We are comparing high-cruise speed and for that high aspect ratio is bad.
OK, I understand that now. Thanks! 
I think high altitude is a possible solution for the Monsun. The supposed 17,000 ft service ceiling would allow it.
Silvaire wrote:
he higher aspect ratio wing is on the slower plane
That makes sense. We are comparing high-cruise speed and for that high aspect ratio is bad.
High aspect ratio gives lower induced drag at low IAS, but has an overall higher parasitic drag.
Where you can see for a given low-lift , low Cl (high IAS) , low AR gives lower drag than high AR
[Edited to use a better-illustrating graph]
Another factor on that one is wing aspect ratio (Monsun AR 6.8 vs RV-7 ’s stubbier 5.2). That may ammount to a 10% wing drag delta in cruise, but still not enough to explain the apparent 30% drag delta.
And not only that but the higher aspect ratio wing is on the slower plane… although if the RV-7s stubby wing could be expected to provide more efficient low AoA cruise it’d be good to learn about it.
The RV-9 has a higher aspect ratio wing than the relatively stubby RV-7 wing, I believe the idea with the RV-9 is to reduce drag and use a smaller engine, while sacrificing aerobatic capability to do so.
Airborne_Again wrote:
Interesting! The study is a bachelor degree thesis from the Royal Institute of Technology in Stockholm
Small world!
Airborne_Again wrote:
it appears they did not take the landing gear into account at all
IF that is the case, then fuselage+gear drag would be a higher proportion, but still not the dominant factor. Fuselage itself is in general 15-30% total parasitic drag, according to other statistical studies, but the point being made that fuselage cross section and shape are a dominant differentiating factor in cruise drag is still independent of the fixed LDG type.
Now if we talk landing gear, that is indeed a big differentiating factor, as you say. Fairing or retracting make a big delta vs fixed unfaired.
The retractable nosegear in the Monsun is thus also an interesting factor adding to @Silvaire ’s puzzle!
Another factor on that one is wing aspect ratio (Monsun AR 6.8 vs RV-7 ’s stubbier 5.2). That may ammount to a 10% wing drag delta in cruise, but still not enough to explain the apparent 30% drag delta.
It must be the addition of all the different items that adds up to that 30%, but I was hoping to find a clear smoking gun!
Antonio wrote:
From this study fulltext01_pdf it seems that, after taking into account the actual wet surfaces of each main parts of the airframe, the fuselage only accounts for 23% of the total parasitic drag:
Interesting! The study is a bachelor degree thesis from the Royal Institute of Technology in Stockholm. (Honestly, as an associate professor who frequently reviews degree papers, I’d say it’s quite a bit on the thin side for a two-person degree thesis, but of course that doesn’t make the contents as such less interesting.)
From glancing through the paper, it appears they did not take the landing gear into account at all. That could well explain the discrepancy between their theoretical model and the flight tests as the landing gear has a major effect on drag.
According to the Archer II POH, cruise speed decreases by 8 knots if the aircraft is flown without wheel fairings. That would correspond to about 8% higher speed with fairings compared to no fairings. In my experience the effect is not quite that large but it is certainly large enough that you can’t ignore it in flight planning.
Yep, he has the original McCauley CS prop.
I should also make clear that if you look around on the net for real world data, 135 kts cruise as quoted by the manufacturer for his top spec 160 HP/retracting nose gear version of the plane is on the high end for the type, 125 kts is more typical but then you get into the exact configuration, IAS versus TAS and power settings etc. 135 kts TAS at altitude cruise is probably a fair number. My friends plane is actually considerably faster, likely because he removed all the antennas, faired in the main landing gear much better and so on… the usual stuff that can be done to an existing plane.
The RV cruise speed and climb data provided by Vans is reliable (as well as being much higher). If you look at the RV some things are obviously much better, the canopy, cowling and wing root fairings are good examples but the RV wing itself appears visually to be worse.
Thanks much for your thoughts I’d really like to understand light aircraft aerodynamic design better.
Silvaire wrote:
Monsun is Airfoil: NACA 64215 at root, NACA 64212 at tipMonsun NACA 64013.5
Hey, well spotted @Silvaire!
Then I guess we are still….Raiders of the lost drag!
Silvaire wrote:
My uneducated guess would have been that the Monsun wing design and wing loading would be better than the RVs at speed, and that the RV wing would be better at high AoA, not the opposite.
But that difference is small…we are looking for a big smoking gun here…as in 30-40% drag big…
Perhaps it is also in the prop? Does your friend fly the original prop?
Silvaire wrote:
I’m no expert in this field, more of a student!
I don’t want to ever stop being one! Everybody sais that was the greatest time of their lives, so why stop it?
Monsun is Airfoil: NACA 64215 at root, NACA 64212 at tip
Monsun NACA 64013.5
@Antonio, I’ve studied more closely what you posted. I think the first quote is correct for the plane based on memory. FWIW the Monsun wing section certainly looks thinner and more symmetrical on the plane than the NACA 64013.5 as plotted in post 33… could that plot be the wrong wing section for the plane?
These are the 64215 (root) and 64212 (tip) sections I found at the link you posted, and the wing section does look visually like this on the plane to me – but I don’t know enough to do anything with the link beyond searching for the airfoil by name.
As @arj1 noted, the RV has 121 sq ft wing area, the Monsun has 110 sq ft and slightly more weight leading to a higher wing loading.
My uneducated guess would have been that the Monsun wing design and wing loading would be better than the RVs at speed, and that the RV wing would be better at high AoA, not the opposite. But I’m not claiming to know anything and I’m no expert in this field, more of a student! It would great to understand the reality of this particular low AoA drag comparison.
If nothing else my friend in Florida would love to know as he’s been tweaking his plane to go faster since the 1970s and is still flying it at age 80. I think it’s probably the fastest of the breed.
Peter wrote:
I was in all personally.
no doubt Peter… but probably not at the same time, nor space, and not reading off the very same instruments for all of them 
I was in all personally.
