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Thorp T-18 Frequently Asked Questions

1. Can the A/C be flown with the canopy open?

In short no, the negative pressure at the trailing edge of the canopy forces it shut in flight. It is possible to “crack” the canopy open a few inches but not recommended.

2. Gravity feed vs. boost pumps?

The location of the Thorp’s main tank allows a very simple gravity feed system. If the fuel system has been installed correctly and the following “rules” have been followed boost pumps are not necessary.

Gravity Feed “Rules”

No sharp turns i.e. avoid 90 deg fittings, stick with 45 deg (AN823) or straight (AN816)
3/8 lines throughout, no reductions or increase in hose size. No bushings, reducers etc, all components should accept a #6 pipe thread or a #6 flared tube.

#6  fittings only

No check valves, fuel flow transducers etc.

K.I.S.S.! The system should follow as direct a path from the tank to the carburetor as possible no sharp turns, loops etc.

Carefully size the lines, hoses such that they have the necessary flex component but at the same time have no excess length, as direct a path as possible.

3. Options for “extra” fuel, aux tanks/wing tanks?

Wet Wings (either inboard or outboard)… (Outboard if you have a "C" wing) 24 or 32 gallons respectively (or less).

Baggage compartment tank (check the old NL's), usually around 10-15 gallons.

Tom Hunters fuel pods (I have a set, they work great) either 12 or 16 gallons depending on the size.

Portable pax seat tank

Some have made under the seat tanks, not much fuel can be stored there.

4. Can the center tunnel be removed to make more room?

Yes, there is a popular modification that runs the rudder cables out board rather than down the center. This allows the removal of the forward tunnel and a reduced size of the center tunnel.

5. Electric trim options?

Yes! Electric trim is quite popular.  The 12V Grainger motor 2L009 is the leading choice for pitch trim. Most use a Ray Allen servo mounted in the Aileron for roll trim.

6. Can electric flaps be installed?

Yes,  the 1 x 2" Stroke 150lb Force Linear Actuator (FA-150-S-12-2") from Firgelli Automations makes an excellent flap motor.

7. Is a Lycoming engine the only option for a power plant?

The Thorp was designed around 4 cylinder parallel valve Lycoming powerplants. Some have installed angel valve IO-360s, Continental 4 cylinder, Blanton V6 and Subaru auto conversions though although it is not recommended.

8. Can I get plans for the S-18 or the T-18?

S-18 Kits and plans are available from Thorp Central (www.thorpcentral.com).

T-18 Plans are available from  (to be announced early 2018).

9. Are there local builders I can talk to?  Thorp list Ambassadors?

Sure, try the “Thorp Ambassadors” link.

10. Can I get my PPL in a Thorp?

It has been done but the Thorp is definitely not a trainer. The best course would be to get one’s PPL in a proper training aircraft and transition to the Thorp later.

11. If I buy a Thorp are there any instructors to teach me how to fly it? Is there a list of instructors?

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12. How much will it cost to build a Thorp?

Depending on your scrounging abilities and equipment requirements Thorps can be built for as little as 30K in 2011 dollars. Pretty amazing considering once complete you’ll have and airplane that outperforms “Kit Planes” costing four times that much!

13. Why should I buy a Thorp vs. an RV?

This is a tough one; it’s really to each his/her own. Most Thorp drivers are pretty passionate about their airplanes. Here's a quote from a fellow T-18 owner who also built an RV-7A.

"I will attempt to give an unbiased opinion on the T/S-18 and the RV series.

I have just returned from a breakfast flight in my T-18 with S-18 wings.  As always, it lept into the air with two aboard and promply cruise climbed to 2,000 agl at about 1,000 ft/min.  We leveled off and with 24 squared power, indicated 187 mph.  After a fine airport breakfast on the seaside in Venice, Florida, proceded back to LAL.  I decided to push it up at 1000 ft msl and got just over 200 mph (two aboard, hot day, full internal fuel).  I have easily broken the red-line of 214 mph straight and level (autopilot on a cool day, solo).

With a roll rate of 180 degrees per second, light aerobatics are a joy in the Thorp.  While not a great soft field plane, you can get it in and out of a short sod strip without too much pucker factor.

Contrasted with the RV series, you will find the RVs have about 120 degree per second roll rate with a decided ratcheting effect with full deflection.  The RVs must be flown all the time as they exhibit neutral ailron feel much like a helicopter.  Some are better than others depending on the trim spring tension.  The Thorp just sits there like a 40# robin.

As far as speed is concerned, they are about even with a lot depending on weight and trim.  The tandem RVs are a bit faster due less flat plate area, given the same power and prop selection.

Pitch moments in the RVs can vary widely with CG.  The RV-6 is particularly pitch sensitive.  The RV-7 is more mild mannered due to a longer tail arm and the -8 flies well if you mind the balance with two aboard.  It is a totally different plane with two aboard.  The -9 is again, a different plane as it was designed with a docile airfoil.

The Thorp has the edge in ground handling as you can easily see over the nose in the three point attitude, which you can't in the taildragger RVs.  the nose roller RVs are OK on the ground except for the excessive braking required to steer, especially in a cross wind.  Takeoff in the nose rollers, particularly in the RV-7 can be tricky as the ground attitude yields a negative angle of attack.  This causes the nose to pop over center at lift-off and can cause a high pitch up at lift off if you are not ready.  Most of us use full aft stick from the beginning of the take off roll blending it out as the tail becomes effective.

The RVs have more room and more fuel (you need to switch tanks as you burn off).  The T/S-18 has more baggage capacity and a more tolerant CG range.

In the final analysis, I have owned a T-18 and an RV-7.  I still have the T-18."

14. Is a Thorp difficult to fly?

The Thorp is an honest airplane. It is not “twitchy”, “sensitive” or in any way shape or form a difficult airplane to fly. It is however first and foremost a taildragger and like all taildraggers must be flown all the way to the hangar and second a high performance airplane that must be treated as such, “low and slow” is a situation no Thorp driver should find himself in, period.

15. If I build a Thorp is there a materials list?

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16. I am a big/tall guy, how will I know if I fit?

Thorps are not “big” airplanes but can be configured for big and or tall pilots. Many of our fellow T-18 pilots are over 6ft and 200lbs. That being said most Thorps are built to suit the original builder. If you are planning on purchasing a used T-18, make you try in on first!

17.  How wide is the A/C? (Cockpit) S-18/T-18.

The T-18 is 38” wide at the shoulders; The S-18 (T-18CW) is 40” wide at the shoulders.

18. How wide is the S-18 on a trailer?  Is it Highway legal?  Are there any S-18’s now being kept at home?

The S-18 (or T-18 with the C wing) is street legal with the wings in the folded position. Several T-18/S-18 owners keep their airplanes at home and save many $$ on hangar rent!

19. What kind of tools do I need to build a Thorp?

The Thorp was designed to be built with common sheet metal working tools.  Any of the starter kits from Aircraft Spruce, Avery Tools or ATS should be more than enough.

20. How long to build a Thorp vs. an RV?

T-18’s have been scratch built in as little time as 6 months. However the vast majority have taken longer. On average 5 years of continuous part time work seems to be a good number.

21. Why can I get a Thorp for ½ the cost of an RV? Is the RV that much better?

This is a tough question. Used T-18’s are sold for way below what the market should demand. Most T-18’s are not equipped like comparable RV’s on the market today. Another reason is support, for many years Thorps have all but been forgotten and as a result many believe support does not exist for the design. This organization and website hope to put an end to that!

22. Are there any Thorp Fly-ins where I can see a Thorp?

The Thorp group is well represented at Sun n Fun in the Spring and well as Oshkosh Airventure in the summer. In addition we have at least two “Thorp only” events in the Spring and Fall. The Fall gathering is held the second weekend in October at Kentucky Dam State Park (M34). The Spring gathering varies.

23. Where can I get a ride in a Thorp?

This one is easy! Find a Thorp owner and just ask. We’re all very proud of our airplanes!

24. Is the S-18 (folding wing) more difficult to build than the standard T-18?  What are the advantages of the folding wing vs. the standard T-18 wing?  Better performance?  Cost savings for the S-18 (at home) vs. hanger rent for the T-18 at the airport?

The S-18 (T-18C) wing is more complex than the standard T-18 wing. The primary advantage of the “C” wing is aesthetics, trailer ability and a higher G loading. The “C” wing is stressed at +6-3 at 1500lbs vs. the standard wing at 1250lbs. If you plan to keep your aircraft at home the “C” wing can save quite a bit in hangar rent as well!

25. Can you do aerobatics in a T-18/S-18?  What are the G limits?

The Thorp is stressed for aerobatics (+6/-3 G’s at 1250 for the standard wing and 1500 lbs for the “C” wing). In practice though, the design is too clean to be a serious aerobatic aircraft. An inexperienced aerobatic pilot can find himself over VNE and overstressing the aircraft without much warning. In short Rolls, Loops etc are fine, more advanced aerobatics are not advised.

26. Are there parts suppliers for the A/C? S-18/T-18?

Yes, currently Throp Central (www.thorpcentral.com) supplies parts/kits for the S-18 and  (to be announced early 2018) supplies parts for the T-18.

27. If I want to buy a Thorp is there someone (T-18 guy) that can inspect it for me before I buy it?

Check the “Thorp Ambassadors” link to the left; if you do not see someone in your area you can use the “Thorp Buyers Guide” link as a guide.

28. What should I expect to pay for a good “used” Thorp?

Thorps vary in price mostly based on engine size.  Generally Thorp prices vary based on first engine size (0-290s at the bottom, 0-320s in the middle and 0-360s at the high end), then equipment (avioncs, extra fuel, CS prop) and finally a Wide Body or "C" wing can add a little as well.

29. Is there an inspection list that I can use for a Thorp that I am thinking of buying?

Sure try the “Thorp Buyers Guide” to the left.

30. Can the Thorp be built to LSA weight specifications?


31. Can the T-18 wing be removed to make it portable?

Yes the T-18 wing was designed to be removed easily.

32. Can you install an Auto Pilot in the T-18?

Yes many have.

33. How much can a T-18/S-18 carry?  Fuel, pax, bags?

The “standard” Thorp is capable of hauling 2 adults 80lbs baggage and full fuel.