved for homing on your home base.
The airbases had the multiple ground set equivalents of the SCR-522, but with higher power and mast antennae.

Channel A was normally used for air-air with local squadrons and GCI
Channel B was normally used for air-ground with local tower and other-than-local towers, and air-air with other-than-local squadrons
Channel D was generally used for air-ground with DF stations and GCI when fitted with 'pip-squeak' IFF

C-Channel was also briefed Command Channel between Bomber Command ship (s) leading a wing and the Fighter Commander leading Groups tasked to escort.
B-Channel also for Air-Sea Rescue
A-Channel also for Ground Control Center communications with bomber and fighter groups. Each fighter group was on separate frequency and each Bomb Wing was on separate frequency.

True Air Speed Vs Indicated Air Speed

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JDCAVE

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JDCAVE

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When the US Army Air Forces flew de Havilland Mosquitos

drgondog

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When the US Army Air Forces flew de Havilland Mosquitos

MIflyer

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Barrett

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yosimitesam

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SaparotRob

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