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What contributes to which amount of drag ? / speed improvements

I think you got it mostly figured. Aside from the obvious rigging, clean leading edges and slightly tail heavy CG (reduces the drag caused by trim), I’d add the following to the list before starting to even consider any speed mods.

1. cooling drag. The baffles have to be tight and every hole on the top of the engine should be sealed so there are no leaks. Leaks in the cowling area apparently cause the largest drag especially in our older design airplanes with large cowling holes and lots of leaks. I think a lot of speed can be gained by simply working on making things tight.

2. Sharp edges. All parts where there is a sudden sharp turn like wing roots, cowling exits, antenna roots etc. if once could turn all sharp edges into smooth curves this would help a lot.

3. Stuff that are round and exposed to clean air flow. A big example is exhaust pipe but not much can be done there. But even if you can add some fairing to any round shape, tube that is exposed to clean airflow, it also helps.

After this, I’d fist consider flap and aileron gap seals and take it from there

Switzerland

One thing that always bothered me is the invisible plume of exhaust gases blown at a sharp angle to flight direction. I envision this to be a conical tube of steadily increasing cross-section that is bent backwards by airspeed and absolutely trashes the airflow on the fuselage downstream. Can anyone point us to some tuft testing?

LSGG, LFEY, Switzerland

By far the biggest thing is cockpit volume. This is why small 2-seat or 1-seat planes go so fast

After that, I think it is a lot of money for small improvements. Flush rivets should make a difference but are expensive to do.

Aerials certainly make a big difference, in the quantity found on a typical IFR plane. It would be great if one could have a composite vertical stabiliser and then one could do what airliners do and hide them in that.

Interesting that restricting the cowling airflow makes a lot of difference. How much difference to max cruise do cowl flaps make?

Administrator
Shoreham EGKA, United Kingdom

Flyingfish wrote:

One thing that always bothered me is the invisible plume of exhaust gases blown at a sharp angle to flight direction.

That was discussed here however without real outcome. I personally think that exhaust design rather addresses engine necessities (and possible CO issues), which both supersede a perfect airflow. In any case, (in EASA land) there are few options to legally amend the exhaust piping, so like cockpit volume Peter pointed out it’s one of the things which more or less has to be accepted. There’s not much to do here.

Germany

Peter wrote:

How much difference to max cruise do cowl flaps make?

Cannot tell that yet. As soon as I’m done checking my baffle seals I might aim for max cruise. So long we typically fly at 150 KTAS and see around 4-5 knots decrease between fully closed and fully open. But it’s complex, because opening the cowl flaps also induces a very slight up tendency so you trim down a bit. Don’t know which contributes the most, cooling airflow, simply drag of the flaps or new trim setting. Opened cowl flaps reduce CHT currently by about 20°F, that’s about what the CHT goes down when reducing power, resulting in the same speed.

Germany

On a C182 G1000 the cowl flaps made ~3 knots difference between open and closed. The school renting it would leave them permanently half open to not worry about the position

Very early in the development of the Spitfire the exhausts were angled almost completely aft, which added the equivalent of 30hp in thrust (3% increase). Someone told me the Powerflow exhaust added 10hp to their Cardinal (5%), but this isn’t really reducing drag.

I did read recently that aileron and flap gap seals only start to make a significant difference above 300kt. I used to fly a Comanche with speed mods which was maybe 5-6kt faster than book speeds, but the definite improvement was in handling.

On an Aquila A210 (100hp with laminar flow wing) a dirty leading edge did decrease cruise by a little, but it badly affected rate of climb.

EGHO-LFQF-KCLW, United Kingdom

Wonder why a Robin DR400 flys faster than a PA28 with the same engine ?

Robin have done most of the stuff mentioned above, very tight cowl seals, small cooling intakes, smooth cooling outlets with the exhaust ducted to perform a cooling Venturi function, aileron gap seals, AOA washout to reduce induces drag and lots of small smooth fairings to reduce drag.

Wonder why a Robin DR400 flys faster than a PA28 with the same engine ?

Is that comparing like for like i.e. same weight, same fuel flow, both having the same good/bad/totally-shagged/missing wheel fairings?

the cowl flaps made ~3 knots difference between open and closed.

That is a good data point because it directly shows the benefit of having less cooling air. Presumably the same is achieved with smaller front holes and better baffling inside.

I did read recently that aileron and flap gap seals only start to make a significant difference above 300kt. I used to fly a Comanche with speed mods which was maybe 5-6kt faster than book speeds, but the definite improvement was in handling.

It seems this varies. Reports by TB owners suggest the gap seals do nothing for the TB20 but do “something” for the TB9/10.

Administrator
Shoreham EGKA, United Kingdom

On a C182 G1000 the cowl flaps made ~3 knots difference between open and closed

This is my experience also on my TR182.

LFMD, France

The Robin DR400 wing is different from the Pa28 wing. That will have more effect than the condition of fairings etc.

Maoraigh
EGPE, United Kingdom
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