measuring drag in flight

[Greedy]

I am plodding through a aeronautical engineering self education process through access to a technical library. Being a pilot Im starting from a low base.Im hoping someone here can advise me.
I have seen many mathmatical calculations of drag in cruise flight but is there a way to actually measure it in test flying?
Perhaps sensors in the hub of a test propeller or something of the sort?
I have seen pictures of early German jet and rocket aircraft being towed for test purposes. Did they measure the tension in the tow line?

[Genghis the Engineer]

In level flight, if you know thrust, it's the same value. If you tow something in level flight, or strain gauge all the engine mounts, this can do it.

In gliding flight, if you can measure speed and descent rate (both fairly easy) you can calculate drag, since work done is rate of descent x mass x g, and it is also drag x TAS, so drag = Rod*M*g/TAS

(All in consistent scientific units of-course, most usually nowadays, SI, otherwise known as kg.m.S).

You might try this book which many universities now use for the first couple of years of an aero-eng degree.

In reality, most aeroplanes will have a calculated drag value in use, based upon some combination of empirical calculations, CFD, and wind tunnel testing. There's an organisation called ESDU who are probably the main supplier of drag calculation methods, and most aeronautical engineering students will spend a great deal of "quality" time with ESDU data sheets.

G

[George Koumis]

If this formula is correct then during straight & level flight, where RoD=0, this formula would yield Drag=0 (!!!)

Please accept my doubts about what you say...

george

[Genghis the Engineer]

I said GLIDING flight, so zero engine thrust. Straight and level flight (in still air at constant speed) needs thrust.

G

[Greedy]

Thanks for the replies and the recommendation of the John Anderson Jr book.
I have read a number of his other books- History of Aerodynamics , and another on the history of aircraft technology. He is a very readable author for those without formal training or strong calculus skills. I will be on the lookout for it.
It has seemed to me that the "book" figures of the aircraft I have flown(Saab 340 and CRJ 200 being the largest) have been a little on the optimistic side. Could this be attributed to the theoretical computational methods ?

[Genghis the Engineer]

More likely that the book figures are based upon a perfectly flown, brand new test aeroplane, with brand new engines.

Estimated drag is used for design work, but ultimately book performance figures are based on flight test results.

G

[asmccuk]

More than 50 years ago I remember participating in measured descent performance tests on the Twin Pioneer over Prestwick, and later on the 748 over Woodford, with all engines shut down/feathered as a means of confirming the airframe drag. I believe it was part of the continual discussion/argument between airframe manufacturer and engine manufacturer as to whose drag or power calculations were valid!

The internal quietness without engines was amazing, but so were the creaking noises from the airframe!

Fascinating times!

[D120A]

As a former Twin Pin operator I am fascinated - what was the answer, in lb?

I recall doing rate of descent measurements in a Dove on the Flight Test course at Cranfield in the 60s (only one engine shut down of course). Always remembered the answer - at the datum speed and height, the drag was 630lb!

Happy days indeed.:ok:

[Algy]

asmccuk,

amazing stuff - tell us more....

[Greedy]

Thanks especially to Genghis and asmccuk.
Having thought about Genghis's suggestion of the gliding lift drag ratio I realised that that a fairly acccurate zero thrust would have to be set on the propellers otherwise you could be measuring some "disking" or alternatively residual thrust. Feathering both (all) is obviously the answer for the supremely confident !
Strain guages on the engine mountings would be my prefered option.

[Rivet gun]

I'm no expert and can only go on what I've read. This book

Amazon.com: Performance of Light Aircraft (Aiaa Education Series) (9781563473302): John T. Lowry: Books


suggests a method involving glide tests. The idea is that in order to construct a drag curve you need various things you can measure (mass, wing area, aspect ratio) and two "unknowns" which are the parasite drag coeficient, Cd0, and the inducued drag efficiency factor. If you knew these you could construct the traditional quadratic drag curve and hence predict the best glide angle and the speed at which this occurs.

If you do some glide tests over a range of speeds, you should be able to determine the best glide speed and corresponding best glide angle by trial and error. You can then work backwords to determine Cd0 and the efficiency factor.

Of course this still leaves the problem of shutting down the engine(s) and accounting for any residual prop drag. It also assumes that the traditional drag curve is a good model.

[asmccuk]

Responses for D120A post 8, and Algy post 9;

Sorry folks it is all so long ago that I do not remember the drag results on the Twin Pioneer or the 748. On both aeroplanes this was during a busy development period doing drag assessments plus engine out climb performance checks and engine cooling compliance checks.

Before the 748 tests in December 1960, the preparation included practice 'deadstick' landings with one engine feathered and one at a nominal zero thrust setting. On the day we did four measured descents, so we managed to unfeather and restart each time without drama.

asmccuk

[somishah]

As a former operator of Twin Pin fascinates me - What was the response that the nail?

I remember measuring speed (http://city-news.org) of descent in a dove in flight test at Cranfield in the 60s (one engine stopped of course). Remember always the answer - the speed and height of reference, the characteristics were £ 630!

[barit1]

Greedy:Strain guages on the engine mountings would be my prefered option.

Depends, of course, on how the cowl is attached. If the cowl hangs on the engine, its drag (or thrust) will be included in the force picked up by strain gauges. But if the cowl hangs directly on the airframe, with the engine "floating" inside, then the gauge thrust = true total drag.

[cwatters]

I suppose artificially increasing drag slightly by a known amount (tow something with a strain guage in the rope?) and noting the change in velocity might allow it to be calculated. Would have to be something small so that that the velocity change wasn't too great otherwise you can't assume that engine power and prop efficiency are the same at both speeds.

Power1 = drag x velocity1
Power2 = (drag + extra drag) x velocity2

Assume constant power so Power1 = Power 2

drag x velocity1 = (drag + extra drag) x velocity2

rearrange to give

Drag = Extra Drag x velocity2 / (velocity1-velocity2)

Main problem with this approach is that you want (velocity1-velocity2) to be large to minimise error in the calculation but yet small so that engine and prop power is constant or you can't equate them.

Some practical issue I suspect.

[Greedy]

cwatters,
I think there is still a problem in your final re arrangement of the formula.
It still contains two unknowns- Drag and the incremental increase in drag with a towed object.
I suspect it could be solved using a differential equation however my maths is not good enough to go down this path.
Changing the approach to something I read on the calculation of drag on a car you could:
1 set up level flight at some speed of interest
2 Quickly and accurately set zero thrust. Keep the aeroplane in level flight
3 Time the change in speed over a small speed range to compute acceleration.
4 Calculate Drag Force from F= m a

The problems with this are that it assumes deceleration is linear (which it is not ) and accurately setting zero thrust.

Greedy

[Mad (Flt) Scientist]

The problems with this are that it assumes ... accurately setting zero thrust.

If you were able to accurately set zero thrust, you'd be able to accurate also know the thrust for level flight, and simply fly at a trimmed speed.

In fact, that's the more common approach - to ASSUME that thrust is known, and use that to define drag. After all, in terms of usable performance numbers, what you want is the condition with thrust, so even if you could somehow define drag directly, you'd still need to combine that with thrust data to give some kind of useful performance data to the crew.

[Greedy]

Mad (Flt) scientist,
Yes understand that drag determination can be achieved by calculating thrust delivered by a prop in level flight. I believe formulas exist that have been established from empirical data for both turboprop and turbojet aircraft.

In attempting to do a quick (and rough) calculation in the aircraft I currently fly could I use:
percentage of maximum torque x percentage of maximum RPM x Rated power (at sea level) to establish the power delivered to the prop at some cruise condition above sea level ?

Then multiply the answer by a assumed prop efficiency of .8 to determine power converted to thrust by the prop?
eg .7 max torque x .8 max rpm x 4000 eshp at sea level = 2240 hp delivered

2240 x .8 prop efficiency = 1792 Hp converted to forward speed.

Would this approach hold for all cruise altitudes?