Ramifications of AC 103-7 Appendix 2, stall speed

Hi Dean.

     I dug into my archives and came up with an example for a Kolb Firefly.

     Step 1. Add the weight factors-[empty weight= 254 lbs, FAA pilot weight= 170 lbs, fuel weight= 30 lbs] = 454 lbs.

      Step 2. Divide total wt/wing area, 454/112.9=4.02 lbs/sqft.

      Step 3. Select lift factor that matches the wing profile:

          1.4= Single/double surface with camber less than 7% and all semi-symmetrical airfoils without flaps.

          1.6= Flat bottom, double surface with camber of 7 % or more.

          1.8= Single surface with camber of 7% or more or double surface with flaps up to 50% of the total wingspan.

           2.0= Double surface with flaps more than 50% of the total wingspan.

       Step  4. Use the Stall Speed Graph in Appendix 2.                                                                                                                            The Firefly uses the lift factor of 2.0, so on the 2.0 line the maximum wing loading it can have is 3.92 lbs/sqft. We are too heavy at 4.02, so must either decrease the total weight by 12lbs. ( reduce the empty weight of the aircraft, only use a          3 gallon tank or convince the FAA that you really only weigh 158 lbs.)or increase the wing area by 2.9 square feet                 ( without going over 254 lbs)

Question: "Can I be assured that the companies that sell the plans for aircraft that claim they are legal under FAR 103 have documentation demonstrating Technical Standards Committee Findings that they have demonstrated the power off stall speed does not exceed 24 knots? "

 Answer: No. You are the builder and responsible for meeting the requirements of 103. You can do this in any manner acceptable to the FAA. Use AC 103-7 or by flight test. If you want to be certain you comply send the flight test results to the FAA for review and comment.

This is one of the reasons we need and do not have a turn key UL on the market. Any manufacturer that gets large enough will be challenged and put out of business if they do not comply.

The stall speed requirement of FAR 103 is even more of an issue that the empty weight. It is more commonly ignored and more difficult to prove or disprove.

There are other methods that may be employed to increase CL max. Adding slats or high lift wing tips would likely be among the best solutions. For more high lift suggestions see TWIT (The Wing Is The Thing). Witold Casper claimed to get lift coefficients up to 25. Unbelievable but it does shed light on what may be possible. Watch the birds and you will see what I mean.

Ultralights are where the action is. It is only here that experiment continues without burdensome oversight.

Question: If one designs and builds an Ultralight how does one verify that it meets FAR 103 requirements? Answer: Design calculations followed by flight test. Use the Boeing method. If the design does not meet the predictions, keep modifying and testing until it does. Do not give up. Keep notes of your flight tests and progress if any. You may want to do a stall speed test on every flight. If you present this data to the FAA it will demonstrate that you are serious about complying.

Have you heard the phrase “close enough for government work” and “close only counts in horse shoes and hand grenades”. If you just can’t quiet prove the stall speed limit is met, apply to the FAA for an exemption. The same goes for weight. If enough people followed these guides, we could get appropriate relief and a good nights sleep.

I second Grants response.

  You have only two ways to provide proof that you meet the stall speed requirements... IF you are asked.

1. provide any evidence satisfactory to the inspector.  That's a pretty wide door.  There are no limits to how you do it if the evidence is satisfactory!  Timing gates, data logger, two witnesses with stop watches.  You name it.  The FAA quickly realized the ideal, and the reality, are different.  Timing ultralight planes flying as slow as possible through timing gates suddenly seemed like a bad idea.  Back then the manufacturers asked the same questions and the FAA decided after 103 was published to come up with a paper method which should produce fewer lower altitude stalls.

2.  The charts and tables are optional but not required.  You do not have to comply with them.  They cannot cover all possible designs.  The FAA would prefer you use the tables but if you do not, it's up to them to ask, or not ask, for some other proof of stall speed.   It became the part of 103 everyone knew could not be enforced without a lot of trouble.  In practical terms it's not needed.  Checking for fuel tank size, number of seats, and empty weight takes care of nearly all of the non-compliant vehicles.

If you have a 253 lbs ultralight with 5 gallons that stalls at 30mph just enjoy it and don't worry.

BTW the top speed is also seldom an issue because it is feasible and permitted to install a different prop or change the throttle control to limit power to a 253 lb ultralight so it does not go over 63mph for the testing.  Limiting devices should not be configured in a way that could be defeated by the pilot while in flight.

--Dan

It's my understanding that the charts and graphs are pretty generous compared to the real world. One of the easiest and lightest high lift devices to add to your wing that didn't exist when Part 103 was written would be vortex generators. Has anyone out there used them on a true UL? There have been some significant stall speed reductions on Challengers, Tornadoes and the like.

The truth is that if it looks like a UL and has a single seat the rest of the requirements are pretty much a moot point other than having a 15 gallon fuel tank from what I am hearing.  The FAA is not going to have you demonstrate the max or stall speed or produce a chart reference.   If it has two seats and a large fuel tank you better have your aircraft paper work in order.  Roger

NACA Reseaarch Memorandum RM A50L12  Preliminary Investigation of the Delay of Turbulent Flow Separation by Means of Wedge-Shaped Bodies dated 03 01 1951 Declassified 09 20 1954 compaires lift drag and pitching moment for a basic airfoil and airfoil with wedges and with vanes. Basic airfoil stalled at 1.3 CL and 14 degrees AOA. Wedges and vanes stalled at 20 degrees AOA and reached about 1.9 CL. Wedges were slightly better than vanes in CL and worse in CD but variation was small overall. Drag increase with vanes was minimal, with wedges CD increase was measurable.

You could get above 3.9 psf with the right flaps and/or slats. If you did it just right, you could maybe do it with just a flap. 3.9 at the legal stalling speed is a Cl of about 2.

http://img90.imageshack.us/img90/5545/86393flapsta8.jpg

It's kind of unfortunate that Appendix two doesn't apply to interesting designs:

 The values provided here are for relatively square, rectangular wings; they are not valid for
noticeably swept or tapered wings.

I suppose you could have a wing that was actually square. That might be interesting! I think low AR has some real potential for ultralights.

While I appreciate the fact that you might be able to design a legal ultralight from scratch , i.e. 253.9999 lbs. empty weight, with a 3.9 lb. / sq. ft. wing loading by using high lift devices to meet the stall speed requirement as defined in Appendix 2 of AC 103.7, but that was not the intent of my post.  There are many designs that are sold as "legal" ultralights that clearly don't meet the criteria because they don't have any semblance of a flap or other high lift device and vortex generators are not taken into account in Appendix 2.

For instance the new Belite line has a wing area of about 100 sq. ft.  (I am using Belite for demonstration purposes, there are many other designs such as the Max 103 by JDT, and the Legal Eagle ultralights that have the same problem.)  At least the Belite is one of the few designs with flaperons, so you could actually use the 2.0 curve, but with a wing loading of 4.5 lbs. / sq. ft., using the Appendix 2 formula, there is no way that it will meet the 24 knot criteria of the table.  As the Belite web site says, "FAR 103 COMPLIANCE IS ALWAYS THE RESPONSIBILITY OF THE OPERATOR, NOT BELITE AIRCRAFT." so if the FAA ever questioned you, you would have to do the technical standards review.

It just seems ludicrous to have "Regulations" that are not realistic in light of the commercial enterprises that are operating under them today.  It is clear that FAR 103 was intended for aircraft with wing loadings of less than 3 lbs. / sq. ft. operating with low power 2 cycle engines.  But today the average commercial ultralight airplane has a wing loading in the 4+ lbs. / sq. ft. range and 2 cycle engines are disappearing because of pollution standards, so people are turning to 4 cycle engines.  On top of that the speed limit is unenforceable even if you could measure it accurately.

I am troubled by commercial operations that appear unable to meed the letter, let alone the spirit, of the FARs.  They design ultralight aircraft that would have to go through a Technical Standards Committee review process to demonstrate that their designs can meet the 24 knot stall speed, but they leave it up to the buyer to assume that liability.  They even advertise versions with 50 HP engines that are "limited" to a 38 HP "ground limit", whatever that means, to assure that the aircraft does not exceed the 62 MPH ultralight airspeed limit.  If you are going to all of the trouble and expense to put a 50 HP engine in your aircraft, I assume you intend to use it because it certainly weighs more than a comparable 35 HP model.  How does that satisfy the spirit of FAR 103?

Dean, you are expressing frustrations that have been around since the beginning of  Part 103 or even before. I built my old Mitchell Wing B-10 with a 28 hp Rotax 277 even though the factory recommended a 20 hp Zenoah and it originally flew with a 10 hp Mac. I am your typical homebuilder- take a really neat design and stick a bigger motor in it. If a manufacturer can produce an aircraft that meets Part 103, they shouldn't be held responsible if the buyer of that aircraft changes it in any way that causes it to no longer meet Part 103. In addition, I believe, if a manufacturer is trying to sell you an aircraft that you feel isn't legal- don't buy it. One of the founding principles of Part 103 was Self-Regulation. I just leafed through the premier issue of Ultralight Magazine- Oct. 1981. Very few of the "air vehicles" offered for sale had the conventional "airplane" configeration. There were Pterodactyls, Eagles, Weedhoppers, Mitchell Wings, Quicksilvers, Tomcats, Kasperwings, Lazairs, etc., etc. Today there only a few aircraft that are being sold as ultralights. Part of the reason, I believe, is that the market- mostly private pilots "moving down"- wanted an enclosed cockpit, wheel brakes instead of drag-your-heel brakes, and yes, bigger engines, so that un-airplane looking designs fell out of favor. Unfortunately, it seems to be very difficult to make a conventional looking aircraft meet Part 103. So, the challenge is to design an airframe that meets Part 103, but isn't so weird or different that it can satisfy enough customers to stay in business. Trikes and PPC's have created their own markets, but have also grown well past Part 103. I, personally, believe that a low aspect ratio tractor flying wing would be a viable design- example- Milt Hatfield's "Little Bird'. So, hang in there, stay true to the principle of Self-Regulation and if necessary create your own "air vehicle".

While I agree that the low aspect ratio flying wing is a good approach, you can have a totally compliant, moderately conventional looking aircraft that's significantly lighter than the rules. Even in wood. (Skypup, for instance) With aluminum and maybe $200 or so worth of graphlite, or perhaps just a few struts and wires, I'm sure a larger conventional aircraft would be compliant. Would help if someone did a light 4 stroke with redrive, as has already been done in Europe. I forget the name, but there's one over there with 35 or 40hp based on a Briggs. Souped up, but perhaps they've beefed up the appropriate bits. And that's a lot more power than an efficient ultralight with an efficient prop should need. If your prop was any good, with such an engine you ought to be able to climb at 1200 fpm or something at 500 lbs gross. The industrial engines are made for even more severe service than aircraft engines. (Not the same engine the Luciole uses without a redrive.)

I never said that I wanted to buy a commercial ultralight and modify it so it wouldn't meet Part 103.  I understand that any modifications to the "configuration, components, engine, or propeller arrangement" as outlined in AC 103-7 would be my responsibility.  My question was, if I bought a commercial design that claims to be a legal ultralight, and built it as designed, can I expect that it complies with FAR 103 as claimed, especially the stall speed.  As far as I can tell from AC 103-7, Appendix 2, many current commercial designs do not, and none of them claim that they have performed an accepted Technical Standards Review as defined by AC 103-7 (24) (b) (3) which would provide the operator evidence that the ultralight meets the stall speed limit, since it can't be shown by reference to Appendix 2.

And check on not buying an aircraft that I don't feel is legal.  That was the primary reason for asking the question.  I would not buy an aircraft from a commercial establishment that I can't prove is legal without major hassle.  I'm just amazed that the commercial vendors can put out a product without showing conformance to FAR Part 103, especially when it is obvious that the aircraft will not meet the stall speed requirement by reference to the chart in Appendix 2.

Sorry, Dean. I was referring to a generic "you" not necessarily "you" in particular. Of the hundreds of ultralights, sportplanes and homebuilts I've seen over the years, virtually none of them matched the factory's demonstrator aircraft. Even bolt together ultralights like Quicksilvers have many options/additions that result in no two being exactly alike. That's why each aircraft has to qualify on its own as it is in the category that the owner wishes to operate it under. Remember that the stall speed limit is determined by the gross weight of the aircraft. The FAA's standard pilot weight is considered to be 170 lbs. If "you" weighed significantly less than that, can "you" as the only owner/pilot of an ultralight use a reduced gross weight to determine the stall speed? I don't know. My earlier example of the Kolb Firefly shows that 12 lbs can make the difference between being legal or not on the stall speed chart. Does the FAA really care that much? I don't know. It could be quite expensive to find out.

I have responded to this dicussion by putting an analysis of the Belite stall speed on my blog.  A direct link to my blog post is here:

http://jameswiebe.blogspot.com/2010/06/belite-coefficient-of-lift-and-stalling.html

Hope it's helpful...

James

James -

"Rather than write in blogs that I'm not aware of, why don't people pose good questions to me directly?"

Because the question was never about a Belite ultralight specifically.  The question was about meeting FAR 103 through AC-103.7.  Since the Belite apparently does not fit in the table in AC-103.7 per se, because there is no CL line for what you claim is the CL for the Belite wing, the question becomes how would an owner prove that it met the 24 knot stall speed?  Especially considering that Belite states clearly on their web site that it is up to the owner to prove that the airplane meets FAR 103 specs if questioned by the FAA and Belite, does not provide:

"(3) A recognized technical standards committee's findings documented as provided in paragraph 23 will usually be considered acceptable. A committee may issue their findings in relation to a given model of ultralight which are then included by the manufacturer in the sale of the ultralight. The subsequent operators of that model of ultralight may use those findings without having another inspection made, provided that there are no changes or modifications to the configuration, components, engine, or propeller arrangements of the basic model originally reviewed by the committee and any artificial means of restricting maximum airspeed is installed and operational."

Your analysis is very enlightening.  Why doesn't Belite submit it to a "recognized technical standards committee" and offer that document to buyers?  Then there would be no question.  In fact why don't all ultralight manufacturers provide that document?  Or work to change FAR 103 to something that is clearly and easily measurable, which was the whole point of this topic.

Hi Dean,

Yeah, that one comment I made was out of the CG envelope, so I removed it  My apologies. 

The problem is, no one at the FAA is paying attention to Part 103.  I've asked them how many people are assigned to it.  The answer is zero.  And no one knows what a 'recognized technical standards committee' is.  There is no definition that I'm aware of. 

So, I would submit the info to a recognized committee, if I knew of one.

The next truly irritating fact is that the majority of ultralights are still operated outside of weight limitations -- based on the number of 'fat' ultralights for sale, etc.  I doubt that very few original Kitfox Lite were ever operated legally under part 103.  I have a friend who owns one, it's N numbered.  His weighs 330 pounds dry.

Also, the FAA mandates that the operator of the Part 103 vehicle is responsible for its lawful compliance and operation.  We merely restate the obvious on our website.

-- James

A couple of other comments:

1)  "Why don't all ultralight mfgs provide that document?" 

possible answers:

a)  their aircraft aren't compliant with part 103

b)  they can't find a committee, and don't know what it is

c)  they can't perform the calculation

d)  or, they assume that no one at the FAA cares

2) "...work to change FAR part 103?"

a)  the rumblings I get is that any attempt to change it will cause it to end up in a form we won't like

b) the only thing that has a hope of changing it, IMHO, is a rewrite to accommodate electric ultralights

James

I find this interesting since a "Recognized Technical Standards Committee" is defined in AC-103.7 Section 3 Definitions:

b.  Recognized Technical Standards Committee.  This term refers to a group of at least three persons technically qualified to determine whether a given ultralight meets the requirements for operations under Part 103, as follows:

(1) It is recognized by a national pilot representative organization,

(2) It is comprised of persons not directly associated with the manufacture and/or sale of the make of ultralight being inspected, and

(3) It conducts its review and documents the findings in accordance with the guidance provided in this circular. -

I would assume that "a national pilot representative organization" would be EAA or USUA or even AOPA.  It only mentions pilots, not engineers.

The "guidance provided in this circular" is outlined in Appendix 4, Sample Documentation of Technical Standards Committee Findings.

I would think that the documentation that you provided in your blog could be verified by a group of aero engineers that are members of EAA and familiar with ultralights.  The only thing I don't understand is the mathematical derivation that you used for stall speed is not the method used in Appendix 2 to arrive at the graph of stall speed.

Hi Dean,

I think the hitch is the determination of what is 'recognized by a national pilot representative organization.' 

EAA or the USUA would be the top choices.  From my discussions with an AOPA editor, I don't think they care about ultralights.  The EAA does care a great deal:  they have run some great Belite coverage and will be running more in the next issue, (or so I hear.)

If the EAA would 'recognize' a ultralight tech committee, we'd be in great shape!

I'll check the Appendix 2 math and try and figure out the variation.

James