Need education on airframe mods

[AWainwright]

I'm looking at buying a used Kitfox as my first KF, then building one as a second airplane. But I'll admit, the more I look, the more confused I get.

Put the flamethrowers away for a second, but I had looked at a meticulous Kitfox 5 with a Subaru on it. With 370 hours on the airframe and a VERY clean install, I think he got one of the "good" NSI engines and gearbox, but of course the airplane is heavy. I've also looked at a Lycoming KitFox 6.

I've been flying since '86, commercial, CFI, ASEL/AMEL/Glider and have my A&P. I have a BS in Aeronautical Engineering, but I'll admit I've used the education more in engineering project management than design and the education is 20 years old. My theory knowledge seems to be causing me more stress than maybe is necessary, so I thought I'd come here to the Forums and get experience and practical knowledge.

So... the 1º forward sweep on airframes that had the heavier engines on them? In theory a forward sweep can increase performance, but can also have poor side effects in out of coordination yaw (retreating wing drag increases, making it a balancing act to stay coordinated), it can also negatively affect stall/spin characteristics. More so if the rigging isn't perfect. There are plenty of gliders out there with negative sweep, but I have no experience with them.

Being an A&P a Lycoming is attractive, and being experimental I can put on electronic ignition, fuel injection, port and polish, etc... bringing the engine ahead 50 years or so in technology. And designed to run 100LL I have 50 hour oil changes without lead issues in a Rotax, Subaru, or other. If I got the Subaru airplane I could always put on a Lycoming with FWF, Engine, prop, cowl if I ran into NSI issues.

So, pick the flame throwers back up and let me have it....

Wing sweep? Issue? Things to look out for? Performance gain? Unforgiving, or don't notice?

Without the conversation of the NSI, that's been discussed on other threads, heavier engine choices like the NSI or Lycoming, who likes the heavier weight w/ forward CG (more stable?) and who doesn't (light is almost always better in aviation, but not always) and why?

One more thing, this first kitfox is either a 3-6 year ownership experience, selling it after building a 7, or I may keep both. Selling it means how does the above affect value and ease of sale?

I've reorganized my hangar so I can get a KF and my 182 in there, and I have funds, I just need to feel comfortable in a purchase decision :-)

Thanks for your experience and wisdom!
-Andrew

[jimcarriere]

Regarding forward wing sweep and stability (and stall and spin characteristics), consider that the recommended c.g. range for flight is 9.5-16" aft of the datum for no sweep vs 9.5-14.75" for 1° forward sweep (note that the datum is the wing leading edge at the wing root).

The source for this is the assembly manual, dated 2003, for my not-yet-finished S7. I don't know if the numbers are any different for the S7's siblings (S5, S6, SS, etc.).

That's about all I can give you for a partial but informed (partially) reply- hope it helps

[AWainwright]

c.g. range for flight is 9.5-16" aft of the datum for no sweep vs 9.5-14.75" for 1° forward sweep (note that the datum is the wing leading edge at the wing root).

Jim, thanks. So sweeping the wings forward doesn't actually change the forward (9.5") limit?

[War Eagle]

"One more thing, this first kitfox is either a 3-6 year ownership experience, selling it after building a 7, or I may keep both. Selling it means how does the above affect value and ease of sale?"

From what I have seen in the market place, I have formed the opinion that a Kitfox with Rotax 4 stroke engine (i.e. 912 series or 914) brings the highest resale dollar (everything else being equal) when compared to the other less common engines.

There are just so many of them out there and they have provided a very good track record.

[HighWing]

...but I had looked at a meticulous Kitfox 5 with a Subaru on it. With 370 hours on the airframe and a VERY clean install, I think he got one of the "good" NSI engines and gearbox, but of course the airplane is heavy. I've also looked at a Lycoming KitFox 6.

What I almost always think of when these questions come up is the number of hours on the airplane. Vague memory tells me that the NSI factory support was discontinued back in the mid to late 90s. 370 hour on a fifteen year old airplane means to me that it was not particularly fun to fly. This might just be my prejudice showing but a friend who started building about the same time I did - 1993, has less than 50 hours on his NSI Model IV to this day - and he has been trying to sell it for almost forever. Then again, I am in the middle of a long email conversation with a buddy from the original Kitfox list days and he is still flying his NSI powered Model V with 500 hours. I also have friends who have a thousand or more hours on Rotax powered Kitfoxes and love them.

So... the 1º forward sweep on airframes that had the heavier engines on them? In theory a forward sweep can increase performance, but can also have poor side effects in out of coordination yaw (retreating wing drag increases, making it a balancing act to stay coordinated), it can also negatively affect stall/spin characteristics. More so if the rigging isn't perfect.

I also like to check the history. I have no personal experience with the wing sweep except to say that issues regarding wing sweep on forums such as this are non existent and the sweep has been around for years.

One more thing, this first kitfox is either a 3-6 year ownership experience, selling it after building a 7, or I may keep both. Selling it means how does the above affect value and ease of sale?

No opinion really except to say that the same questions you are having will be in the mind of any perspective buyer when the time comes to sell. I would feel pretty comfortable thinking that with the Rotax engine you will most likely be pretty close to getting your investment back on resale in that period of time.
Lowell

[jtpitkin06]

The following is an excerpt from a communication to John McBean in 2011. John confirmed the factory test airplane with the Rotec Radial and later the Lycoming O-233 does not have forward wing sweep.

Some of the communication is edited for clarity and in the interest of brevity.

Kitfox Model 7 Super Sport – Wing Sweep Considerations

The Kitfox Super Sport Assembly Manual recommends building the aircraft with forward wing sweep when using engines heavier than the Rotax 912. While the intent of forward sweep is to offset possible forward CG conditions with heavier engines, sweeping the wings forward may not be necessary.

For the purpose of predicting weight and balance with different powerplants, we ran calculations for engines weighing 40 lbs and 75 lbs more than a Rotax. We used identical loadings shown in the samples provided by Kitfox LLC for most adverse forward and most adverse aft CG. (Section M, Weight and Balance, page 88, of the Kitfox Super Sport Assembly Manual)

Most Adverse Aft CG

The Kitfox LLC sample loading for most adverse aft CG easily reaches the aft limit of 16.0 with two passengers , full fuel and 110 lbs of cargo. This is not an unusual loading for two people on a weekend trip. [Exhibit 1]

We predict the aircraft utility will improve with heavier engines; in particular, the usability of the cargo compartment.

With a 40 pound heavier engine, we can load the aircraft with full fuel, 2 passengers, and maximum cargo of 150 pounds. The aircraft remains within the CG envelope. In this case, a heavier engine allows full utilization of the cargo compartment. [Exhibit 2 and 3]

Most Adverse Forward CG

Next we look at the sample loading as provided by Kitfox LLC for most adverse forward CG with a Rotax engine installation. In this example the aircraft has a single pilot, minimum fuel and no cargo. It shows a CG at 12.1 inches. This figure is well within the forward CG limit of 9.5 inches. [Exhibit 4]

If we install a 40 pound heavier engine the aircraft CG moves forward to 10.26 and is still within the forward CG limit of 9.5 inches. [Exhibit 5]

Test for Extreme Engine Weight Increase

In the final two conditions we assume 75 pounds additional engine weight. A simple relocation of accessories keeps the aircraft within CG limits. For example, the ship’s battery is typically located near station -6.0 under the glare shield, or in some installations, further forward in the engine compartment. Relocating a 25 pound battery behind the seat keeps the aircraft within CG limits. [Exhibits 6 and 7]

Wing Sweep Not Required

It should be noted that none of the examples require wing sweep to keep the aircraft within the CG envelope. We find, not only is forward sweep not required; but, there are several advantages to heavier engines installed without forward sweep. Specifically:

Increased stability in pitch.
Greater utility of the cargo compartment.
Operator is less likely to inadvertently load aircraft beyond aft CG limit.
Extreme forward CG conditions easily corrected with ballast or accessory move.
Less yaw induced rolling moment from forward sweep dihedral effects.
Engine interchanges are possible without re-rigging wings.

Possible CG Envelope Error

We believe there is an error in the CG envelope shown in the Model 7 SS manual for aircraft that elect to use forward swept wings.

Sweeping wings forward one degree moves the mean aerodynamic chord,(MAC), forward by 1.09 inches. Doing so moves the aft center of gravity (CG) limit from 16.0 inches to 14.81 inches. The Kitfox manual conservatively shows the aft limit at 14.75 inches when using forward wing sweep. However, the manual shows the original forward envelope limit of 9.5 inches unchanged when using forward sweep. We believe this is an error. Aerodynamically, if MAC moves forward as a result of relocating the wings, the entire CG envelope moves forward. The new forward limit with forward sweep should be 8.41 inches if all other conditions remain static.

In summary, the use of forward swept wings is probably not necessary for most engine installations up to 75 pounds heavier than the Rotax.


John Pitkin
Greenville, TX

[AWainwright]

John,
Thanks! What great info!

Does anyone know if there was full spin testing completed with the forward swept wing design?

Seem like 0º sweep is better for loading options, and makes sense that stability would be improved with 0º sweep over a 1º sweep.

Is anyone with the forward sweep willing to do a quick flight test and share? In straight and level unaccelerated and coordinated flight, remove your hand from the stick, apply slight right rudder.... does the airplane tend to roll left, neutral, or right? Often a forward sweep design results in reverse dutch roll effect (right rudder rolls left initially, left rudder rolls right initially).

Still most curious about spin testing though.

Thanks again for the very useful info!
Andrew

[HighWing]

Sometimes I am really curious.

Example - Adjustable rudder pedals for the very rare occasion when someone else might fly my airplane. I loaned it out twice in nine years for check rides and both times the pilot(s) used my pedal configuration. Considering, of course, carrying the extra weight of the apparatus for the other 900 hours.

Example, planned forward CG for the times you will want 150 lbs. in the baggage sack. During the Arab oil embargo years ago, the Airline in the family loaded every flight to max aft CG for fuel economy. This is one reason I put five lbs. of ballast in the tail due to a -within the envelope but never-the-less - near the forward limit CG. I didn't want to make every solo flight an excessively trimmed up elevator flight

In other words, are we willing to compromise fuel economy on every flight for the ability to carry max baggage or the chance someone with longer legs might want to fly my airplane. 150 lbs. is a "ton" of baggage. Much more than my wife and I carry when flying commercially for a week away from home - I just asked, our last across the country flight we had likely 110 lbs including carry on.

I'm not trying to suggest there is only one way to do things or that one person's preference is necessarily bad, but only to suggest that every decision we make for a desired result, will likely have unintended consequences as well. And those should be brought into the thought process. An excellent example of this is the heavy engine leading to wing sweep leading to possible stability issues.
Lowell

[redbowen]

I have a series 5 with a lycoming O-290D and the wing is swept. It's flight characteristics are outstanding. Stalls are straight ahead with very little wing drop even when uncoordinated. I would bet you would have a very hard time noticing the difference between swept or not.
As far as value, it is true that others will have the same concerns as you about the heavier engines, but there will be others who share your desire to have a certified aircraft engine. Nothing against the Rotax, but at the time I was shopping, the Rotax was still a bad word. It is now, of course, a proven platform. The main reason I took over the project of my series 5 is because it had a lycoming. I fly with a friend who has a series 5 with the Rotax and I can say we both have a blast. Don't over think it. Bottom line you will love the kitfox as long as the engine is running regardless of the brand.

[kmach]

Hi Andrew,

I can only talk from my personal view and experience, when I was looking for a Kitfox I too found some with the subaru engines and they were heavy. with not enough useful load for my liking. and then there was what to do for , and who to go to ,for support if required.

I ended up setting parameters that I wanted in my kitfox purchase and when one came up for sale with all I desired, it was very easy to make the purchase decision !

Of course then you have to change some things anyways! to improve or maximize!