PDA

View Full Version : Fighter Design & Development Issues



Blutarski2004
03-23-2005, 04:13 PM
I promised someone here on Ubi that I would post some info from a book called FIGHTER FACTS AND FALLACIES, by John G Lee (Asst Director of Research, United Aircraft Corporation), written in 1942.

Without transcribing the entire book, I will point out the important comments. Some will be obvious. Some are not so. Please note that when the author compares something, he does so on the assumption that the two aircraft in question are equal in all other regards.

WING LOADING
1. At low altitudes, the plane with smaller wings and higher wing-loading is faster and has a better climb rate than the plane with larger wings and lower wing-loading.
2. At high altitudes the plane with larger wings and lower wing-loading is faster and has a better climb rate.
3. The plane with lower wing loading will generally climb at a steeper angle.
4. If the increase in wing area is achieved by increasing span and holding chord constant, considerable improvement in performance is achieved. If vice-versa, only a small advantage is achieved.

MANEUVERABILITY & RESPONSE
1. Maneuverability is the measure of space required by an a/c to execute a given maneuver.
2. Response is the measur eof the rapidity with which an a/c follows the movements of the pilot's controls.
3. The greater the wingspan, the more the a/c resists rolling and the greater will be the time to enter or exit a turn
4. stick forces at high speeds are one of the most difficult problems for the a/c designer.

POWER LOADING
1. At all altitudes the airplane with the lower power-loading will outperform it otherwise equal opponent in every way.
2. Air-cooled powerplants are lighter per hp than liquid-cooled engines; the difference in drag is small.
3. Propellers may be designed for speed or for climb. Speed-oriented propellers may cause an a/c with apparently good power-loading characteristics to perform poorly on take-off and low level climb.
4. At altitude, the supercharged a/c is superior in all ways; at sea level, the unsupercharged a/c is slightly better.
5. Supercharged air will become so hot that it will cause engine knock; hence the need for inter-cooling.
6. Engine exhaust thrust may amnount to 10 pct of total propelling force at top speed; the best exhaust thrust is obtained by individual stacks fitted to each cylinder; this cannot be fitted to turbo-supercharged engines.

STREAMLINING
1. The streamlined a/c is faster and will have a longer range, but it will not climb much better.
2. Streamlining is extremely complicated. The drag of small items (mirrors, radio masts, etc) can produce a tremendous multiplier effect by upsetting smooth airflow over large areas of surface.
3. Some of the most frequent contributors to major drag problems are improper wing filleting, sharp corners around the front of the cockpit windshield, external air scoops, disturbances at wing leading edge.
4. Compressibility effects do not necessarily occur over the entire a/c. It only requires that some portion of the a/c structure produce a compression shock wave to massively increase drag effects.
5. Interior streamlining of cooling radiator passages for liquid-cooled engines and cowling ducts on air-cooled engines are important contributors to overall "cleanliness" of a design.
6. At lower altitudes the differences in drag between liquid-cooled and air-cooled powerplants are essentially moot. At high altitudes the ducting problems are less well understood (is this a possible explanation for P38 problems?!?).
7. Induced drag is produced by vortices of positive pressure air escaping from beneath the wingtips and is a major drag component at cruising speed and in the climb. Weight increases this effect; high altitudes increases the effect; lower span loading reduces this effect. Hence a/c designed for operation at high altitude normally feature great wing spans.

PROPELLERS
1. Propeller efficiency can be more than 86 pct under optimal design and flight regime conditions, but these conditions do not always obtain.
2. A propeller optimized for slower speeds will improve take-off and climb performance, but will cause a sacrifice in top speed. The reverse is the case as well.
3. A propeller able to handle 2,000 hp at 40,ooo ft altitude would be big enough to handle 8,000 hp at sea level; it is not possible to design a propeller eto be efficient at both extremes.
4. Fast turning propellers are better at translating engine hp into thrust than slower turning propellers.
5. Velocity of propeller tips cannot exceed the speed of sound without producing very large drag effects. Therefore propellers are limited in diameter by their rotational speed and vice versa.

The book closes out with a useful matrix of inter-relationships between design features which shows in simple terms the typical effect upon a/c performance when a particular performance factor is reduced by 10 percent -

WING LOADING REDUCED BY 10 PCT
-2 pct top speed
-1 pct climb at sea level
+1 pct service ceiling
-9 pct take-off run
-5 pct landing speed
+1 pct range
-10 pct turn radius

POWER LOADING REDUCED BY 10 PCT
+3 pct top speed
+13 pct climb at sea level
+3 pct service ceiling
-0 pct take-off run
-5 pct landing speed
(see note 3) range
(see note 4) turn radius

SPAN LOADING REDUCED BY 10 PCT (see note 6)
-0 pct top speed
+2 pct climb at sea level
+3 pct service ceiling
-0 pct take-off run
-0 pct landing speed
+3 pct range
(see note 4) turn radius

CRITICAL SUPERCHARGER ALTITUDE REDUCED BY 10 PCT
(see note 2) top speed
-0 pct climb at sea level
-4 pct service ceiling
-0 pct take-off run
-0 pct landing speed
+0 pct range
-0 pct turn radius

DRAG REDUCED BY 10 PCT
+3 pct top speed
+1 pct climb at sea level
+1 pct service ceiling
-0 pct take-off run
-0 pct landing speed
+5 pct range
(see note 4) turn radius

GROSS WEIGHT REDUCED BY 10 PCT
+1 pct top speed
+14 pct climb at sea level
+4 pct service ceiling
-13 pct take-off run
-5 pct landing speed
(see note 5) range
-10 pct turn radius

Note 1: Assume wing area changes while holding span to chord ratio constant. This produces a 10 pct reduction in wing loading and a 5 pct reduction in span loading simultaneously.

Note 2: A 10 pct reduction in critical altitude will produce a 2 pct slower maximum speed at the lower critical altitude. Speed at the previous critical altitude will be 7 pct slower.

Note 3: Reduction in power loading does not per se affect range, but it will permit take-off with a heavier fuel load, which will then increase range.

Note 4: Under certain circumstances the drag of an airplane can be so great that the engine power is insufficient to pull it around in as tight a turn as would otherwise be possible. In these cases a lower power-loading, lower span-loading, or a reduction in drag will all serve to reduce turn radius.

Note 5: If the increase in gross weight is all in fuel, range will increase roughly in proportion to the amount of fuel added. If not, a slight reduction in range will result.

Note 6: A reduction in wing-loading, power-loading, or span-loading in this table is equivalent to an increase in wing area, power, or wing span respectively.


- - - - -


Hope this proves useful and/or interesting.

faustnik
03-23-2005, 05:02 PM
Yes, it is interesting for those of us who are not "real pilots". Thanks Blutarski! http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

Buzzsaw-
03-23-2005, 06:18 PM
Salute Blutarski

Excellent contribution to the board.

If a lot of those who posted here bothered to read these notes first, they would realize they have no reason to question Oleg's modelling.

HerrGraf
03-23-2005, 07:05 PM
Yes indeed. A very fine and factual post that quite a few members should read. Nice when someone posts usefull information for all.

WWMaxGunz
03-23-2005, 09:07 PM
Very excellent! Shows me that some people knew more about compression long before the end
of the war than I had thought. Airflow over the body of the plane will always be faster
than the plane is moving, and the props really get it sooner the faster they turn... what
he says about shock waves over some portion affecting drag massively, the compression from
the props in high speed like dives is I wonder affecting airflow over the plane behind?

I remember when Oleg had replied to someone posting performance for a 190 variant to show
that it was undermodelled and he pointed out that the data posted was for that 190 with a
different prop than in the sim. IIRC, the prop modelled was best at different altitude.

it all makes me wonder how many fansites take best specs of actually different planes and
makes one chart of them or just one story of "it could do this" and "it could do that".
The same site can even have impressive sources and all "facts" and yet create a false
picture.

Thanks for taking the time to post that, LB!

EDIT: Almost forgot! Was that written or published in 1942? Or both? And how long all
that was known for to get written and published at all. The books generally lag behind
the knowledge. Given the title, it was not for the guys at the cutting edge, was it?

stathem
03-24-2005, 02:41 AM
Bump, excellent post. ty.

I often wonder what a single engined (piston) fighter would look like if someone sat down with today's CFD, materials, and engine technologies and designed one from scratch.

Blutarski2004
03-24-2005, 07:21 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by WWMaxGunz:
Very excellent! Shows me that some people knew more about compression long before the end
of the war than I had thought. Airflow over the body of the plane will always be faster
than the plane is moving, and the props really get it sooner the faster they turn... what
he says about shock waves over some portion affecting drag massively, the compression from
the props in high speed like dives is I wonder affecting airflow over the plane behind?

I remember when Oleg had replied to someone posting performance for a 190 variant to show
that it was undermodelled and he pointed out that the data posted was for that 190 with a
different prop than in the sim. IIRC, the prop modelled was best at different altitude.

it all makes me wonder how many fansites take best specs of actually different planes and
makes one chart of them or just one story of "it could do this" and "it could do that".
The same site can even have impressive sources and all "facts" and yet create a false
picture.

Thanks for taking the time to post that, LB!

EDIT: Almost forgot! Was that written or published in 1942? Or both? And how long all
that was known for to get written and published at all. The books generally lag behind
the knowledge. Given the title, it was not for the guys at the cutting edge, was it? <HR></BLOCKQUOTE>



..... Yup, the book was definitely intended for a general non-technical audience. I picked it up because it provided in one place a good basic overview of a very complicated subject. I certainly learned a lot from reading it. The item which caught my eye was the author's comment that the proper meaning of internal ducting design for high altitude operation was not well understood. This is a strong clue to me as to what actually was ailing the P38. When you read Freeman, they kept fiddling with the inter-coolers and radiators - First they had it too cool, then they had it too hot (or vice-versa). Their fixes sound a lot like a trial and error approach. My suspicion is that the P38 design was probably about a half step beyond the existing edge of technology.

The copyright on the book (ist ed.) was 1942. Since the preface was dated 01 Sep 42, the book must have been physically published either at the very end of 42 or early in 43.

WWMaxGunz
03-24-2005, 07:05 PM
Oh. Preface being the last part written, I think (since so many refer to contents later in
the book), then it may have been a vacation project for the guy. I'd like to find a copy
of that one myself!

Yeah, the P-38 was ahead of its' time as it could have benefitted from semiconductor
temperature guages... bimetal strips might have spoiled the airflow inside or taken too
long to read temperature change through the duct metal. And that's if the T/C could be
controlled dynamically to vary temperature. There's one used in Gulfstream III's to
provide cooling, for example, and it really can cool the whole plane down.

karost
03-25-2005, 08:20 AM
Wow... Excellent contribution to the community. http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

in "FIGHTER FACTS AND FALLACIES "

hummm. ....did the book said something about "STALL" characteristic ? some thing like a...
- one G stall speed relation to gravity theory.
- or about vertical altitude lose ( acceleration vertical speed ) relation to horizontal air speed.

it would be very good if anyone have info. and like to share here to light up my eyes .... http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif


by the way , Blutarski2004 ... Thanks. http://forums.ubi.com/images/smilies/11.gif

BigKahuna_GS
03-26-2005, 12:29 PM
S!

Great post Blutarski Thanks http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif !

I read a series of of articals like this in Aviation History magazine several years ago. All aircraft designs are a series of comprimises to achieve an overall goal or specific operational parameter. It also demonstrates how difficult is to design the perfect plane that is capable of doing all things well while factoring in combat radious.

It also shows how important it is to match propeller design to powerplant. The P38 intercooler ducting problem was first to hot then too cold in the ETO. On top of this, the operation of the engines while cruising was incorrect complicating finding a soulution and overall determining what was the problem. Operating the engines at high rpm and low MAP for long distances seperated the TEL in the intake manifold and created heat problems while lowering fuel effieciency. The end result was catistrophic engine failures.

Lucky Lindy on the other hand showed the P38 Pilots in the SWPA to use very low RPM settings and higher MAP. On the whole the P38s in the PTO did not experience the same engine problems even though operating at the same high altitudes. Some P38 groups were tasked with intercepting Dinah recon planes at 40,000ft.

The P38s in the PTO had a very high reliability factor , the majority of engines problems in the ETO were found to be incorrect engine operation and poor fuel quality.


__________________________________________________ ________________________
stathem posted Thu March 24 2005 01:41
Bump, excellent post. ty.
I often wonder what a single engined (piston) fighter would look like if someone sat down with today's CFD, materials, and engine technologies and designed one from scratch.
__________________________________________________ _________________________



I have wished for a long time someone would do this. Could you imagine a WW2 fighter with high tech engine brake horsepower ratings coupled with composite aircraft materials, silicon chip electronics and all incorperated into a sleek NASA wind tunnel design. Now that would be something.

At least you can see how much the engines, props and aerodynamics of the original designes can be improved upon in unlimited air racing
(see below).

___