[ GENERAL TERMS AND CONDITIONS ] · VERSION 1.0
General Terms and Conditions
This is a convenience translation. Only the German version is legally binding.
PropLab – online propeller design and engineering services of Tplus Engineering GmbH. Version 1.0, as of 21 Oct 2026.
§ 1 Provider and scope
(1) These General Terms and Conditions (“Terms and Conditions”) apply to all contracts for the use of the web-based platform PropLab at https://prop-lab.app and for the digital products and engineering services offered there.
(2) The provider and contracting party for all services is:
Tplus Engineering GmbH
Steinbeisstr. 25
70771 Leinfelden-Echterdingen
Germany
Phone: +49 711 7674197-0
E-mail: info@tplus-engineering.de
(3) The offer is addressed to consumers within the meaning of Section 13 of the German Civil Code (Bürgerliches Gesetzbuch – BGB) and to entrepreneurs (business customers) within the meaning of Section 14 BGB. Provisions that apply only to consumers or only to entrepreneurs are marked as such.
(4) Deviating, conflicting or supplementary terms and conditions of the Customer do not become part of the contract, even if the Provider does not expressly object to them.
(5) Individual agreements, in particular individual quotations and confidentiality agreements, take precedence over these Terms and Conditions.
§ 2 Services offered
(1) PropLab is a web-based platform for the computer-aided design, optimisation and analysis of propellers, in particular for unmanned aircraft.
(2) The services offered comprise:
- 1D propeller design: automated preliminary design and optimisation based on the operating parameters entered by the User, including a calculation report; free of charge within the allowance under § 3(2), beyond that with tokens under § 16.
- CAD models: automated generation and provision of digital geometry data in the STEP and STL formats.
- CFD flow analyses: numerical flow simulations carried out by the Provider’s engineers.
- FEM structural analyses: numerical strength and structural analyses carried out by the Provider’s engineers.
- Results discussions: technical explanation of the calculations carried out.
- Token packages: credit for additional 1D calculations (§ 16).
(3) The specific scope of services is set out in the service description displayed when ordering or in the individual quotation.
(4) The services are computational design and analysis services. What is owed is the professional performance of the calculations using the methods described, for the agreed inputs and operating states, not any particular behaviour of a manufactured propeller in real operation.
(5) Unless expressly agreed, the contract does not include, in particular, the manufacture of physical propellers, experimental investigations, component releases, proof of strength or airworthiness, certifications and official approvals.
§ 3 User account and registration
(1) The use of PropLab requires a user account.
(2) Registration is free of charge. The account can be used to carry out free 1D designs within the allowance displayed, currently two calculations per calendar day and account and no more than six in total per calendar day from any one internet connection. Free calculations require a confirmed e-mail address. Creating several accounts in order to circumvent the free allowance is not permitted. The Provider may change the free allowance and the scope of the free services for the future; tokens already purchased are not affected.
(3) The User provides complete and accurate information, protects their access data against unauthorised use and does not pass them on to third parties. Any misuse that becomes known must be reported to the Provider without undue delay.
(4) When registering, the User chooses whether they are acting privately (consumer) or for business purposes (entrepreneur); before each order subject to payment, they can check and change this information. The actual purpose of the respective contract is decisive. Entrepreneurs confirm with each order subject to payment that they are acting in the exercise of their trade, business or profession.
(5) The free functions may be used by persons aged 16 or over. Orders subject to payment may only be placed by adults or by minors with the consent of their legal representatives.
(6) In the event of material breaches of these Terms and Conditions or of specific security risks, the Provider may temporarily restrict or block access, taking into account the legitimate interests of the User.
§ 4 Conclusion of the contract
(1) The presentation of the services on the website and in the application does not constitute a binding offer by the Provider.
(2) Before an order subject to payment is submitted, the application displays an overview of the selected services, the billing details and the total price. Up to that point, the Customer can identify and correct input errors using the editing functions of the respective steps. By clicking the “Order with obligation to pay” button (German: „Zahlungspflichtig bestellen“), the Customer makes a binding offer to conclude the contract.
(3) CAD files and token packages: The contract is concluded with the contract confirmation, which the Provider displays immediately after the payment process has been completed successfully and sends by e-mail. Entrepreneurs may also order CAD files and token packages on invoice; in that case, paragraph 4 sentences 1 to 3 apply accordingly.
(4) CFD and FEM analyses and results discussions: The Provider acknowledges receipt of the order by e-mail without undue delay; this acknowledgement of receipt does not yet constitute acceptance. The Provider checks the technical details and accepts the order by sending an order confirmation by e-mail. If no order confirmation is sent within five working days of receipt, the Customer is no longer bound by the order. If the Customer chooses immediate payment, they pay the total price when ordering; even then, the contract is concluded only with the order confirmation. If the Provider does not accept the order within this period or rejects it, the Provider refunds the amount paid in full and without undue delay via the means of payment used.
(5) For all services, the Customer may request an individual quotation in the application via “Request quotation”; services marked “on request” (for example unsteady CFD analyses or analyses of the customer’s own geometries) are offered by the Provider only in this way. The request is non-binding. The Provider sends the quotation by e-mail and makes it available in the user account; it is valid for four weeks unless it states otherwise. The Customer accepts the quotation within that period in the application via “Order as quoted”, making the declarations required by law and choosing the payment method. Entrepreneurs choose between payment on invoice and payment using the electronic payment methods displayed; consumers always pay using the electronic payment methods displayed. On acceptance on invoice, the contract is concluded when the “Place binding order” button is pressed; with electronic payment, upon successful completion of the payment process; an order confirmation under paragraph 4 is not required. Entrepreneurs may also accept the quotation in text form, for example by replying to the e-mail; consumers then receive a link to the order page in the application.
(6) The contract confirmation or order confirmation sets out the content of the contract and is sent together with these Terms and Conditions and – for consumers – the cancellation policy applicable to the contract and the model withdrawal form on a durable medium, usually by e-mail.
§ 5 Contract text and language of the contract
(1) The language of the contract is German.
(2) The Provider stores the contract text (order data, the applicable version of these Terms and Conditions and of the cancellation policy, declarations made). The Customer receives it by e-mail with the contract confirmation or order confirmation; the Customer can also view their orders in their user account. The current version of these Terms and Conditions is available at https://prop-lab.app/agb (binding German version; English convenience translation at https://prop-lab.app/en/terms) and can be saved and printed there.
§ 6 Prices and payment
(1) The prices displayed when ordering or the remuneration agreed in the individual quotation apply.
(2) Consumers are shown total prices including statutory VAT. Entrepreneurs are shown net prices plus statutory VAT.
(3) For entrepreneurs established in another member state of the European Union who provide a valid VAT identification number, no German VAT is charged; the recipient of the service is liable for the tax (reverse charge). The Customer is responsible for the accuracy of their information.
(4) CAD files and token packages are paid for when ordering, using the electronic payment methods of a payment service provider that are displayed. If an entrepreneur orders CAD files or token packages on invoice or accepts a quotation for them, the Provider issues the invoice upon release of the delivery (provision of the CAD files or crediting of the tokens) or, on request, in advance.
(5) Consumers pay for CFD and FEM analyses and results discussions when ordering, using the electronic payment methods displayed; they receive the invoice separately by e-mail after the contract has been concluded. Entrepreneurs choose between this immediate payment and payment on invoice; on invoice, the services are invoiced when the results are made available. Services under an individual quotation (§ 4(5)) are invoiced in accordance with the quotation; the Customer chooses the payment method on acceptance. Invoices are payable without deduction within 14 days of the invoice date, unless otherwise agreed in the quotation.
(5a) For orders on invoice, the Provider may withhold the provision of CAD files, including those included in an analysis, until payment has been received.
(6) Invoices are sent electronically, to entrepreneurs in the format prescribed by law in each case.
(7) Entrepreneurs may only exercise a right of set-off with claims that are undisputed or have been established by a final court decision.
§ 7 1D design and CAD files
(1) The 1D design is carried out on the basis of the parameters entered by the User and the calculation methods implemented in PropLab. It serves the computational preliminary design and the comparison of possible propeller geometries.
(2) The results are based on mathematical models, approximation methods and simplifying assumptions, the limits of which are stated in the report. Their informative value depends on the input data and on the validity of the models.
(3) CAD files are provided digitally in the formats stated when ordering. They reproduce the selected design including the selected subsequent geometry adjustments; the 1D design does not evaluate subsequent adjustments.
(4) Neither the 1D design nor a CAD model includes an experimental validation, strength release, airworthiness check or official approval.
(5) The quality owed for the digital products is determined by the service description. Statutory rights in the event of defects remain unaffected.
§ 8 Engineering services
(1) CFD and FEM analyses and results discussions are carried out by qualified engineers of Tplus Engineering GmbH.
(2) The scope owed is determined by the accepted order or the accepted quotation, in particular geometry, operating points, boundary conditions, calculation methods and documentation of results.
(3) The results apply only to the conditions examined and within the limits of the methods used.
(4) If a consumer has not requested that performance begin before the end of the withdrawal period, the Provider begins processing only after that period has expired.
(5) Information on processing times is non-binding unless it has been expressly agreed as binding.
(6) If changes or additional services not covered by the contract become necessary, they will be agreed separately before being carried out for a charge.
(7) The Customer may terminate an accepted order at any time before its completion by notice in text form. The Provider then charges items that have been fully performed in full, items that have been started pro rata according to the work performed up to receipt of the notice of termination, and items not yet started not at all. The statutory right of withdrawal of consumers remains unaffected.
§ 9 Customer cooperation and data
(1) The Customer is responsible for the accuracy and completeness of the inputs and documents they provide. Before starting a calculation, the Customer checks the inputs for plausibility, in particular units and operating conditions.
(2) When uploading files, the Customer ensures that they are entitled to transmit them and to have them processed in accordance with the contract. Unlawful content or content that evidently endangers security must not be uploaded.
(3) The Provider processes geometries and confidential documents provided by the Customer in order to perform the contractual services. They are not published and not passed on to third parties unless this is necessary for performing the services (for example to technically necessary service providers bound to confidentiality) or is required by law.
(4) The Provider may use calculation results and the associated design data from orders permanently and internally, without the Customer’s name and without any other personal reference, to calibrate and improve its calculation methods. These data are not published and not passed on to third parties.
(5) The Provider does not owe the Customer any data backup. The Customer keeps copies of the data they upload and backs up results provided to them themselves (§ 11).
(6) More extensive confidentiality agreements may be concluded separately. Details on the processing of personal data are set out in the Privacy Policy.
§ 10 Rights of use
(1) The PropLab software, its calculation core, the implemented methods, databases and other protected components remain with Tplus Engineering GmbH or the respective rights holders. There is no entitlement to the release of source code, internal models or databases.
(2) Upon full payment – for CAD files included in an analysis, upon payment for the analysis – the Customer receives a non-exclusive right of use, unlimited in time, to purchased CAD files to the following extent:
- use for private and commercial purposes, in particular for the design, production and manufacture of physical propellers;
- reproduction and modification for the Customer’s own design and manufacturing purposes;
- disclosure to commissioned manufacturing service providers exclusively for the production of components on behalf of the Customer; the Customer ensures that they do not use the files for their own purposes or for those of others;
- sale of the physical propellers manufactured from them without any limit on the number of units.
(3) The following are not permitted: selling or passing on the CAD files themselves as a stand-alone digital product, publishing them for general download, and granting sub-licences beyond the scope of paragraph 2.
(4) The Customer receives a non-exclusive right of use, unlimited in time, to 1D reports, CFD and FEM reports and other results for their development, design and manufacturing purposes. The Customer may pass them on unchanged as documentation to their customers, testing bodies and authorities.
(5) There is no entitlement to the exclusivity of a propeller geometry. The Provider may also calculate and provide identical or similar geometries for other customers.
(6) Rights to geometries and content provided by the Customer remain with the Customer or the respective rights holders; the Customer grants the Provider the rights of use required to perform the services.
(7) Anyone who manufactures, places on the market or operates propellers on the basis of the results or CAD files is themselves responsible for doing so, in particular for material and manufacturing quality, testing, marking and any required approvals, and is deemed to be the manufacturer to that extent within the meaning of the provisions on product safety and product liability.
§ 11 Provision and retrieval period
(1) Results, reports and CAD files are made available for retrieval in the user account.
(2) They remain available for repeated retrieval for 90 days from the date on which they are first made available, unless otherwise agreed. The application displays the end of the retrieval period.
(3) After the 90 days have expired, retrieval may no longer be possible. The Customer downloads the files within this period and backs them up themselves.
(4) The end of the retrieval period does not affect the rights of use granted to files already provided. Statutory claims, in particular in the event of defects, and statutory retention obligations remain unaffected.
§ 12 Availability
(1) PropLab is a web application; using it requires an internet connection and an up-to-date browser.
(2) Temporary interruptions, for example due to maintenance, security measures or technical faults, are possible. The Provider endeavours to provide the service reliably; a particular level of availability is owed only if expressly agreed. Statutory claims relating to digital products remain unaffected.
§ 13 Technical responsibility
(1) Apart from the computational design, the suitability of a propeller for a specific use depends on materials, manufacturing processes, material properties, loads and real operating conditions.
(2) Before physical propellers are manufactured and operated, additional tests, proofs of strength, experimental investigations and practical trials are usually required. The Provider’s services do not replace them unless they are expressly part of the contract.
(3) The manufacturer or operator is responsible for compliance with the regulations on the manufacture and operation of aircraft and their components.
§ 14 Liability
(1) The Provider is liable without limitation in cases of intent and gross negligence, for damage resulting from injury to life, body or health, under the German Product Liability Act (Produkthaftungsgesetz), in the case of fraudulently concealed defects and to the extent of any guarantee assumed.
(2) For services provided free of charge, in particular the free 1D design and the free user account, liability is otherwise excluded.
(3) For services provided against payment, the Provider is liable for slight negligence only in the event of a breach of essential contractual obligations, and limited to the damage typical for the contract and foreseeable at the time the contract was concluded. Essential contractual obligations are obligations whose fulfilment makes the proper performance of the contract possible in the first place and on whose fulfilment the Customer may regularly rely. Otherwise, liability for slight negligence is excluded.
(4) Towards entrepreneurs, the Provider is not liable in the cases of paragraph 3 for lost profits and indirect damage, insofar as these were not typically foreseeable at the time the contract was concluded.
(5) In the cases of paragraphs 2 and 3, the Provider is liable for the loss of data only up to the amount of the effort that would have been required to restore the data from a proper backup made by the Customer.
(6) The above limitations also apply to the personal liability of the Provider’s employees, representatives and vicarious agents.
(7) Towards entrepreneurs, claims for defects become time-barred one year after the service has been provided; this does not apply in the cases of paragraph 1.
(8) Entrepreneurs indemnify the Provider against third-party claims arising from the manufacture, placing on the market or operation of propellers by the Customer or on the Customer’s behalf, insofar as the Provider is not itself liable for the damage under paragraphs 1 to 3. The indemnity includes the reasonable costs of legal defence.
(9) Mandatory statutory consumer rights remain unaffected.
§ 15 Right of withdrawal for consumers
(1) Consumers have a statutory right of withdrawal for contracts concerning CAD files, CFD and FEM analyses, results discussions and token packages. The conditions, time limits and consequences are set out in the cancellation policy at https://prop-lab.app/en/withdrawal (binding German version: https://prop-lab.app/widerruf), which is displayed before the order is placed and sent with the contract confirmation.
(2) In addition to letter or e-mail, consumers can also declare their withdrawal without signing in, using the withdrawal function at https://prop-lab.app/vertrag-widerrufen. A model withdrawal form is available at https://prop-lab.app/en/withdrawal-form (binding German version: https://prop-lab.app/widerrufsformular).
(3) For CAD files, the right of withdrawal expires, subject to the statutory requirements, when provision begins; for services and token packages, upon full performance. The declarations required for this are obtained separately when ordering. For CFD and FEM analyses and results discussions, the declaration on early commencement is voluntary; without it, the Provider begins after the withdrawal period has expired.
(4) Entrepreneurs have no statutory right of withdrawal.
§ 16 Token packages
(1) Token packages are credit for additional 1D calculations. Package sizes and prices are as displayed when ordering.
(2) The tokens are credited to the user account upon conclusion of the contract (§ 4(3)) or, for an order on invoice, upon release by the Provider.
(3) One token entitles the User to one 1D calculation beyond the free allowance. The free allowance is used first, then one token per calculation. The token is debited when the calculation starts; before the start, the application shows whether a token will be used.
(4) If a calculation is aborted because of a technical error, or if the User cancels it before it begins, the token is credited back. A calculation that delivers no results for lack of feasible designs but provides an evaluation of the limits is deemed to have been performed.
(5) Tokens do not expire. Later price changes do not apply to tokens already purchased.
(6) Tokens are tied to the user account and are not transferable. A payout is made only in accordance with paragraphs 7 and 8.
(7) If a consumer withdraws from the contract for a token package, the Provider refunds the pro rata package price for the unused tokens; for tokens already used, the consumer pays compensation for their value in the amount of the pro rata package price. The right of withdrawal expires when all tokens have been used.
(8) If the Customer closes their user account or the Provider terminates it, the Provider refunds the pro rata package price for unused tokens upon request.
(9) The pro rata package price is the package price paid divided by the number of tokens in the package.
§ 17 Term and termination of the user account
(1) The contract for the free user account runs for an indefinite period.
(2) The User may have their account closed at any time, for example by e-mail to info@tplus-engineering.de.
(3) The Provider may terminate the contract for the free user account by giving four weeks’ notice in text form. The right to terminate for good cause, in particular in the event of misuse, remains unaffected.
(4) The termination of the account does not affect individual contracts already concluded, existing claims or statutory retention obligations. § 16(8) applies to unused tokens.
§ 18 Countries and legal restrictions
(1) Online orders subject to payment are possible for customers with a billing address in Germany and in the other member states of the European Union. The Provider offers services to customers in other countries on request after individual review.
(2) The Provider does not provide any services that would violate sanctions, embargoes or export control provisions. The Customer uses results and files, and passes them on to others, only in compliance with these provisions.
§ 19 Consumer dispute resolution
Tplus Engineering GmbH is neither willing nor obliged to participate in dispute resolution proceedings before a consumer arbitration board.
§ 20 Applicable law and place of jurisdiction
(1) The law of the Federal Republic of Germany applies, excluding the UN Convention on Contracts for the International Sale of Goods (CISG). For consumers, this choice of law applies only insofar as it does not deprive them of the protection afforded by mandatory provisions of the law of the state of their habitual residence.
(2) If the Customer is a merchant (Kaufmann), a legal entity under public law or a special fund under public law, the place of jurisdiction is Stuttgart.
§ 21 Final provisions
(1) For contracts concluded, the version of these Terms and Conditions agreed at the time the contract was concluded applies. Changes for future contracts are published on the platform.
(2) If a provision of these Terms and Conditions is invalid, the remainder of the contract remains valid; the invalid provision is replaced by the statutory provisions.