From FAR 1: Category:
(1) As used with respect to the certification, ratings, privileges, and limitations of airmen, means a broad classification of aircraft. Examples include: airplane; rotorcraft; glider; and lighter-than-air; and
(2) As used with respect to the certification of aircraft, means a grouping of aircraft based upon intended use or operating limitations. Examples include: transport, normal, utility, acrobatic, limited, restricted, and provisional.
Class
(1) As used with respect to the certification, ratings, privileges, and limitations of airmen, means a classification of aircraft within a category having similar operating characteristics. Examples include: single engine; multiengine; land; water; gyroplane; helicopter; airship; and free balloon
So, I assume you hold a rotorcraft (category), helicopter (class) certificate, right?
If you wish to fly a fixed wing airplane, you will need to be rated for airplane (category), single engine land (class), for example.
From part 61:
§ 61.63 Additional aircraft ratings (other than on an airline transport pilot certificate).
(a) General. To be eligible for an additional aircraft rating to a pilot certificate, for other than an airline transport pilot certificate, an applicant must meet the appropriate requirements of this section for the additional aircraft rating sought.
(b) Additional category rating. An applicant who holds a pilot certificate and applies to add a category rating to that pilot certificate:
(1) Must have received the required training and possess the aeronautical experience prescribed by this part that applies to the pilot certificate for the aircraft category and, if applicable, class rating sought;
(2) Must have an endorsement in his or her logbook or training record from an authorized instructor, and that endorsement must attest that the applicant has been found competent in the aeronautical knowledge areas appropriate to the pilot certificate for the aircraft category and, if applicable, class rating sought;
(3) Must have an endorsement in his or her logbook or training record from an authorized instructor, and that endorsement must attest that the applicant has been found proficient on the areas of operation that are appropriate to the pilot certificate for the aircraft category and, if applicable, class rating sought;
(4) Must pass the required practical test that is appropriate to the pilot certificate for the aircraft category and, if applicable, class rating sought; and
(5) Need not take an additional knowledge test, provided the applicant holds an airplane, rotorcraft, powered-lift, or airship rating at that pilot certificate level.
(c) Additional class rating. Any person who applies for an additional class rating to be added on a pilot certificate:
(1) Must have an endorsement in his or her logbook or training record from an authorized instructor and that endorsement must attest that the applicant has been found competent in the aeronautical knowledge areas appropriate to the pilot certificate for the aircraft class rating sought;
(2) Must have an endorsement in his or her logbook or training record from an authorized instructor, and that endorsement must attest that the applicant has been found proficient in the areas of operation appropriate to the pilot certificate for the aircraft class rating sought;
(3) Must pass the required practical test that is appropriate to the pilot certificate for the aircraft class rating sought;
(4) Need not meet the specified training time requirements prescribed by this part that apply to the pilot certificate for the aircraft class rating sought unless the person holds a lighter-than-air category rating with a balloon class rating and is seeking an airship class rating and
(5) Need not take an additional knowledge test, provided the applicant holds an airplane, rotorcraft, powered-lift, or airship rating at that pilot certificate level.
§ 61.129 Aeronautical experience.
(a) For an airplane single-engine rating. Except as provided in paragraph (i) of this section, a person who applies for a commercial pilot certificate with an airplane category and single-engine class rating must log at least 250 hours of flight time as a pilot that consists of at least:
(1) 100 hours in powered aircraft, of which 50 hours must be in airplanes.
(2) 100 hours of pilot-in-command flight time, which includes at least—
(i) 50 hours in airplanes; and
(ii) 50 hours in cross-country flight of which at least 10 hours must be in airplanes.
(3) 20 hours of training on the areas of operation listed in §61.127(b)(1) of this part that includes at least—
(i) 10 hours of instrument training of which at least 5 hours must be in a single-engine airplane;
(ii) 10 hours of training in an airplane that has a retractable landing gear, flaps, and a controllable pitch propeller, or is turbine-powered, or for an applicant seeking a single-engine seaplane rating, 10 hours of training in a seaplane that has flaps and a controllable pitch propeller;
(iii) One cross-country flight of at least 2 hours in a single-engine airplane in day VFR conditions, consisting of a total straight-line distance of more than 100 nautical miles from the original point of departure;
(iv) One cross-country flight of at least 2 hours in a single-engine airplane in night VFR conditions, consisting of a total straight-line distance of more than 100 nautical miles from the original point of departure; and
(v) 3 hours in a single-engine airplane in preparation for the practical test within the 60-day period preceding the date of the test.
(4) 10 hours of solo flight in a single-engine airplane on the areas of operation listed in §61.127(b)(1) of this part, which includes at least—
(i) One cross-country flight of not less than 300 nautical miles total distance, with landings at a minimum of three points, one of which is a straight-line distance of at least 250 nautical miles from the original departure point. However, if this requirement is being met in Hawaii, the longest segment need only have a straight-line distance of at least 150 nautical miles; and
(ii) 5 hours in night VFR conditions with 10 takeoffs and 10 landings (with each landing involving a flight in the traffic pattern) at an airport with an operating control tower.
According to this, you can count a lot of your helicopter PIC time, but you'll have to have at least 50 hours PIC in a fixed wing airplane, and follow the other specified requirements.
And, this also suggests you'll have to complete the fixed wing cross countrys. There's more to that than navigation, actually.
Also, take a look at the PTS for those tasks you'll have to complete. The key there is the matrix at the front, which shows what tasks must be completed on the practical test if you alread hold certain ratings.
You won't have to do nearly as much flying as someone with no certificates, in any case.
A helicopter of any kind will not substitute for a tailwheel airplane for that endorsement, no more than a tailwheel endorsement would qualify you to land a helicopter. It isnt' the approach that's most critical in a tailwheel aircraft.
MTV