Drag and altitude

[Romani]

A question for the flight experts here. Modern airliners fly very high because jets are similar to rockets and at high speeds drag is a very important consideration ant at nearly stratospheric altitudes the air is so thin that drag is much reduced and fuel consumption too.

Back to WWI I've been reading about German high altitude recon 2-seaters, and how in some cases they were able to keep a speed of about 100 mph at 20,000 ft making them hard to catch by , in other circumstances, faster and more aerodynamic single seater fighters (if only because they are smaller) and I'm wondering, conventional wisdom is that Great War biplanes are drag monsters with the extra wing, fabric covering, all those struts, bracing wires, exposed cylinders, blunt nose radiators... etc. And that is true for most altitudes, but I wonder if at the top altitudes, specially in late war, say 15,000 to 20,000 ft (5.000 to 6.000 meters) airplanes are actually reaping a unexpected side benefit from the thinner air decreasing the relative importance of the drag factor?

I am sure there must be a formula for this!

[Dan_San_Abbott]

Romani:
In order for these machines to fly at high altitudes, they had to have an engine that could provide the power at high altitudes. The engine that accomplished this was the 260 Ps Maybach MbIVa. The Rumpler RuMb, later the Ru.C.VII could reach 7300 meters (23944 ft). The early Ru.C.IV with the 260 Ps Mercedes D.IVa was also a high flyer in early 1917 attaining 6800 meters, (22304 ft.) The Rumpler family of high flyers, derived from the Ru.C.IV, were a cleanly aerodynamic design with a low draw, high lift air foils, out performed Allied fighters. Had the war continued into 1919, German aircraft would have been performing high altitiude reconnaissance in the 8000meter to 8800 meter altitude. The drag formula is:
Drag = air density in slugs. cu. ft/2 x velocity in feet/second squared x drag coeffiecent x frontal area in square feet= force in pounds.
Blue skies,
Dan-San

[Kacey]

Originally Posted by Dan_San_Abbott

Romani:
In order for these machines to fly at high altitudes, they had to have an engine that could provide the power at high altitudes. The engine that accomplished this was the 260 Ps Maybach MbIVa. The Rumpler RuMb, later the Ru.C.VII could reach 7300 meters (23944 ft). The early Ru.C.IV with the 260 Ps Mercedes D.IVa was also a high flyer in early 1917 attaining 6800 meters, (22304 ft.) The Rumpler family of high flyers, derived from the Ru.C.IV, were a cleanly aerodynamic design with a low draw, high lift air foils, out performed Allied fighters. Had the war continued into 1919, German aircraft would have been performing high altitiude reconnaissance in the 8000meter to 8800 meter altitude. The drag formula is:
Drag = air density in slugs. cu. ft/2 x velocity in feet/second squared x drag coeffiecent x frontal area in square feet= force in pounds.
Blue skies,
Dan-San

Hello Dan,

How have you been? I hope well, or as well as can be expected.

The formula you gave is correct for an object like a skydiver, parachutist or falling body shape, automobile, bicycle ... etc.

I believe for an aerofoil/airfoil the correct reference area is the square of the chord. For an aeroplane the correct reference area is the wing area.

Respectfully Submitted,

KC

[R Pope]

High flyers need light wing loading. For a fighter to have good altitude performance it would need bigger wings, which would handicap it at lower heights. At altitude, a higher angle of attack is necessary to maintain level flight, and drag increases accordingly.

[Romani]

Thank you for all the answers, I have the suspicion that above 15,000 ft the rules of the game change. Things as the low drag advantage of RAF wire in wing bracing of British planes or the opposite, the high drag of German airplanes wire bracing or the thick wings of the Fokker DVII don't matter as much for better or for worse, engine performance and wing lift being the main factors.

One follow up question, does a high pitch propeller optimized for climbing like in most German airplanes gives some advantage once you are in thin air, or only when climbing?

[YavorD]

What really matters is overall lift-to-drag ratio (L/D, or CL/CD).
There are further details, of course.

• For high speed optimized aeroplane one may choose an airfoil with a best lift-to-drag ratio at lower values of lift coefficient, that is at lower wing angle of attack.
• For high altitude flight, as well as for better manoeuvrability, it is preferable to use an airfoil with best lift-to-drag ratio at higher value of lift coefficient, that is at higher angle of attack. In this case maximum L/D ratio is shifted right on the graph of (L/D) versus alfa (angle of attack). Low level dash speed will be not too good but high altitude performance will be significantly improved, rate of climb and take-off figures too.

Regards,
Yavor