Having a background as a glider pilot I am thinking about to what extent we are able to reduce drag on our planes, to either increase speed or reduce fuel burn. Maybe we can identify and summarize some easy things which really help reducing drag. However, I’ve not come to it and it seems like near-impossible to nail some particular thing down that might increase speed.
As a very basic example, and as my starting point for this, I played a bit with my cowl flaps and found out, that if I opened my cowl flaps I’ll get about the same speed / engine temp as with cowl flaps closed but ~5% lower power setting. It amounts to ~0.4 Gal/h higher fuel flow with open cowl flaps or about 5 knots difference on same ~52% power setting. Now these big flaps are very obvious. But what are results from other factors?
When there is dirt on the leading edge of the wing this supposedly contributes to about 5% of drag in a glider, which is why wing wipers are used in competition gliders. However, the influence on a glider with the highly effective wing will be much higher. I suppose that the percentage is far less on more draggy and thicker wings, like practically all SEPs tend to have, more like 0.5%. I wasn’t able to notice any difference after a long low-level flight where I had so many insects there that the leading edge was close to being all black. With icing it seems to be very visible but other than that seems to be a non-issue. Other opinions?
Some topics were already discussed here, for instance influence of antennae on speed, or a long discussion about flying on the step where a change in AOA improves speed.
Rigging can be a an issue, however as far as I understand it rathes provokes “user input” on the controls to keep the aircraft in level flight. Because control surfaces should be measured every now and then during annual so that in principal an aircraft should be well rigged to the extent that it flies. So rigging seems to be hardly a drag issue, but rather regarding comfort. I don’t know if this counts to “rigging”, but for example during climb (high power, high AOA, low speed) you normally need quite a lot right rudder input, or use rudder trim, to keep the ball centered. On my Comanche using the rudder during climb to keep the ball centered improves climb rate by about 150fpm, we repeatedly tested it on a climb up to FL150. So this is quite simple and effective.
What about CoG? Moving the CoG back during flight is said to improve speed, because the elevator produces less downforce. However I could not find any real information about the contribution for a SEP. There are voices saying it may be up to 5 knots. I’ll investigate on that on one of my next longer flights and will report. Having three rows of seats I can play around with that.
Before my last longer flight I put a broad white tape on the corner of the wing-fuselage-transition. This is very common among gliders to tape just anything, there are good products available. It’s a very flexible tape, so it can even smooth out some “sharp edges”. It’s put in 5 minutes. However, I really can’t tell if any improvement occured, because for this you need identical meteorological conditions.
A huge amount of drag is produced for engine cooling, as I recently learned. Baffles and baffle seals seem to be worth checking in order to reduce amount of engine power needed for cooling. As this is wasted engine energy, I would call it drag.
What about door seals? Would bad door seals have an influence on drag?
My Warrior has a few speed mods (wing fairings, gap seals and somewhat flamboyant wing droop tips), but in a recent check with @Graham this only produced around 3-4 KTAS above book.
The speed mods for the Mooney 20/21 early series appear to be effective, and in effect the 201 is the result.
Another factor that we usually disregard when it comes to cruise speed is aircraft mass which affects induced drag. This has much less effect on light aircraft than on airlines as airliners typically cruise near the least drag speed, where any change in induced drag will be quite noticeable, while light aircraft typically cruise at speed way higher than least drag, where instead form drag and parasitic drag dominate.
However, the POH for the Cessna 172S, does give the effect of mass on cruise speed as a 1 knot increase in TAS for every 125-150 lbs decrease in mass.
UdoR wrote:
Having a background as a glider pilot
The biggest difference IMHO, is that GA aircraft have much lower induced drag at typical cruise speeds than gliders, because the wings on your typical GA aircraft are far to big to be efficient, because of the need to meet the stall speed requirements needed without elaborate and heavy high lift devices.
UdoR wrote:
What about CoG? Moving the CoG back during flight is said to improve speed, because the elevator produces less downforce. However I could not find any real information about the contribution for a SEP. There are voices saying it may be up to 5 knots.
I never saw any difference and it may well be that the increased downforce on the tail plane increase the co efficient of lift on the main wing. It is well known on a biplane that the staggering of the wings (e.g. beech staggerwing) makes a big difference to the lift and stall characteristics of the wings. Canard wing configurations definite interact.
UdoR wrote:
When there is dirt on the leading edge of the wing this supposedly contributes to about 5% of drag in a glider, which is why wing wipers are used in competition gliders. However, the influence on a glider with the highly effective wing will be much higher. I suppose that the percentage is far less on more draggy and thicker wings, like practically all SEPs tend to have, more like 0.5%. I wasn’t able to notice any difference after a long low-level flight where I had so many insects there that the leading edge was close to being all black. With icing it seems to be very visible but other than that seems to be a non-issue. Other opinions?
I agree, it’s probably related to the small cross-section & thin wings of gliders, at efficient speeds that would make more difference?
In SEP, the wings are usually thick and draggy: 75% power most of the drag is explained by cruise speeds rather than bugs 
however,
- Some efficient airfoils will complain: DA40 & DA42 comes to my mind, they hate rime ice and bugs from grass, even rain on takeoff !
- Some draggy airfoils will not notice: PA18 & C182 comes to my mind, they are not worth cleaning !
The other thing,
- In gliders, the numbers are pretty standard: speed, sink, weight (and thermals strength) and the effectiveness of cleaning the wings and wing wipers can be checked during competitions 
- In SEP there is more variability due to engine & propeller performance, so they leave more room to speculation & confirmation bias and they are rarely checked, all you have are claims in pilots forums
Wash and wax it.
I have no idea of the effect on say my 182. But I do remember a Ryanair flight where I paid for “premium service”, i.e. a pre-reserved seat. I was all alone in row 2, with half a dozen empty rows behind me and the rest of the plane packed full. When I asked if I could move forward to row 1, they told me it was absolutely forbidden because it would upset the balance of the plane. So I think Michael O’Leary thinks it makes a difference.
Ibra wrote:
In SEP there is more variability due to engine & propeller performance, so they leave more room to speculation
Yah, that’s a bit frustrating. It’s near-impossible to really check whether some amendment really changed drag. Or to be more precise, it’s a lot of testing involved if you wanted to obtain precise results. For example, the effect of prop RPM on speed can be tested within a single flight.
Airborne_Again wrote:
Another factor that we usually disregard when it comes to cruise speed is aircraft mass which affects induced drag
That’s true, but I did not point that out because mostly you can’t do anything about it, other than taking only the fuel aboard you really need and leaving all the stuff at home you prob99 won’t need at all. But the effect on cruise speed is in fact huge! Having made long cross-european flights this year with very different load settings (Comanche has nearly 600kg payload) in the beginning I was puzzling to why on some occasions the plane was so much slower, this can make up maybe up to about 7 knots in difference (~4%) or so. I haven’t nailed it down yet to numbers. But I noted that towards the end of the flight when 200kg of fuel are spent the plane is performing notably better.
So for anyone not aware about it this could be one factor to be put on a “checklist” on how to reduce drag. To just keep the weight as low as possible.
Pilot_DAR wrote:
Wash and wax it.
But other than having said that would it be possible to estimate the effect on drag?
UdoR wrote:
Yah, that’s a bit frustrating. It’s near-impossible to really check whether some amendment really changed drag. Or to be more precise, it’s a lot of testing involved if you wanted to obtain precise results. For example, the effect of prop RPM on speed can be tested within a single flight.
I check the followings on every time when I am heavy near MTOW,
1) At idle power at slow IAS range and confirm ROD values, glide performance as per the book?
2) At full power at slow IAS range and confirm ROC values, excess power performance as per the book?
When I had access to fleet of 3 aircraft (DR400, C172, DA40), I was puzzled to see variations in fast cruise performance TAS/FF even when 1) & 2) are roughly similar…
It will be good to have a way to attribute % of drag for cruise TAS/FF
